Smooth start of a commutator electric motor circuit. Soft starter: general information, selection tips and application features

Since recently the use of an asynchronous motor has become very widespread due to its simplicity, reliability and low price. This has become the reason for its widespread use in industry. In order to improve its characteristics and extend its service life, there are a large number of different devices capable of adjusting, starting, or protecting the engine. I will talk about one of them in this article.

This device is an electric motor soft start device (abbreviated as soft starter), otherwise called a soft starter, despite the fact that this name can be used for any device capable of soft starting the engine.

The soft starter of modern asynchronous motors replaces all previous methods, such as starting using the “star-delta switching” method, or starting using a rheostat. It is necessary to keep in mind the fact that this method is not cheap, therefore, its use must be justified. It goes without saying that the cost of the device greatly depends on the required power, starting functionality and protective properties and ranges from 2 to 10 thousand rubles, and sometimes more.

Operating principle

When the motor starts, a considerable starting torque appears (due to the need to overcome the load torque on the shaft).

To create this moment, the motors take a large amount of energy from the network, which is one of the starting problems - voltage sag.

This factor may have a negative impact on other energy consumers on the same network. Another unpleasant factor is the possibility of damage to the mechanical parts of the drive due to a sharp starting jerk.

Another problem during startup is created by considerable starting currents. Such currents, when flowing through the motor windings, generate a lot of heat, creating the risk of damage to the insulation of the windings and failure of the motor as a result of a turn short circuit.

To get rid of all such manifestations of a negative nature during engine start, a soft starter is used, which makes it possible to reduce the start currents, resulting in a significant reduction in voltage sags and, as a consequence, heating of the windings.

By reducing starting currents, we reduce the starting torque, which results in softening shocks during starting and, as a result, preserving the mechanical parts of the drive. A very significant advantage of the soft starter is that there are no jerks when starting, and the acceleration is smooth.

By appearance such a device is a rectangular module with medium dimensions, having contacts to which the motor and control circuits are connected. Some of these devices have an LCD screen, indicators and buttons that allow you to set different starting modes, take readings, limit current, etc. In addition, the devices are equipped with a network connector, which is used for programming and data exchange.

Although these devices are called soft-start devices for an electric motor, they allow not only starting, but also stopping the engine. In addition, they have all kinds of protective functionality, such as, for example, short-circuit protection, thermal protection, monitoring phase loss, excess starting currents and changes in supply voltage. In addition, the devices have a memory in which errors that occur are recorded. Therefore, using the network connector, they can be read and decrypted.

The implementation of a smooth start of motors using these devices occurs through a slow increase in voltage (while the motor accelerates smoothly) and a decrease in starting currents. The parameters that are subject to adjustment are, as a rule, primary voltage, acceleration time and stopping time. Making the primary voltage too small is not beneficial, because at the same time, the starting torque is significantly reduced, for this reason it is set within 0.3-0.6 of the nominal value.
At start, the voltage quickly rises to the preset start voltage, after which, during the set acceleration time, it slowly increases to the nominal value. At this time, the engine smoothly but quickly accelerates to the required speed.

Now such devices are manufactured by many enterprises (mostly foreign). They have many functions and can be programmed. However, with all this, they have one big disadvantage - a fairly high cost. But it is possible to create similar device and with your own hands, then it will cost much less.

Do-it-yourself soft start device for an electric motor

I'll give you one of them possible schemes similar device. The basis for constructing such a device can be a phase-type power regulator, made in the form of a KR1182PM1 microcircuit. In this circuit there are three of them (one for each phase). The diagram is shown in the figure below.

This circuit is designed to work with a 380V*50Hz motor. The motor windings are connected in a “star” and connected to the output circuits of the circuit (they are designated L11, L2, L3). The common point of the motor windings is connected to the mains neutral terminal (N). The output circuits are made on back-to-back pairs of imported thyristors, which have fairly high performance at a low price.

Power comes to the circuit after the main switch g1 closes. But the engine won't start yet. The reason for this is the de-energized windings of the relays k1-k3, as a result of which pins 3 and 6 of the microcircuits are shunted by their normally closed contacts (through resistances r1-r3). As a result of this, capacitors c1-c3 are not charged, and the microcircuits do not generate control pulses.

The circuit is started by closing the toggle switch sa1. This leads to the supply of 12 volts to the relay windings, which, in turn, makes it possible to charge the capacitors and, as a result, increase the opening angle of the thyristors. With this, a smooth rise in voltage of the motor windings is achieved. When the capacitors are fully charged, the thyristors will open to the greatest angle than the rated engine speed will be achieved.

To turn off the engine, it is enough to open the contacts sa1, which will cause the relays to turn off and the process will go in the opposite direction, ensuring engine braking.

Write comments, additions to the article, maybe I missed something. Take a look at, I will be glad if you find something else useful on my site. All the best.

Solid state soft starters (SSRVs) are designed to reduce the damaging effects of surge currents that cause mechanical stress in equipment and system components. At ABB Inc. The main emphasis is on expanding the functions of “soft” starters, which can also be used as motor protective shutdown devices. The operation of such starters is based on monitoring the motor current, voltage and temperature. A new approach to solving the problem is to smoothly increase the torque, rather than the voltage on the motor. The soft starter calculates the real stator power, its losses, etc. as a result, the actual power transferred to the rotor. It is important that the motor torque no longer depends directly on the voltage supplied to the motor or on its mechanical characteristics. The increase in torque occurs in accordance with the timed acceleration schedule. Low-voltage "soft" starters from Eaton (S752. Triac TS112 and circuits on it SB01 and S811) use a pulse-width modulated (PWM) voltage with an amplitude of 24 V to control the contactor winding. In this case, in steady state, the device consumes only 5 W. Danfoss Ci-tronic motor management devices cover a range of up to 20 kW (depending on input voltage). The smallest soft starter module MCI-3 is only 22.5 mm wide. The MCI-15 module is designed to operate with a motor with a power of up to 7.5 kW at a voltage of 480 V. An important characteristic of SSRV starters is the smooth stop of the motor. ABB's PST Series soft starters include a plain text HMI interface for easy soft stop setting centrifugal pumps, crushers, mixers, etc. The devices continuously monitor engine torque to determine when to start...

For the diagram "Device for protecting the electric motor from overheating"

Protection of electric motors from overcurrent is carried out by thermal relays built into magnetic starters. In practice, there are cases of failure due to overheating at the rated current value, at elevated temperatures environment or difficult heat exchange conditions, and the thermal relays do not operate. ...

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Failures of hand-held power tools that sometimes occur - grinders, electric drills and jigsaws - are often associated with their high starting current and significant dynamic loads on gearbox parts that occur when the engine starts abruptly. Soft start device collector The electric motor described in has a complex circuit, it contains several precision resistors and requires painstaking setup. By using the KR1182PM1 phase regulator microcircuit, it was possible to produce a much simpler device for a similar purpose that does not require setup. You can connect to it without any modification any hand-held power tool powered by a single-phase 220 V, 50 Hz network. Start and stopping the engine are carried out by the power tool switch, and in its off state the device does not consume current and can remain connected to the network indefinitely. Scheme The proposed device is shown in the figure. The XP1 plug is plugged into the power socket, and the power tool's power plug is inserted into the XS1 socket. You can install and connect in parallel several sockets for tools that work alternately. When the motor circuit of a power tool is closed by its own switch, voltage is supplied to the phase regulator DA1. Capacitor C2 begins to charge, and the voltage across it gradually increases. As a result, the delay in turning on the internal thyristors of the regulator, and with them the VSI triac, in each subsequent half-cycle of the mains voltage decreases, which leads to a smooth increase in the current flowing through the motor and, as a result, an increase in its speed. With the capacitance of capacitor C2 indicated in the diagram, acceleration to maximum speed takes 2...2.5 s, which practically does not create a delay in operation, but completely eliminates thermal...

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Consumer Electronics PUMP CONTROL UNITTo periodically fill the reservoir or, conversely, remove liquid from it, you can use a device that fundamentally scheme which is shown in Fig. 1, and the design is in Fig. 2. The use of reed sensors in it has some advantages - there is no electrical contact between the liquid and electronic unit, which allows it to be used for pumping condensation water, a mixture of water and oils, etc. In addition, the use of these sensors increases the reliability of the unit and the durability of its operation. Fig.1 In automatic mode, the device operates as follows. When the liquid level in the tank increases, the ring permanent magnet 8 (Fig. 2), which is attached to the rod 6 connected to the float 9, approaches the upper level reed switch 3 (SF2 in the diagram) from below and causes it to close. SCR VS1 opens, relay K1 is activated, turning on the pump motor with contacts K1.1 and K1.2 and self-blocking with contacts K1.3 (if the relay is not clearly self-blocking, its winding must be bypassed with an oxide capacitor with a capacity of 10... T160 current regulator circuit 50 µF).Puc2 The pump pumps out the liquid, its level in the reservoir decreases, approaching the set lower level. The magnet approaches Gorkom 2 (SF3 according to the diagram) of the lower level and causes it to short-circuit. SCR VS2 opens, relay K2 is activated and its contacts K2.1 break the circuit of the control electrode of the SCR. The thyristor closes, turning off the pump motor. If, after closing the contacts of reed switch 3 and turning on the pump, for some reason the liquid level continues to rise, the alarm reed switch 4 closes and the electric bell NA1 sounds. When the liquid level changes, the rod together with the float 9 performs reciprocating movements in the guide rings 7. 5 studs serve for...

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For the amateur radio designer, phase-inverted cascade. A single-transistor phase-inverted cascade provides the same output voltages, but the output resistances are not equal. This drawback is eliminated in the cascade, fundamentally scheme which is shown in the figure. A current generator is made on transistor T1. As a result, a high-resistance internal resistance of the generator is connected in parallel with resistor R6. A resistance is connected in parallel with resistor R5 collector transition of transistor T2, many times greater than the resistance of resistor R1. Thus, the output resistances will be determined by the resistances of resistors R5 and R6. When using the elements indicated on schematic diagram, and transistors with a static gain of 60 (transistors T1) and 30 (transistors T2), the cascade provided a gain of approximately 4.8. The device can use transistors MP40 (T1) and KT315 (T2). "Radio fernsehen eleckfronik" (GDR), 1974, N 13...

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, sources uninterruptible power supply 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 fundamental electrical diagram UPP. 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 circuit breaker QF1 with specially selected characteristics.

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” - absence 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 the 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
ok.

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 have to increase the power 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, properties of transformer steel may change so much so 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 already completely unhealthy, because may 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 a powerful and bulky industrial equipment, However, it is not. 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, do not forget that soft start systems cannot regulate engine speed 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.

Video: Soft start, adjustment and protection of the commutator. engine

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 is the starting option allows you to achieve a current almost a third lower than when starting an 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.

Video: Connecting a three-phase asynchronous electric motor with a star or triangle

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 has its own functionality and final cost respectively.

With such schemes, usually it is possible to reduce the starting current up to two or three nominal. 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 startup, and braking, smooth start 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 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 scheme is the most reliable and universal system smooth start of electric motors. Maximum power, controlled by such a device of motors, is limited solely by the maximum thermal and electrical endurance of the triacs used. His versatility allows you to implement a lot of functions such as: dynamic brake, kickback or limit balancing magnetic field and current.

An important element of the last of the mentioned circuits is the bypass contactor, which was mentioned earlier. He allows you to ensure the correct thermal conditions of the electric motor soft start system, 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 you can assemble a decent homemade starter which will extend the life of your power tool and household appliances for many years.