Electric motor soft start microcircuit. We make a smooth start of a power tool with our own hands. Frequency control of rotation speed

Who wants to strain, spend their money and time on re-equipment of devices and mechanisms that already work perfectly? As practice shows, many do. Although not everyone in life encounters industrial equipment equipped with powerful electric motors, they constantly encounter, albeit not so voracious and powerful, electric motors in everyday life. Well, everyone probably used the elevator.

Electric motors and loads - a problem?

The fact is that virtually any electric motor, at the moment of starting or stopping the rotor, experiences enormous loads. The more powerful the engine and the equipment it drives, the greater the costs of starting it.

Probably the most significant load placed on the engine at the time of start-up is a multiple, albeit short-term, excess of the rated operating current of the unit. After just a few seconds of operation, when the electric motor reaches its normal speed, the current consumed by it will also return to normal levels. To ensure the necessary power supply, it is necessary to increase the capacity of electrical equipment and conductive lines, which leads to their rise in price.

When starting a powerful electric motor, due to its high consumption, the supply voltage “drops”, which can lead to failures or failure of equipment powered from the same line. In addition, the service life of power supply equipment is reduced.

If emergency situations occur that result in engine burnout or severe overheating, the properties of transformer steel can change so much that after repair the engine will lose up to thirty percent of its power. Under such circumstances, it is no longer suitable for further use and requires replacement, which is also not cheap.

Why do you need a soft start?

It would seem that everything is correct, and the equipment is designed for this. But there is always a “but”. In our case there are several of them:

• at the moment of starting the electric motor, the supply current can exceed the rated one by four and a half to five times, which leads to significant heating of the windings, and this is not very good;
• starting the engine by direct switching leads to jerks, which primarily affect the density of the same windings, increasing the friction of the conductors during operation, accelerates the destruction of their insulation and, over time, can lead to an interturn short circuit;
• the aforementioned jerks and vibrations are transmitted to the entire driven unit. This is not healthy at all because it can cause damage to its moving parts: gear systems, drive belts, conveyor belts, or just imagine yourself riding in a jerking elevator. In the case of pumps and fans, this is the risk of deformation and destruction of turbines and blades;
• We should also not forget about the products that may be on the production line. They may fall, crumble or break due to such a jerk;
• Well, and probably the last point that deserves attention is the cost of operating such equipment. We are talking not only about expensive repairs associated with frequent critical loads, but also about a significant amount of inefficiently spent electricity.

It would seem that all of the above operating difficulties are inherent only in powerful and bulky industrial equipment, however, this is not so. All this can become a headache for any average person. This primarily applies to power tools.

The specific use of such units as jigsaws, drills, grinders and the like require multiple start and stop cycles over a relatively short period of time. This operating mode affects their durability and energy consumption to the same extent as their industrial counterparts. With all this, we should not forget that soft start systems cannot regulate the operating speed of the motor or reverse their direction. It is also impossible to increase the starting torque or reduce the current below that required to start rotating the motor rotor.

Options for soft start systems for electric motors

Star-delta system

One of the most widely used starting systems for industrial asynchronous motors. Its main advantage is simplicity. The engine starts when the windings of the star system are switched, after which, when the normal speed is reached, it automatically switches to delta switching. This starting option allows you to achieve a current that is almost a third lower than when starting the electric motor directly.

However, this method is not suitable for mechanisms with low rotational inertia. These, for example, include fans and small pumps, due to the small size and weight of their turbines. At the moment of transition from the “star” to the “triangle” configuration, they will sharply reduce the speed or stop altogether. As a result, after switching, the electric motor essentially starts again. That is, in the end, you will not only not achieve savings in engine life, but also, most likely, you will end up with excessive energy consumption.

Electronic motor soft start system

A smooth start of the engine can be done using triacs connected to the control circuit. There are three schemes for such connection: single-phase, two-phase and three-phase. Each of them differs in its functionality and final cost, respectively.

Using such circuits, it is usually possible to reduce the starting current to two to three rated ones. In addition, it is possible to reduce the significant heating inherent in the aforementioned star-delta system, which helps to increase the service life of electric motors. Due to the fact that the engine starting is controlled by reducing the voltage, the rotor accelerates smoothly and not abruptly, as with other circuits.

In general, engine soft start systems are assigned several key tasks:

• the main one is to reduce the starting current to three to four rated ones;
• reducing the motor supply voltage, if appropriate power and wiring are available;
• improvement of starting and braking parameters;
• emergency network protection against current overloads.

Single-phase starting circuit

This circuit is designed to start electric motors with a power of no more than eleven kilowatts. This option is used if it is necessary to soften the shock at start-up, but braking, soft starting and reducing the starting current do not matter. Primarily due to the impossibility of organizing the latter in such a scheme. But due to the cheaper production of semiconductors, including triacs, they have been discontinued and are rarely seen;

Two-phase starting circuit

This circuit is designed to regulate and start motors with a power of up to two hundred and fifty watts. Such soft start systems are sometimes equipped with a bypass contactor to reduce the cost of the device, however, this does not solve the problem of phase supply asymmetry, which can lead to overheating;

Three-phase starting circuit

This circuit is the most reliable and universal soft start system for electric motors. The maximum power of motors controlled by such a device is limited solely by the maximum temperature and electrical endurance of the triacs used. Its versatility allows you to implement a lot of functions, such as: dynamic brake, reverse pickup or balancing of magnetic field and current limitation.

An important element of the last of the mentioned circuits is the bypass contactor, which was mentioned earlier. It allows you to ensure the correct thermal conditions of the soft start system of the electric motor, after the engine reaches normal operating speed, preventing it from overheating.

The soft start devices for electric motors that exist today, in addition to the above properties, are designed to work together with various controllers and automation systems. They have the ability to be activated by command from the operator or the global control system. Under such circumstances, when the loads are turned on, interference may appear that can lead to malfunctions in the automation, and therefore it is worth paying attention to protection systems. The use of soft start circuits can significantly reduce their influence.

Do-it-yourself soft start

Most of the systems listed above are actually not applicable in domestic conditions. Primarily for the reason that at home we extremely rarely use three-phase asynchronous motors. But there are more than enough commutator single-phase motors.

There are many schemes for smooth starting of engines. The choice of a specific one depends entirely on you, but in principle, having a certain knowledge of radio engineering, skillful hands and desire, it is quite possible to assemble a decent homemade starter that will extend the life of your power tools and household appliances for many years.

elektro.guru

Starting an induction motor smoothly is always a difficult task because starting an induction motor requires a lot of current and torque, which can burn out the motor winding. Engineers are constantly proposing and implementing interesting technical solutions to overcome this problem, for example, using a star-delta connection circuit, an autotransformer, etc.

Currently, similar methods are used in various industrial installations for the uninterrupted operation of electric motors.

Why do we need UPP?

The principle of operation of an induction electric motor is known from physics, the whole essence of which is to use the difference between the rotation frequencies of the magnetic fields of the stator and rotor. The magnetic field of the rotor, trying to catch up with the magnetic field of the stator, contributes to the excitation of a large starting current. The motor runs at full speed, and the torque value also increases along with the current. As a result, the winding of the unit may be damaged due to overheating.

Thus, it becomes necessary to install a soft starter. Soft starters for three-phase asynchronous motors allow you to protect units from the initial high current and torque that arise due to the sliding effect when operating an induction motor.

Advantages of using a circuit with a soft starter (SPD):

1. reduction of starting current;
2. reduction in energy costs;
3. increasing efficiency;
4. relatively low cost;
5. achieving maximum speed without damaging the unit.

How to start the engine smoothly?

There are five main soft starting methods.

• High torque can be created by adding an external resistance to the rotor circuit as shown in the figure.

• By including an automatic transformer in the circuit, the starting current and torque can be maintained by reducing the initial voltage. See the picture below.

• Direct starting is the simplest and cheapest method because the induction motor is connected directly to the power source.
• Connections using a special winding configuration - the method is applicable for motors intended for operation under normal conditions.

• Using SCP is the most advanced method of all the methods listed. Here, semiconductor devices such as thyristors or SCRs, which control the speed of an induction motor, successfully replace mechanical components.

Commutator motor speed controller

Most circuits for household appliances and electrical tools are based on a 220 V commutator motor. This demand is explained by its versatility. The units can be powered from direct or alternating voltage. The advantage of the circuit is due to the provision of effective starting torque.

To achieve a smoother start and have the ability to adjust the rotation speed, speed controllers are used.

You can start an electric motor with your own hands, for example, in this way.

Conclusion

The soft starters are designed and created to limit the increase in the starting technical parameters of the engine. Otherwise, undesirable phenomena may lead to damage to the unit, burning of windings or overheating of operating circuits. For long-term service, it is important that the three-phase motor operates without voltage surges, in soft start mode.

As soon as the induction motor reaches the required speed, a signal is sent to open the circuit relay. The unit becomes ready to operate at full speed without overheating and system failures. The presented methods can be useful in solving industrial and domestic problems.

electricdoma.ru

Smooth start of an asynchronous electric motor. Design and principle of operation

Asynchronous electric motors, in addition to the obvious advantages, have two significant disadvantages - a large starting current (up to seven times more than the rated current) and a jerk at the start. These shortcomings negatively affect the condition of electrical networks, require the use of circuit breakers with an appropriate time-current characteristic, and create critical dynamic loads on equipment.

Everyone is familiar with the effect of starting a powerful asynchronous motor: “the voltage sags and everything around the electric motor shakes. Therefore, to reduce negative impacts, methods and schemes have been developed to soften the jerk and make the start of an asynchronous motor with a squirrel-cage rotor smoother.

Methods for smooth starting of asynchronous motors

In addition to the negative impact on the power circuit and the environment, the starting impulse of an electric motor is also harmful to its stator windings, because the moment of increased force during startup is applied to the windings. That is, the jerking force of the rotor puts intense pressure on the winding wires, thereby accelerating the wear of their insulation, the breakdown of which is called an interturn short circuit.

Illustration of the principle of operation of an asynchronous electric motor

Since it is structurally impossible to reduce the starting current, methods, circuits and devices have been invented to ensure a smooth start of an asynchronous motor. In most cases, in industries with powerful power lines and in everyday life, this option is not mandatory - since voltage fluctuations and starting vibrations do not have a significant impact on the production process.

Graphs of current changes during direct starting and using soft starters

But there are technologies that require stable, not exceeding standard parameters, both power supply and dynamic loads. For example, this could be precision equipment operating on the same network with voltage-sensitive electricity consumers. In this case, to comply with technological standards for soft starting of the electric motor, various methods are used:

• Star-delta switching;
• Starting using an autotransformer;
• asynchronous motor soft start devices (USM).

The video below lists the main problems that arise when starting an electric motor, and also describes the advantages and disadvantages of various soft starters for squirrel-cage asynchronous electric motors.

Industrial soft starters for electric motors of various powers

Introduction to the soft start principle

In order to smoothly start an asynchronous electric motor as efficiently as possible and at minimal cost by purchasing ready-made soft starters, you must first become familiar with the operating principle of such devices and circuits. Understanding the interaction of physical parameters will allow you to make the optimal choice of soft starter.

Using soft starters, it is possible to reduce the starting current to a value of three times the rated value (instead of a seven-fold overload)

For a smooth start of an asynchronous electric motor, it is necessary to reduce the starting current, which will have a positive effect on both the load on the electrical network and the dynamic overloads of the motor windings and drive mechanisms. They achieve a reduction in starting current by reducing the supply voltage of the electric motor. A reduced starting voltage is used in all three methods proposed above. For example, using an autotransformer, the user independently lowers the voltage at startup by turning the slider.

By lowering the voltage at the start, you can achieve a smooth start of the electric motor

When using star-delta switching, the line voltage on the motor windings changes. Switching is carried out using contactors and a time relay designed for the time the electric motor starts. A detailed description of the soft start of an asynchronous electric motor using star-delta switching is available on this resource at the specified link.

Star-delta switching circuit using contactors and time relays

Theory of soft start

To understand the principle of a smooth start, it is necessary to understand the law of conservation of energy required to spin the rotor shaft of an electric motor. In a simplified way, we can consider the acceleration energy to be proportional to power and time, E = P*t, where P is power equal to current multiplied by voltage (P = U*I). Accordingly, E = U*I *t. Since in order to reduce the starting torque and reduce the load on the network it is necessary to reduce the starting current I, then while maintaining the level of energy spent, it is necessary to increase the acceleration time.

Increasing the acceleration time by reducing the starting current is only possible with a small load on the shaft. This is the main disadvantage of all UPPs

Therefore, for equipment with difficult starting conditions (large load on the shaft during startup), special electric motors with a wound rotor are used. You can learn about the properties of these engines from the corresponding section in the article on this resource by clicking on the link.

Motor with phasic rotor, required for heavy-duty equipment

It is also necessary to take into account that during a soft start, increased heating of the windings and electronic power switches of the starting device occurs. To cool semiconductor switches, it is necessary to use massive radiators, which increase the cost of the device. Therefore, it is appropriate to use a soft starter for short-term acceleration of the engine with further bypassing of the switches with direct mains voltage. This mode (bypass switching) makes the electronic soft start device for asynchronous motors more compact and cheaper, but limits the number of starts in a certain interval due to the required time for cooling the keys.

Block diagram of shunting power semiconductor switches (bypass)

Main parameters and characteristics of the soft starter

Below in the text there will be diagrams of soft-start devices for study and self-manufacturing. For those who are not ready to soft-start an asynchronous electric motor with their own hands, relying on a finished product, information about the existing types of soft starters will be useful.

Example of an analog and digital soft starter, in modular design (mounted on a DIN rail)

One of the main parameters when choosing a soft starter is the power of the electric motor being serviced, expressed in kilowatts. Equally important is the acceleration time and the ability to adjust the start interval. All existing soft starters have these characteristics. More advanced soft starters are universal and allow you to configure soft start parameters in a wide range of values ​​relative to engine characteristics and process requirements.

An example of a universal softstarter

Depending on the type of soft starter, they may contain various options that increase the functionality of the device and allow you to control the operation of the electric motor. For example, with the help of some soft starters it is possible not only to smoothly start an electric motor, but also to brake it. More advanced soft starters protect the engine from overloads and also allow you to regulate the rotor torque during start-up, stop and operation.

An example of differences in the technical characteristics of different soft starters from the same manufacturer

Types of soft starters

According to the connection method, soft starters are divided into three types:

Do-it-yourself SCP

For self-production of a soft starter, the do-it-yourself soft start circuit for an asynchronous motor will depend on the capabilities and skills of the craftsman. Independent mitigation of starting overloads using an autotransformer is available to almost any user without special knowledge, but this method is inconvenient due to the need to manually adjust the start of the electric motor. On sale you can find inexpensive soft start devices that you will have to connect to the power tool yourself, without having in-depth knowledge of radio engineering. An example of work before and after the soft starter, as well as its connection, is shown in the video below:

An example circuit of a relatively simple two-phase soft starter

Modern soft starters have inside a complex electronic filling consisting of many electronic parts operating under the control of a microprocessor. Therefore, to make a similar soft starter with your own hands using circuit diagrams available on the Internet, you need not only the skill of a radio amateur, but also the skills of programming microcontrollers.

infoelectrik.ru

Soft start device for motors - Website for repair, connection, installation of electricians with your own hands!

Hello, my dear readers. In this article we will look at possible options for smooth engine starting.

It has long been no secret to anyone that all electric motors at the moment of starting suffer from one unpleasant disease - large starting currents. There is no way to treat this without special “medicines”. In short, the total (or equivalent) resistance of an inductor (motor winding, as a special case) consists of active (resistance of the coil to direct current) resistance and inductive (reactive), which depends on the frequency of the alternating voltage and inductance. You can read more about resistance in this article.

This is where the cause of engine illness lies. When the engine develops rated speed, the inductive reactance is very large, and, therefore, the sum of the active and reactive resistances is also large, but when the engine is stopped, the inductive reactance is practically zero, only the active remains, and it is small. According to Ohm's law, the current in a circuit is inversely proportional to the resistance, i.e. the smaller it is, the greater the current. Well, where there are high currents, don’t expect anything good. A large current means a large force, and a large force generally tries to break everything in its path. This is where soft starters come in handy.

One of the options can be considered the use of frequency converters. The advantage of this soft start method is the ability to adjust engine speed within very precise limits, flexible adjustment of start time, the ability to remotely adjust speed and start, use in dependent circuits (when the speed of revolutions is controlled by some device, sensor, etc.). The only disadvantage of this method is the price and complexity of setup in some models. Well, quite often it happens that we buy an expensive “toy” and use 15 percent of what it can do.

There is another, quite interesting, but at the same time inexpensive way to launch smoothly. But there is one small catch. The motor must be selected so that, with the “triangle” connection method, it is suitable for our voltage, that is, if we have three phases with a voltage between them of 380 volts, then the motor should be 660/380 volts. The principle is that when connected by a star, the engine runs more smoothly and, by the way, does not develop its rated power. When the windings are connected in a triangle, the engine delivers the declared power in full, but at the same time “breaks away”. This circuit allows you to spin the engine on a “star” at actually a reduced voltage (that is, the engine in our example needs 660 volts when connected to a star, and we give it 380), and then we switch it to a triangle, but it already works at the nominal or close to the rated speed and a strong current surge will not occur.

The simplicity of the scheme has a number of disadvantages. It is best to use not two machines, but a switch that will switch contacts. Because if you turn on two machines at once, a short circuit will occur. Another drawback is that with such a scheme it is quite difficult to organize reversal, only if you make another control unit, but only in reverse. Well, the general disadvantage of asynchronous three-phase motors is that when connected in a triangle, the temperature of the motor is higher and it works harder than when connected in a star, but this is understandable, since it produces full power.

Another way is to use rheostats. The difficulty is that they must be powerful, there must be three of them, and they must be adjusted at the same time. We will look at the operating principle below.

The world does not stand still and the electronics industry has come up with a solution for such cases. This solution is called "softstarter". To put it roughly, this is almost a frequency converter, but incredibly simple. It does not have the same programming capabilities as the inverter. We will now find out what opportunities there are.

Operating principle of a motor soft starter

It's simple. Let us remember Ohm's law: the current in a circuit is directly proportional to the voltage, which means that in order to reduce the current, it is necessary to reduce the voltage. This is exactly what a softstarter does. Essentially, this is a replacement for the rheostats we talked about above. The schematic diagram of such a device may well look like this:

We see a set of microcircuits that control thyristor switches, which limit the voltage supplied to the motor. In this case, the scheme is very primitive; the time interval here is rigidly set and not adjustable. Modern models have various settings.

The operating principle is simple. The circuit sets a certain initial voltage (30-60% of the nominal) and sets the time during which this initial voltage rises to the nominal value. What you should pay attention to when choosing such a device. First of all, of course, power (in critical cases it makes sense to take a margin of at least 30%, this allows you to hope that the device will work longer), the second parameter is the restart time (this indicator indicates after what period time, you can restart a completely stopped engine). The remaining parameters will be determined only by your appetite and your requests. Well, as usual – a wish: good luck with your creations!

jelektro.ru

Scheme of a thyristor soft start device for an asynchronous electric motor

Alexander Sitnikov (Kirov region)

The circuit discussed in the article allows for shock-free starting and braking of the electric motor, increasing the service life of the equipment and reducing the load on the electrical network. A soft start is achieved by regulating the voltage on the motor windings with power thyristors.

Soft start devices (SFDs) are widely used in various electric drives. The block diagram of the developed soft starter is shown in Figure 1, and the operation diagram of the soft starter is shown in Figure 2. The basis of the soft starter is three pairs of back-to-back thyristors VS1 - VS6, connected to the break of each phase. Soft start is carried out due to gradual

increasing the mains voltage applied to the motor windings from a certain initial value Un to the nominal Unom. This is achieved by gradually increasing the conduction angle of thyristors VS1 - VS6 from the minimum value to the maximum during the time Tstart, called the start time.

Typically, the value of Unat is 30...60% of Unom, so the starting torque of the electric motor is significantly less than if the electric motor is connected to full mains voltage. In this case, the drive belts are gradually tensioned and the gear wheels of the gearbox are smoothly engaged. This has a beneficial effect on reducing the dynamic loads of the electric drive and, as a result, helps to extend the service life of the mechanisms and increase the interval between repairs.

The use of a soft starter also makes it possible to reduce the load on the electrical network, since in this case the starting current of the electric motor is 2–4 rated motor current, and not 5–7 rated current, as with direct starting. This is important when powering electrical installations from energy sources of limited power, for example, diesel generator sets, uninterruptible power supplies and low-power transformer substations

(especially in rural areas). After the start-up is completed, the thyristors are bypassed by a bypass (bypass contactor) K, due to which during the time Trab the thyristors do not dissipate power, which means energy is saved.

When the engine is braking, the processes occur in the reverse order: after the contactor K is turned off, the conduction angle of the thyristors is maximum, the voltage on the motor windings is equal to the mains voltage minus the voltage drop across the thyristors. Then the conduction angle of the thyristors during the time Ttorm decreases to the minimum value, which corresponds to the cut-off voltage Uots, after which the conduction angle of the thyristors becomes zero and voltage is not applied to the windings. Figure 3 shows current diagrams of one of the motor phases with a gradual increase in the conduction angle of the thyristors.

Figure 4 shows fragments of the electrical circuit diagram of the soft starter. The full diagram is available on the magazine's website. For its operation, a voltage of three phases A, B, C of a standard 380 V network with a frequency of 50 Hz is required. The windings of the electric motor can be connected either by a star or a delta.

Inexpensive devices of type 40TPS12 in TO-247 housing with direct current Ipr = 35 A are used as power thyristors VS1 - VS6. The permissible current through the phase is Iadd = 2Ipr = 70 A. We will assume that the maximum starting current is 4Ir, which means that Inom< Iдоп/4 = 17,5 А. Просматривая стандартный ряд мощностей электродвигателей, находим, что к УПП допустимо подключать двигатель мощностью 7,5 кВт с номинальным током фазы Iн= 15 А. В случае, если пусковой ток превысит Iдоп (по причине подключения двигателя большей мощности или слишком малого времени пуска), процесс пуска будет остановлен, поскольку сработает автоматический выключатель QF1 со специально подобранной характеристикой.

Damping RC chains R48, C20, C21, R50, C22, C23, R52, C24, C25 are connected in parallel to the thyristors, preventing false switching on of the thyristors, as well as varistors R49, R51 and R53, absorbing overvoltage pulses over 700 V. Bypass relays K1, K2, K3 type TR91-12VDC-SC-C with a rated current of 40 A shunt the power thyristors after the start is completed.

The control system is powered from a transformer power supply powered from the phase-to-phase voltage Uav. The power supply includes step-down transformers TV1, TV2, diode bridge VD1, current-limiting resistor R1, smoothing capacitors C1, C3, C5, noise suppression capacitors C2, C4, C6 and linear stabilizers DA1 and DA2, providing voltages of 12 and 5 V, respectively.

The control system is built using a DD1 microcontroller type PIC16F873. The microcontroller issues control pulses for thyristors VS1 – VS6 by “igniting” optosimistors ORT5-ORT10 (MOC3052). To limit the current in the control circuits of thyristors VS1 - VS6, resistors R36 - R47 are used. Control pulses are applied simultaneously to two thyristors with a delay relative to the beginning of the phase-to-phase voltage half-wave. Synchronization circuits with mains voltage consist of three identical units, consisting of charging resistors R13, R14, R18, R19, R23, R24, diodes VD3 - VD8, transistors VT1 - VT3, storage capacitors C17 - C19 and optocouplers OPT2 - OPT4. From output 4 of optocouplers OPT2, OPT3, OPT4, pulses with a duration of approximately 100 μs are received at the inputs of the microcontroller RC2, RC1, RC0, corresponding to the beginning of the negative half-wave of phase voltages Uab, Ubc, Uca.

The operation diagrams of the synchronization unit are shown in Figure 5. If we take the top graph as the mains voltage Uav, then the middle graph will correspond to the voltage on capacitor C17, and the bottom graph will correspond to the current through the photodiode of the ORT2 optocoupler. The microcontroller registers the clock pulses arriving at its inputs, determines the presence, the order of alternation, the absence of “sticking” of phases, and also calculates the delay time of the thyristor control pulses. The inputs of the synchronization circuits are protected from overvoltage by varistors R17, R22 and R27.

Using potentiometers R2, R3, R4, parameters corresponding to the soft starter operation diagram shown in Figure 2 are set; respectively, R2 – Tstart, R3 – Tbrake, R4 – Unstart Uots. The setpoint voltages from the motors R2, R3, R4 are supplied to the inputs RA2, RA1, RA0 of the DD1 microcircuit and converted using an ADC. The starting and braking times are adjustable from 3 to 15 s, and the initial voltage is adjustable from zero to a voltage corresponding to the thyristor conduction angle of 60 electrical degrees. Capacitors C8 - C10 are noise suppressing.

The “START” command is issued by closing contacts 1 and 2 of the XS2 connector, and a log appears at output 4 of the optocoupler OPT1. 1; capacitors C14 and C15 suppress oscillations arising due to “bouncing” of contacts. The open position of contacts 1 and 2 of the XS2 connector corresponds to the “STOP” command. Switching the launch control circuit can be realized with a latching button, toggle switch or relay contacts.

Power thyristors are protected from overheating by a B1009N thermostat with normally closed contacts located on the heat sink. When the temperature reaches 80°C, the thermostat contacts open, and a log level is sent to the RC3 input of the microcontroller. 1, indicating overheating.

LEDs HL1, HL2, HL3 serve as indicators of the following states:

• HL1 (green) “Ready” – no emergency conditions, ready to launch;
• HL2 (green) “Operation” – a flashing LED means that the soft starter is starting or braking the engine, constant light means it is working on bypass;
• HL3 (red) “Alarm” – indicates overheating of the heat sink, absence or “sticking” of phase voltages.

The bypass relays K1, K2, K3 are turned on by supplying a log to the microcontroller. 1 to the base of transistor VT4.

Programming of the microcontroller is in-circuit, for which connector XS3, diode VD2 and microswitch J1 are used. Elements ZQ1, C11, C12 form the clock generator start circuit, R5 and C7 are the power reset circuit, C13 filters noise along the microcontroller power buses.

Figure 6 shows a simplified algorithm for the operation of the soft starter. After initializing the microcontroller, the Error_Test subroutine is called, which determines the presence of emergency situations: overheating of the heat sink, inability to synchronize with the mains voltage due to phase loss, incorrect connection to the network or strong interference. If the emergency situation is not recorded, then the Error variable is assigned the value “0”, after returning from the subroutine the “Ready” LED lights up, and the circuit goes into standby mode for the “START” command. After registering the “START” command, the microcontroller performs an analogue-to-digital conversion of the setpoint voltages on the potentiometers and calculates the Tstart and Ustart parameters, after which it issues control pulses for the power thyristors. At the end of the start-up, the bypass is turned on. When the engine is braking, control processes are performed in reverse order.

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Smooth start of an electric motor - ElectrikTop.ru

Electric motors are the most common electrical machines in the world. Not a single industrial enterprise, not a single technological process can do without them. Rotation of fans, pumps, movement of conveyor belts, movement of cranes - this is an incomplete, but already significant list of tasks solved with the help of engines.

However, there is one nuance in the operation of all electric motors without exception: at the moment of start, they briefly consume a large current, called starting current.

Why is the starting current of an electric motor dangerous?

When voltage is applied to the stator winding, the rotor rotation speed is zero. The rotor must be moved and spun to the rated speed. This requires significantly more energy than is needed for the nominal operating mode.

Under load, inrush currents are higher than at idle. The mechanical resistance to rotation from the mechanism driven by the engine is added to the weight of the rotor. In practice, they try to minimize the influence of this factor. For example, for powerful fans, the dampers in the air ducts automatically close at the time of startup.

At the moment the starting current flows from the network, significant power is consumed to bring the electric motor to its nominal operating mode. The more powerful the electric motor, the more power it needs to accelerate. Not all electrical networks tolerate this regime without consequences.

Overloading the supply lines inevitably leads to a decrease in network voltage. This not only makes starting the electric motors even more difficult, but also affects other consumers.

And the electric motors themselves experience increased mechanical and electrical loads during startup processes. Mechanical ones are associated with an increase in torque on the shaft. Electrical ones, associated with a short-term increase in current, affect the insulation of the stator and rotor windings, contact connections and starting equipment.

Methods for reducing inrush currents

Low-power electric motors with inexpensive ballasts start quite well without the use of any means. Reducing their starting currents or changing the rotation speed is not economically feasible.

But, when the influence on the operating mode of the network during the startup process is significant, the inrush currents require reduction. This is achieved through:

• application of electric motors with wound rotor;
• using a circuit to switch windings from star to delta;
• use of soft starters;
• use of frequency converters.

One or more of these methods is suitable for each mechanism.

Electric motors with wound rotor

The use of asynchronous electric motors with a wound rotor in work areas with difficult working conditions is the most ancient form of reducing starting currents. Without them, the operation of electrified cranes, excavators, as well as crushers, screens, and mills, which rarely start when there is no product in the driven mechanism, is impossible.

Reducing the starting current is achieved by gradually removing resistors from the rotor circuit. Initially, at the moment the voltage is applied, the maximum possible resistance is connected to the rotor. As the time relay accelerates, one after another they turn on contactors that bypass individual resistive sections. At the end of acceleration, the additional resistance connected to the rotor circuit is zero.

Crane motors do not have automatic stage switching with resistors. This happens at the will of the crane operator moving the control levers.

Switching the stator winding connection diagram

In the brno (winding start distribution block) of any three-phase electric motor there are 6 terminals from the windings of all phases. Thus, they can be connected either in a star or in a triangle.

Due to this, some versatility in the use of asynchronous electric motors is achieved. The star connection circuit is designed for a higher voltage level (for example, 660V), and the triangle connection for a lower voltage level (in this example, 380V).

But at a rated supply voltage corresponding to a delta circuit, you can use a star circuit to pre-accelerate the electric motor. In this case, the winding operates at a reduced supply voltage (380V instead of 660), and the inrush current is reduced.

To control the switching process, you will need an additional cable in the electric motor, since all 6 winding terminals are used. Additional starters and time relays are installed to control their operation.

Frequency converters

The first two methods cannot be applied everywhere. But the subsequent ones, which became available relatively recently, make it possible to smoothly start any asynchronous electric motor.

A frequency converter is a complex semiconductor device that combines power electronics and elements of microprocessor technology. The power part rectifies and smoothes the mains voltage, turning it into constant voltage. The output part of this voltage forms a sinusoidal one with a variable frequency from zero to the nominal value - 50 Hz.

Due to this, energy savings are achieved: the units driven into rotation do not operate with excessive productivity, being in a strictly required mode. In addition, the technological process has the opportunity to be finely tuned.

But it is important in the spectrum of the problem under consideration: frequency converters allow smooth starting of the electric motor, without shocks and jerks. There is no starting current at all.

Soft starters

A soft starter for an electric motor is the same frequency converter, but with limited functionality. It works only when the electric motor accelerates, smoothly changing its rotation speed from the minimum specified value to the nominal one.

To prevent useless operation of the device after the acceleration of the electric motor is completed, a bypass contactor is installed nearby. It connects the electric motor directly to the network after the start is complete.

When performing equipment upgrades, this is the simplest method. It can often be implemented with your own hands, without the involvement of highly specialized specialists. The device is installed in place of the magnetic starter that controls the start of the electric motor. It may be necessary to replace the cable with a shielded one. Then the parameters of the electric motor are entered into the device’s memory, and it is ready for action.

But not everyone can handle full-fledged frequency converters on their own. Therefore, their use in single copies is usually meaningless. The installation of frequency converters is justified only when carrying out a general modernization of the electrical equipment of the enterprise.

electrictop.ru

Do-it-yourself soft start of an electric motor

Electric motor soft start device

One of the main disadvantages of asynchronous electric motors with a squirrel-cage rotor is the presence of high starting currents. And if theoretically, methods for reducing them have been well developed for quite a long time, then almost all of these developments (the use of starting resistors and reactors, switching from star to delta, the use of thyristor voltage regulators, etc.) were used in very rare cases.

Everything has changed dramatically in our time, because... Thanks to the progress of power electronics and microprocessor technology, compact, convenient and efficient soft starters for electric motors (soft starters) have appeared on the market.

Soft start devices for asynchronous motors are devices that significantly increase the service life of electric motors and actuators operating from the shaft of this motor. When supply voltage is applied in the usual way, processes occur that destroy the electric motor.

The starting current and voltage on the motor windings, during transient processes, significantly exceed the permissible values. This leads to wear and breakdown of winding insulation, burning of contacts, and significantly reduces the service life of bearings, both the motor itself and the devices sitting on the electric motor shaft.

To provide the necessary starting power, it is necessary to increase the rated power of the supply electrical networks, which leads to a significant increase in the cost of equipment and excessive consumption of electricity.

In addition, a sag in the supply voltage at the time of starting the electric motor can lead to damage to equipment operated from the same power sources; the same sag causes serious damage to power supply equipment and reduces its service life.

At the moment of starting, the electric motor is a serious source of electromagnetic interference that disrupts the operation of electronic equipment powered from the same electrical networks or located in close proximity to the motor.

If an emergency occurs and the motor overheats or burns out, then, as a result of heating, the parameters of the transformer steel will change so much that the rated power of the repaired motor can be reduced by up to 30%, as a result, this electric motor will be unsuitable for use in its original place.

The soft start device for electric motors combines the functions of soft starting and braking, protection of mechanisms and electric motors, as well as communication with automation systems.

A soft start using a soft starter is realized by slowly raising the voltage to smoothly accelerate the engine and reduce starting currents. The adjustable parameters are usually the initial voltage, acceleration time and deceleration time of the electric motor. A very small starting voltage can greatly reduce the starting torque of the motor, so it is usually set to 30-60% of the rated voltage.

When starting, the voltage increases abruptly to the set value of the initial voltage, and then smoothly rises to the nominal value during the specified acceleration time. The electric motor will then smoothly and quickly accelerate to its rated speed.

The use of soft starters makes it possible to reduce the inrush current to minimum values ​​and reduces the number of relays and contactors used. switches. Provides reliable protection of electric motors from emergency overload, overheating, jamming, phase loss, and reduces the level of electromagnetic interference.

Soft starters for electric motors are simple to design, install and operate.

An example of a connection diagram for an electric motor soft starter

When choosing a soft starter, consider the following:

1. Motor current. It is necessary to select a soft starter based on the full load current of the motor, which should not exceed the maximum load current of the soft starter.

3. Mains voltage. Each soft starter is designed to operate at a specific voltage. The power supply voltage must correspond to the softstarter's rated value.

Soft starters

download price list download manual

Soft start is one of the essential conditions for safe and long-term operation of three-phase asynchronous electric motors.

LD1000 Series

The LD1000 series soft starter provides smooth acceleration and deceleration of the electric motor, thereby reducing the load on the electrical network and the starting mechanisms. LD1000 implements this task by limiting the starting current and torque by smoothly increasing the supplied voltage to the electric motor.

If you are not sure which soft starter to choose, our managers will always help you by calling +7 495 981-54-56.

Only here you can buy soft starters at an optimal price-quality ratio!

Main technical characteristics:

• Supply voltage 380V, 50 Hz
• Limits starting current to 450% of rated motor current
• Bypass contactor control (bypass system)
• Motor protection (short circuit, overvoltage, undervoltage, overload, phase loss, overcurrent, etc.)
• Operating temperature from 0 to +50˚С, relative air humidity no more than 95% without condensation
• Maximum acceleration time 60 s.

Cooling fan soft start

Finally, I had a free minute and I decided to make another device for my car) This time I got to the engine cooling system fan. In the standard version, when the VSOD is turned on, the on-board network voltage drops. When I installed the device I had made, I got a smooth increase in the current in the motor winding when it was turned on, eliminating a sharp jump in current, as well as dips and sudden drops in the voltage of the on-board network

P.S. This device is placed as close as possible to the fan, otherwise interference may occur that will interfere with the normal operation of the vehicle.

Application of the KR1182PM1 microcircuit. Smooth start of the electric motor

Electric motor soft starters

Soft starting of an electric motor has recently been used more and more often. Its areas of application are varied and numerous. These are industry, electric transport, utilities and agriculture. The use of such devices can significantly reduce starting loads on the electric motor and actuators, thereby extending their service life.

Starting currents

Starting currents reach values ​​7–10 times higher than in operating mode. This leads to a voltage drop in the supply network, which negatively affects not only the operation of other consumers, but also the engine itself. The start-up time is delayed, which can lead to overheating of the windings and gradual destruction of their insulation. This contributes to premature failure of the electric motor.

Soft start devices can significantly reduce the starting load on the electric motor and the electrical network, which is especially important in rural areas or when the engine is powered from an autonomous power plant.

Overload of actuators

When the engine starts, the torque on its shaft is very unstable and exceeds the rated value by more than five times. Therefore, the starting loads of the actuators are also increased compared to operation in steady state and can reach up to 500 percent. Instability of the starting torque leads to shock loads on the gear teeth, shearing of keys and sometimes even twisting of the shafts.

Electric motor soft start devices significantly reduce starting loads on the mechanism: the gaps between the gear teeth are smoothly selected, which prevents their breakage. Belt drives also smoothly tension the drive belts, which reduces wear on the mechanisms.

In addition to a smooth start, the smooth braking mode has a beneficial effect on the operation of mechanisms. If the engine drives the pump, then smooth braking avoids water hammer when the unit is turned off.

Industrial soft starters

Soft starters are currently produced by many companies, for example Siemens, Danfoss, Schneider Electric. Such devices have many functions that are user programmable. These are acceleration time, deceleration time, overload protection and many other additional functions.

With all the advantages, branded devices have one drawback - a fairly high price. However, you can create such a device yourself. At the same time, its cost will be small.

Soft start device based on KR1182PM1 microcircuit

The first part of the article talked about the specialized microcircuit KR1182PM1. representing a phase power regulator. Typical circuits for switching it on, soft start devices for incandescent lamps, and simply load power regulators were considered. Based on this microcircuit, it is possible to create a fairly simple soft-start device for a three-phase electric motor. The device diagram is shown in Figure 1.

Figure 1. Scheme of the motor soft start device.

A soft start is carried out by gradually increasing the voltage on the motor windings from zero to the nominal value. This is achieved by increasing the opening angle of the thyristor switches over a time called the startup time.

Description of the circuit

The design uses a three-phase electric motor 50 Hz, 380 V. The star-connected motor windings are connected to the output circuits indicated in the diagram as L1, L2, L3. The center point of the star is connected to the network neutral (N).

The output switches are made on thyristors connected back-to-back - in parallel. The design uses imported 40TPS12 type thyristors. At a low cost, they have a fairly high current - up to 35 A, and their reverse voltage is 1200 V. In addition to them, the keys contain several more elements. Their purpose is as follows: damping RC circuits connected in parallel with the thyristors prevent false switching on of the latter (in the diagram these are R8C11, R9C12, R10C13), and with the help of varistors RU1 RU3 switching noise, the amplitude of which exceeds 500 V, is absorbed.

DA1 DA3 microcircuits of type KR1182PM1 are used as control nodes for output switches. These microcircuits were discussed in some detail in the first part of the article. Capacitors C5 C10 inside the microcircuit form a sawtooth voltage, which is synchronized with the network. The thyristor control signals in the microcircuit are generated by comparing the sawtooth voltage with the voltage between microcircuit pins 3 and 6.

To power relay K1 K3, the device has a power supply, which consists of only a few elements. This is transformer T1, rectifier bridge VD1, smoothing capacitor C4. At the output of the rectifier, an integrated stabilizer DA4 type 7812 is installed, providing an output voltage of 12 V, and protection against short circuits and overloads at the output.

Description of the operation of the soft starter for electric motors

Mains voltage is supplied to the circuit when power switch Q1 is closed. However, the engine does not start yet. This happens because the windings of relay K1 K3 are still de-energized, and their normally closed contacts bypass pins 3 and 6 of microcircuits DA1 DA3 through resistors R1 R3. This circumstance prevents capacitors C1 C3 from charging, so the microcircuit does not generate control pulses.

Putting the device into operation

When the toggle switch SA1 is closed, the 12 V voltage turns on relay K1 K3. Their normally closed contacts open, which makes it possible to charge capacitors C1 C3 from internal current generators. Along with the increase in voltage on these capacitors, the opening angle of the thyristors also increases. This achieves a smooth increase in voltage on the motor windings. When the capacitors are fully charged, the switching angle of the thyristors will reach its maximum value, and the rotation speed of the electric motor will reach the rated speed.

Engine shutdown, smooth braking

To turn off the engine, open switch SA1. This will turn off relay K1 K3. They are normal - the closed contacts will close, which will lead to the discharge of capacitors C1 C3 through resistors R1 R3. The discharge of the capacitors will last for several seconds, during which time the engine will stop.

When starting the engine, significant currents can flow in the neutral wire. This happens because during smooth acceleration the currents in the motor windings are non-sinusoidal, but there is no need to be particularly afraid of this: the starting process is quite short-lived. In steady-state mode, this current will be much less (no more than ten percent of the phase current in nominal mode), which is due only to the technological dispersion of winding parameters and phase imbalance. It is no longer possible to get rid of these phenomena.

Details and design

To assemble the device, the following parts are required:

Transformer with a power of no more than 15 W, with an output winding voltage of 15-17 V.

Relays K1 K3 are suitable for any 12 V coil with a normally closed or switching contact, for example TRU-12VDC-SB-SL.

Capacitors C11 C13 type K73-17 for an operating voltage of at least 600 V.

The device is made on a printed circuit board. The assembled device should be placed in a plastic case of suitable dimensions, on the front panel of which switch SA1 and LEDs HL1 and HL2 should be placed.

Motor connection

The connection between switch Q1 and the motor is made with wires whose cross-section corresponds to the power of the latter. The neutral wire is made of the same wire as the phase wires. With the component ratings indicated in the diagram, it is possible to connect motors with a power of up to four kilowatts.

If you plan to use a motor with a power of no more than one and a half kilowatts, and the start-up frequency will not exceed 10 15 per hour, then the power dissipated by the thyristor switches is insignificant, so radiators can not be installed.

If you plan to use a more powerful engine or the starts will be more frequent, you will need to install thyristors on radiators made of aluminum strip. If the radiator is supposed to be used as a common one, then the thyristors should be isolated from it using mica spacers. To improve cooling conditions, you can use heat-conducting paste KPT-8.

Checking and setting up the device

Before switching on, first of all, you should check the installation for compliance with the circuit diagram. This is the basic rule, and you cannot deviate from it. After all, neglecting this check can lead to a bunch of charred parts, and for a long time discourage you from doing experiments with electricity. The errors found should be eliminated, because after all, this circuit is powered from the network, and it is not to be trifled with. And even after this check, it is still too early to connect the engine.

First, instead of the engine, you should connect three identical incandescent lamps with a power of 60-100 W. During testing, it is necessary to ensure that the lamps ignite evenly.

The unevenness of the switching time is due to the scatter in the capacitances of capacitors C1 C3, which have a significant tolerance on capacitance. Therefore, it is better to immediately select them using the device before installation, at least with an accuracy of up to ten percent.

The shutdown time is also determined by the resistance of resistors R1 R3. With their help you can adjust the shutdown time. These settings should be made if the spread in the on-off time in different phases exceeds 30 percent.

The engine can be connected only after the above checks have passed normally, not to say even perfectly.

What else can be added to the design?

It has already been said above that such devices are currently produced by different companies. Of course, it’s impossible to replicate all the functions of branded devices in such a homemade device, but you can still probably copy one.

We are talking about a so-called bypass contactor. Its purpose is as follows: after the engine has reached its rated speed, the contactor simply bridges the thyristor switches with its contacts. The current flows through them, bypassing the thyristors. This design is often called a bypass (from the English bypass - bypass). For such an improvement, additional elements will have to be introduced into the control unit.

Sources.

Characteristic of any electric motor during the startup process is a multiple excess of current and mechanical load on the driven equipment. At the same time, overloads of the supply network also occur, creating a voltage drop and deteriorating the quality of electricity. In many cases, a soft starter (soft starter) is required.

The need for smooth starting of electric motors

The stator winding is an inductance coil consisting of active resistance and reactive. The value of the latter depends on the frequency of the supplied voltage. When the engine starts, the reactance changes from zero, and the starting current has a large value, many times greater than the rated one. The rotation torque is also high and can destroy the driven equipment. During braking mode, current surges also appear, leading to an increase in the temperature of the stator windings. In the event of an emergency caused by motor overheating, repairs are possible, but the parameters of the transformer steel change and the rated power is reduced by 30%. Therefore, a soft start is necessary.

Starting an electric motor by switching windings

The stator windings can be connected in star and delta. When all ends of the motor windings are brought out, you can switch the star and delta circuits from the outside.

The soft start device for an electric motor is assembled from 3 contactors, a load relay and a time relay.

The electric motor starts in a star configuration when contacts K1 and K3 are closed. After an interval specified by the time relay, K3 is turned off and the delta circuit is connected by contactor K2. At the same time, the engine reaches full speed. When it accelerates to rated speed, the starting currents are not so large.

The disadvantage of the circuit is that a short circuit occurs when two circuit breakers are turned on simultaneously. This can be avoided by using a switch instead. To organize reverse, another control unit is needed. In addition, according to the "triangle" circuit, the electric motor heats up more and works harder.

Frequency control of rotation speed

The electric motor shaft is rotated by the magnetic field of the stator. The speed depends on the frequency of the supply voltage. The electric drive will work more efficiently if the voltage is additionally changed.

The soft start device for asynchronous motors may include a frequency converter.

The first stage of the device is a rectifier, which is supplied with voltage from a three-phase or single-phase network. It is assembled on diodes or thyristors and is designed to generate a pulsating DC voltage.

In the intermediate circuit, the ripples are smoothed out.

In the inverter, the output signal is converted into a variable signal of a given frequency and amplitude. It works on the principle of changing the amplitude or width of the pulses.

All three elements receive signals from the electronic control circuit.

Operating principle of the soft starter

An increase in starting current by 6-8 times and torque requires the use of a soft starter to perform the following actions when starting or braking the engine:

• gradual increase in load;
• reduction of voltage drop;
• control of starting and braking at certain times;
• reduction of interference;
• protection against voltage surges, phase loss, etc.;
• increasing the reliability of the electric drive.

The motor soft starter limits the amount of voltage supplied at the moment of starting. It is adjusted by changing the opening angle of the triacs connected to the windings.

Starting currents must be reduced to a value no more than 2-4 times higher than the nominal value. The presence of a bypass contactor prevents the triacs from overheating after it is connected after the motor has spun up. Switching options are single-, two- and three-phase. Each circuit is functionally different and has a different cost. The most advanced is three-phase regulation. It is the most functional.

Disadvantages of soft starters based on triacs:

• simple circuits are used only with light loads or at idle start;
• prolonged startup leads to overheating of windings and semiconductor elements;
• The shaft rotation torque is reduced and the engine may not start.

Types of AMR

The most common regulators are open-loop regulators on two or three phases. To do this, the voltage and start time are preset. The disadvantage is the lack of torque control based on the engine load. This problem is solved by a device with feedback, along with additional functions of reducing the inrush current, creating protection against phase imbalance, overload, etc.

The most modern soft starters have continuous load monitoring circuits. They are suitable for heavily loaded drives.

Selection of soft starter

Most soft starters are voltage regulators based on triacs, differing in functions, control circuits and voltage change algorithms. Modern models of soft starters use phase control methods for electric drives with any starting modes. Electrical circuits can have thyristor modules for different numbers of phases.

One of the simplest is a soft start device with single-phase regulation through one triac, which only allows softening the mechanical shock loads of motors with a power of up to 11 kW.

Two-phase regulation also softens mechanical shocks, but does not limit current loads. The permissible engine power is 250 kW. Both methods are used based on reasonable prices and the characteristics of specific mechanisms.

Multifunctional soft starter with three-phase control has the best technical characteristics. Here the possibility of dynamic braking and optimization of its operation is provided. The only disadvantages that can be noted are the high prices and dimensions.

Take the Altistart soft starter as an example. You can select models for starting asynchronous motors whose power reaches 400 kW.

The device is selected according to its rated power and operating mode (normal or heavy).

Selection of soft starter

The main parameters by which soft starters are selected are:

• the maximum current strength of the soft starter and the motor must be correctly selected and match each other;
• the parameter for the number of starts per hour is set as a characteristic of the soft starter and should not be exceeded when operating the engine;
• the specified device voltage should not be less than the mains voltage.

Soft starter for pumps

The soft start device for the pump is designed primarily to reduce hydraulic shocks in pipelines. Advanced Control soft starters are suitable for working with pump drives. The devices almost completely eliminate water hammer when pipelines are full, allowing you to increase the service life of the equipment.

Smooth starting of power tools

Power tools are characterized by high dynamic loads and high speeds. Its obvious representative is the angle grinder (angle grinder). Significant inertial forces act on the working disk at the beginning of rotation of the gearbox. Large current overloads occur not only during startup, but also every time the tool is fed.

The soft start device for power tools is used only for expensive models. An economical solution is to install it yourself. This can be a ready-made block that fits inside the tool body. But many users assemble a simple circuit themselves and connect it to the power cable.

When the motor circuit is closed, voltage is applied to the phase regulator KR1182PM1 and capacitor C2 begins to charge. Due to this, triac VS1 is turned on with a delay that gradually decreases. The motor current gradually increases and the speed increases gradually. The engine accelerates in approximately 2 seconds. The power supplied to the load reaches 2.2 kW.

The device can be used for any power tool.

Conclusion

When choosing a soft starter, it is necessary to analyze the requirements for the mechanism and characteristics of the electric motor. Manufacturer's specifications can be found in the documentation supplied with the equipment. There should be no mistakes when choosing, since the functioning of the device will be disrupted. It is important to consider the speed range to select the best drive/motor combination.

An induction motor has the ability to start on its own due to the interaction between the rotating magnetic field flux and the rotor winding flux, causing a high current in it. As a result, the stator draws a large current, which by the time the motor reaches full speed becomes more than rated, which can lead to overheating of the motor and damage. To prevent this, a soft start device for the electric motor is required.

Operating principle of the starter

It consists in the fact that the device regulates the voltage applied to the engine during starting, controlling the current characteristics. For asynchronous motors, the starting torque is approximately proportional to the square of the starting current. It is proportional to the applied voltage. Torque can also be considered to be approximately proportional to the applied voltage, thus by adjusting the voltage during starting, the current drawn by the machine and its torque are controlled by the device and can be reduced.

Using six SCRs in a configuration as shown, the soft starter can regulate the voltage supplied to the motor at startup from 0 volts to rated line voltage. Soft starting of an electric motor can be carried out in three ways:

1. Direct starting using full load voltage.
2. Applying gradually reduced.
3. Application of starting a partial winding using an autotransformer starter.

SCP can be of two types:

1. Open management: The starting voltage is applied with a time delay regardless of the current or motor speed. For each phase, two SCRs are performed first delayed by 180 degrees for the corresponding half-wave cycles (for which each SCR is performed). This delay gradually decreases over time until the applied voltage reaches the nominal value. It is also known as a temporary voltage system. This method does not actually control engine acceleration.
2. Closed loop control: Any characteristics of the motor output such as current or speed are monitored. The trigger voltage is varied accordingly to obtain the required response. Thus, the task of the soft starter is to control the conduction angle of the SCR and control the supply voltage.

Benefits of soft starting

Solid state soft starters use semiconductor devices to temporarily reduce parameters at the motor terminals. This provides motor current control to reduce the motor's torque limit. The control is based on controlling the voltage of the motor terminals on two or three phases.

Several reasons why this method is preferable to others:

1. Increased efficiency: The efficiency of a soft starter system using solid state switches is mainly due to the low voltage condition.
2. Controlled launch: The starting parameters can be controlled by changing them easily, which ensures that it starts without any jerking.
3. Controlled acceleration: Engine acceleration is controlled smoothly.
4. Low cost and size: This is achieved using solid state switches.

Solid State Components

Power switches such as SCRs that are phase controlled for each part of the cycle. For a three-phase motor, two SCRs are connected to each phase. Motor soft start relays must be rated at least three times the line voltage.

A working example of a system for a three-phase asynchronous motor. The system consists of 6 SCRs, control logic in the form of two comparators - LM324 and LM339 to obtain the level and ramp voltage and an opto-isolator to control the application of gate voltage to the SCR on each phase.

Thus, by controlling the duration between pulses or their delay, the controlled SCR angle is controlled and the power supply is regulated during the engine starting phase. The entire process is actually an open-loop control system that controls the timing of the application of gate trigger pulses for each SCR.

SCR Basics

SCR (Silicon Controlled Rectifier) ​​is a high power controlled DC power regulator. Soft starters for asynchronous motors SCR is a four-layer PNPN silicon semiconductor device. It has three external terminals and uses the alternative symbols in Figure 2(a) and has the transistor equivalent circuit in Figure 2(b).​

The main way to use an SCR is as a switch with the anode positive relative to the cathode, controlled when the machine is started.

The main characteristics of SCR can be understood with the help of these diagrams. The motor soft starter can be turned on and made to act as a silicon forward bias rectifier by briefly applying gate current to it through S2. The SCR quickly (within a few microseconds) automatically latches into the on state and remains on even when the gate drive is removed.

This action is shown in Figure 2 (b) the initial gate current is turned on by Q1 and the collector current of Q1 is turned on by Q2, the collector current of Q2 then holds Q1 even when the gate drive is removed. A saturation potential of 1 V or so is created between the anode and cathode.

Only a short gate pulse is required to turn on the SCR. Once the SCR has been latched, it can be turned off again by briefly reducing its plate current below a certain value, typically a few milliamps; in AC applications, shutdown occurs automatically at the zero crossing point in each half cycle.

Significant gain is available between the gate and anode of the SCR, and low values ​​of gate current (typically a few mA or less) can control high values ​​of anode current (up to tens of amplifiers). Most SCRs have anode ratings of hundreds of volts. The characteristics of the SCR gate are similar to those of the transistor junction - the emitter of the transistor (see Fig. 2(b)).

An internal capacitance (several pF) exists between the anode and gate of the SCR, and a sudden increase in voltage appearing at the anode can cause enough signal to break through to the gate to turn on the SCR. This "speed effect" can be caused by power line transients, etc. Speed ​​effect problems can be overcome by running a CR smoothing network between the anode and cathode to limit the rise speed to a safe value.

The AC mains voltage (Fig. 5) is rectified using a passive diode bridge. This means that the diodes fire when the line voltage is greater than the voltage across the capacitor section. The resulting waveform has two pulses during each half-cycle, one for each diode conduction window.

The waveform shows some continuous current as conduction passes from one diode to the next. This is typical when it is used in the DC link of the drive and some load is present. Inverters use pulse wide modulation to create output signals. A triangle signal is generated at the carrier frequency from which the IGBT inverter will switch.

This waveform is compared to a sine waveform at the fundamental frequency that must be delivered to the motor. The result is the U waveform shown in the figure.

The inverter output can be any frequency below or above the line frequency up to the inverter limits and/or the motor mechanical limits. Please note that the drive always operates within the motor slip rating.

Start control process

The timing of the SCR is the key to controlling the voltage output for the soft starter. During startup, the soft starter logic determines when to turn on the SCR. It does not turn on the SCR at the point where the voltage goes from negative to positive, but waits a while after that. This is a known process called "gradual recovery" of SCR. The SCR cut-in point is set or programmed such that the initial torque, initial current or current limit is strictly regulated.

The result of gradual SCR recovery is a non-sinusoidal reduced voltage at the motor terminals, which is shown in the figures. Since the motor is inductive and the current lags the voltage, the SCR remains on and conducts until the current reaches zero. This occurs after the voltage has become negative. Individual SCR voltage output.

When compared with the full voltage waveform, you can see that the peak voltage is the same as the full wave voltage. However, the current does not increase to the same level as when full voltage is applied due to the inductive nature of the motors. When this voltage is applied to the motor, the output current looks like the figure.

Since the voltage frequency is the same as the line frequency, the current frequency is also the same. The SCRs progress to full conduction in stages, gaps in current being filled until the waveform looks the same as the motor.

Such a soft start of an asynchronous electric motor, in contrast to an AC drive, has the characteristics of the current in the network and the motor current are always the same. During startup, the change in current depends directly on the magnitude of the applied voltage. Motor torque varies as the square of the applied voltage or current.

The most important factor in the evaluation is engine torque. Standard engines produce approximately 180% of full load torque at start-up. Therefore, 25% derating will be equal to full load torque. If the motor draws 600% of the full load current when starting, then the current in this circuit will reduce the starting current from 600% to 450% of the load.

Starter connection diagrams

There are two options with which the starter starts the electric motor: a standard circuit and inside a triangle.

Standard scheme. The starter is connected in series with the line voltage supplied to the motor.

Inside the triangle, there is another circuit by which the starter is connected, called the internal delta circuit. In this circuit, the two cables that connect to one of the motors will be connected directly to the I/P power supply, and the other cable will be connected through the starter. One feature of this circuit is that the starter can be used for large motors, such as 100 kW motors, since the phase currents are divided into 2 parts.

The design features of some tools, for example, an angle grinder, result in a high impact of dynamic loads on the engine of the device. To eliminate uneven loads on the electrical appliance and its components, it is recommended to purchase or make your own soft start device (SPD).

general information

In power tools, in which the working part is represented by a disk that rotates at high speed, at the beginning of their operation, inertial forces act on the gearbox axis. This impact entails the following negative aspects:

1. The inertial jerk created as a result of the load on the axle during a sharp start can tear the unit out of your hands, especially if disks of large diameter and weight are used;

Important! Due to such inertial jerks, when working with steel and diamond discs, it is necessary to hold the tool with both hands and be prepared to hold it, otherwise you may be injured if the unit breaks down.

1. A sudden supply of operating voltage to the engine creates a large current overload, which occurs after the unit has reached the minimum speed. This leads to overheating of the motor windings and rapid wear of the brushes. Frequently turning the tool on and off can lead to a short circuit, since there is a high probability of melting the insulating layer of the windings;
2. A sharp increase in speed of an angle grinder or circular saw due to the high torque leads to rapid wear of the gearbox. Sometimes it is possible for the gearbox to jam or even break off its teeth;
3. The overloads that the working disk experiences during a sudden start can lead to its destruction. The presence of a protective casing on such power tools is mandatory.

Important! When starting the angle grinder, the open section of the casing should be on the opposite side from the person in order to protect him from flying fragments in case of possible destruction of the working disk.

To reduce the harmful effects of sudden and dynamic starts on power tools, manufacturers produce models with built-in soft start and speed control.

For information. Such devices are built into units from the middle and high price categories.

A soft starter and speed controller are missing from many power tools found in most households. If you purchase powerful equipment (the diameter of the working disk is more than 20 cm) without a soft starter, a sudden start of the engine will lead to rapid wear of the mechanics and electrical parts, and it is also difficult to hold such a unit in your hands when turning it on. Installing a soft starter is the only way out.

The market for components for power tools offers many models of ready-made soft start units and rotary regulators.

A ready-made soft starter for a power tool can be mounted either inside the case if there is free space, or connected to the power cable. However, you don’t have to buy a finished product, but make it yourself, since the design of this device is quite simple.

Self-production of UPP

To manufacture the most popular soft starter for power tools based on the KR1182PM1R board, you will need the following tools and materials:

• soldering iron with solder;
• phase control microcircuit KR1182PM1R;
• resistors;
• capacitors;
• triacs;
• other auxiliary elements.

In the device, which is obtained according to the diagram above, control occurs through the KR1182PM1R board, and triacs act as the power part.

The advantages of this soft starter assembly are the following:

• ease of manufacture;
• no need for additional settings after assembling the soft starter;
• the soft starter can be installed in any type and model of power tool that is designed for an alternating voltage of 220 V;
• there are no requirements for the removal of a separate power button - the modified unit is activated by a standard button;
• the possibility of installing such a unit inside the equipment or in the break of the power cable with its own housing;
• Any home craftsman who has the basics of soldering and reading microcircuits can make such devices.

Recommendation. The most practical option for connecting a soft starter is to connect it to an outlet that serves as a power source for the power tool. To do this, you will need to connect a power socket to the output of the device (socket XS1 in the diagram), and supply 220V power to the input (socket XP1 in the diagram).

Operating principle of the soft starter

The operating principle of such a soft start unit installed in an angle grinder consists of the following processes:

1. After pressing the start key on the angle grinder, voltage is supplied to the microcircuit;
2. At the control capacitor (C2), a process of smooth increase in electrical voltage occurs: as this element is charged, it reaches operating values;
3. The thyristors located on the control board open with a delay, which depends on the time the capacitor is fully charged;
4. The triac (VS1) is controlled by thyristors and opens with the same delay;
5. In each half of the period of alternating electrical voltage, such a pause decreases, which leads to its smooth supply to the input of the working unit;
6. After turning off the grinder, the capacitor element is discharged by the resistance of the resistor.

It is the processes described above that determine the smooth start of the angle grinder, which eliminates inertial shock to the gearbox due to a gradual increase in disk speed.

The time it takes for the power tool to reach its operating speed is determined only by the capacitance of the control capacitor. If, for example, the capacitor element has a capacity of 47 μF, then a smooth start will be ensured in 2-3 seconds. This time is enough to ensure that the start of using the tool is comfortable, and the tool itself is not subjected to shock loads.

If the resistor has a resistance of 68 kOhm, then the capacitor discharge time will be approximately 3 seconds. After this time period has passed, the soft starter is completely ready for the next cycle of starting the power tool.

On a note. This circuit can be subject to slight modification, which will add the function of a speed controller to the soft starter. To do this, you need to change the regular resistor (R1) to a variable version. By controlling the resistance, you can regulate the power of the electric motor by changing the number of its revolutions.

Other elements of the circuit are intended for the following:

• resistor (R2) is responsible for controlling the amount of electric current that flows through the input of the triac;
• capacitor (C1) is one of the additional components of the control system of the KR1182PM1R board, used in the standard version of the switching circuit.

Tips for assembling the structure and choosing materials:

1. Ease of installation and compactness of the future product can be ensured by soldering condensing elements and resistors directly to the legs of the control board;
2. A triac must be selected with a minimum electrical current of 25 A and an electrical voltage of no more than 400 V. The magnitude of the electric current will completely depend on the power rating of the power tool motor;
3. Due to the soft start of the unit, the current will not exceed the nominal values ​​​​set by the manufacturer. In some cases, for example, jamming of the working disk of an angle grinder, an additional supply of electric current may be required; accordingly, it is better to choose a triac with an operating current that is equal to twice the nominal value of the tool;
4. The power of an angle grinder or other type of tool when working with a soft starter according to the KR1182PM1R scheme should not exceed 5,000 W. This condition is due to the operating characteristics of the board.

There are also other soft start schemes for power tools and various motors, which differ strikingly from each other in all respects: from the installation method and appearance to the connection method and components.

For your information. The above scheme is the simplest and is used everywhere, as it has proven its efficiency and reliability.

Soft start device for power tools - saving money on repairs and complete protection of the main components of the device. Everyone has a choice: buy a UPP or make it yourself. If you have some knowledge of electrical engineering and soldering radio components, then it is recommended to do the assembly yourself, as it is reliable and simple. Otherwise, you should purchase a ready-made soft-start device for power tools at any specialized store or radio market.

Video

Alexander Sitnikov (Kirov region)

The circuit discussed in the article allows for shock-free starting and braking of the electric motor, increasing the service life of the equipment and reducing the load on the electrical network. is achieved by regulating the voltage on the motor windings with power thyristors.

Soft start devices (SFDs) are widely used in various electric drives. The block diagram of the developed soft starter is shown in Figure 1, and the operation diagram of the soft starter is shown in Figure 2. The basis of the soft starter is three pairs of back-to-back thyristors VS1 - VS6, connected to the break of each phase. Soft start is carried out due to gradual

increasing the mains voltage applied to the motor windings from a certain initial value Un to the nominal Unom. This is achieved by gradually increasing the conduction angle of thyristors VS1 - VS6 from the minimum value to the maximum during the time Tstart, called the start time.

Typically, the value of Unat is 30...60% of Unom, so the starting torque of the electric motor is significantly less than if the electric motor is connected to full mains voltage. In this case, the drive belts are gradually tensioned and the gear wheels of the gearbox are smoothly engaged. This has a beneficial effect on reducing the dynamic loads of the electric drive and, as a result, helps to extend the service life of the mechanisms and increase the interval between repairs.

The use of a soft starter also makes it possible to reduce the load on the electrical network, since in this case the starting current of the electric motor is 2 - 4 times the motor current rating, and not 5 - 7 ratings, as with direct starting. This is important when powering electrical installations from energy sources of limited power, for example, diesel generator sets, uninterruptible power supplies and low-power transformer substations

(especially in rural areas). After the start-up is completed, the thyristors are bypassed by a bypass (bypass contactor) K, due to which during the time Trab the thyristors do not dissipate power, which means energy is saved.

When the engine is braking, the processes occur in the reverse order: after the contactor K is turned off, the conduction angle of the thyristors is maximum, the voltage on the motor windings is equal to the mains voltage minus the voltage drop across the thyristors. Then the conduction angle of the thyristors during the time Ttorm decreases to the minimum value, which corresponds to the cut-off voltage Uots, after which the conduction angle of the thyristors becomes zero and voltage is not applied to the windings. Figure 3 shows current diagrams of one of the motor phases with a gradual increase in the conduction angle of the thyristors.

Figure 4 shows fragments of the electrical circuit diagram of the soft starter. The full diagram is available on the magazine's website. For its operation, voltage of three phases A, B, With a standard network of 380 V with a frequency of 50 Hz. The windings of the electric motor can be connected either by a star or a delta.

Low-cost devices of type 40TPS12 in TO-247 housing with direct current Ipr = 35 A are used as power thyristors VS1 - VS6. The permissible current through the phase is Iadd = 2Ipr = 70 A. We will assume that the maximum starting current is 4Ir, which means that Inom< Iдоп/4 = 17,5 А. Просматривая стандартный ряд мощностей электродвигателей, находим, что к УПП допустимо подключать двигатель мощностью 7,5 кВт с номинальным током фазы Iн= 15 А. В случае, если пусковой ток превысит Iдоп (по причине подключения двигателя большей мощности или слишком малого времени пуска), процесс пуска будет остановлен, поскольку сработает автоматический выключатель QF1 со специально подобранной характеристикой.

Damping RC chains R48, C20, C21, R50, C22, C23, R52, C24, C25 are connected in parallel to the thyristors, preventing false switching on of the thyristors, as well as varistors R49, R51 and R53, absorbing overvoltage pulses over 700 V. Bypass relays K1, K2, K3 type TR91-12VDC-SC-C with a rated current of 40 A shunt the power thyristors after the start is completed.

The control system is powered from a transformer power supply powered from the phase-to-phase voltage Uav. The power supply includes step-down transformers TV1, TV2, diode bridge VD1, current-limiting resistor R1, smoothing capacitors C1, C3, C5, noise suppression capacitors C2, C4, C6 and linear stabilizers DA1 and DA2, providing voltages of 12 and 5 V, respectively.

The control system is built using a DD1 microcontroller type PIC16F873. The microcontroller issues control pulses for thyristors VS1 - VS6 by “igniting” optosimistors ORT5-ORT10 (MOC3052). To limit the current in the control circuits of thyristors VS1 - VS6, resistors R36 - R47 are used. Control pulses are applied simultaneously to two thyristors with a delay relative to the beginning of the phase-to-phase voltage half-wave. Synchronization circuits with mains voltage consist of three identical units, consisting of charging resistors R13, R14, R18, R19, R23, R24, diodes VD3 - VD8, transistors VT1 - VT3, storage capacitors C17 - C19 and optocouplers OPT2 - OPT4. From output 4 of optocouplers OPT2, OPT3, OPT4, pulses with a duration of approximately 100 μs are received at the inputs of the microcontroller RC2, RC1, RC0, corresponding to the beginning of the negative half-wave of phase voltages Uab, Ubc, Uca.

Diagrams of the operation of the synchronization unit are shown in Figure 5. If we take the top graph as the mains voltage Uav, then the middle graph will correspond to the voltage on capacitor C17, and the bottom graph will correspond to the current through the photodiode of the optocoupler ORT2. The microcontroller registers the clock pulses arriving at its inputs, determines the presence, the order of alternation, the absence of “sticking” of phases, and also calculates the delay time of the thyristor control pulses. The inputs of the synchronization circuits are protected from overvoltage by varistors R17, R22 and R27.

Using potentiometers R2, R3, R4, parameters corresponding to the soft starter operation diagram shown in Figure 2 are set; accordingly, R2 - Tstart, R3 - Tbrake, R4 - Unstart Uots. The setpoint voltages from the motors R2, R3, R4 are supplied to the inputs RA2, RA1, RA0 of the DD1 microcircuit and converted using an ADC. The starting and braking times are adjustable from 3 to 15 s, and the initial voltage is adjustable from zero to a voltage corresponding to the thyristor conduction angle of 60 electrical degrees. Capacitors C8 - C10 are noise suppressing.

Team “START” is applied by closing contacts 1 and 2 of the XS2 connector, while a log appears at output 4 of the optocoupler OPT1. 1; capacitors C14 and C15 suppress oscillations arising due to “bouncing” of contacts. The open position of contacts 1 and 2 of the XS2 connector corresponds to the “STOP” command. Switching the launch control circuit can be realized with a latching button, toggle switch or relay contacts.

Power thyristors are protected from overheating by a B1009N thermostat with normally closed contacts located on the heat sink. When the temperature reaches 80°C, the thermostat contacts open, and a log level is sent to the RC3 input of the microcontroller. 1, indicating overheating.

LEDs HL1, HL2, HL3 serve as indicators of the following states:

• HL1 (green) “Ready” - no emergency conditions, ready to launch;
• HL2 (green) “Operation” - a flashing LED means that the soft starter is starting or braking the engine, constant light means it is working on bypass;
• HL3 (red) “Alarm” - indicates overheating of the heat sink, absence or “sticking” of phase voltages.

The bypass relays K1, K2, K3 are turned on by supplying a log to the microcontroller. 1 to the base of transistor VT4.

Programming of the microcontroller is in-circuit, for which connector XS3, diode VD2 and microswitch J1 are used. Elements ZQ1, C11, C12 form the clock generator start circuit, R5 and C7 are the power reset circuit, C13 filters noise along the microcontroller power buses.

Figure 6 shows a simplified algorithm for the operation of the soft starter. After initializing the microcontroller, the Error_Test subroutine is called, which determines the presence of emergency situations: overheating of the heat sink, inability to synchronize with the mains voltage due to phase loss, incorrect connection to the network or strong interference. If the emergency situation is not recorded, then the Error variable is assigned the value “0”, after returning from the subroutine the “Ready” LED lights up, and the circuit goes into standby mode for the “START” command. After registering the “START” command, the microcontroller performs an analogue-to-digital conversion of the setpoint voltages
on potentiometers and calculation of parameters Tstart and Ustart, after which it issues control pulses for power thyristors. At the end of the start-up, the bypass is turned on. When the engine is braking, control processes are performed in reverse
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