Soft starter. Softstarters: the right choice. Single-phase starting circuit

Soft Starter Toshiba TMC7 - an example of a soft starter

At the words "soft starter" a person who is far from electronics arises an association - something soft, stuffed with foam rubber or cotton wool.

But let's take a serious look at this wonderful device, find out what is inside it and which side to approach it from.

What is a soft starter?

The term “soft” does not refer to the starter itself, but to the starting of the motor, which is connected through such a starter.

This means, as a rule, an asynchronous electric motor with a squirrel-cage rotor. This is the most common type of engine. According to my observations, in 95% of cases, induction motors are used in industrial equipment.

Choice

When choosing a soft starter, it is quite logical to be guided primarily by the power of the connected electric motor.

However, if the motor has difficult starting conditions, as well as with frequent switching on / off, a headroom is necessary.

The fact is that the soft starter is designed in such a way that it cannot pull the motor for a long time at a voltage below the rated voltage. Since thyristors are used for this, and they are heated. And they need time to cool down and prepare "mentally" for the next start or stop. During normal operation, when the engine is running at nominal, the thyristors are fully open, the voltage on them tends to zero, and they practically do not heat up.

In powerful soft starters, in order not to strain the thyristors after the motor reaches the nominal value, a bypass contactor (bypass) is used, which can be either built-in or external.

main parameters

1. Acceleration time (front ramp)... The name speaks for itself. The shorter the acceleration time, the more difficult it is for the motor and the less sense it makes to use a soft starter. The usual acceleration time is 10 ... 20 sec. The longer this time, the more difficult it is for the soft starter - thyristors cannot work in this mode for a long time, they heat up. Another name for this parameter is the slope of the acceleration characteristic.

2. Deceleration (deceleration) time, rear ramp... The same, but the voltage gradually decreases. Another name is the slope of the braking characteristic.

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3. Initial voltage. If this value is set low, then the engine will smoothly pick up speed. If it is very small, it may not move at all. It is optimal to set such a minimum level at which the motor is guaranteed to start rotating when turned on.

4. Current limits. Here the principle is the same as that of a thermal relay, which. Only the relay cannot endure for a long time, and turns off the starting circuit, and the soft starter limits the motor current at a set level. For example, during acceleration, the current can be 120-140% of the nominal for some time, this is normal. The current will be kept at the limiting level, then the voltage will continue to increase to the nominal value.

5. Rated current. This parameter is used to protect the motor during operation, and is similar to the operation of a thermal relay - it turns off the motor if the current exceeds the setting.

Connection diagram

Schemes for switching on soft starters may differ for different models, but the meaning is the same.

I will highlight the main points.

1. Three phases at the input, three phases at the output.

2. Start / stop control system - two-wire (switch) or three-wire (two buttons, Start and Stop):

3. Internal alarm relay, which indicates an error (eg overheating or overload) and opens the corresponding one.

Scheme for switching on a soft starter

For more information about switching circuits and an example of a real application of soft starters -.

Settings

Let's take a closer look at the front panel of the Toshiba TMC7 Softstarter, appearance which is shown at the very beginning of this article.

Toshiba TMC7 Soft Starter - Front Panel

Reset- error reset.

Trip codes- error codes that are indicated in a certain number of flashes of the Ready LED.

Here is the number of blinks and the corresponding error:

Power section problem
Start time exceeded
Motor overload
Overheating of the engine
Phase imbalance
Input frequency is out of range 40 ... 72 Hz
Phase sequence error
Communication error (when using an optional module)

Current ramp- Current rise at start-up, in percent and in seconds.

Motor FLC- motor current, as a percentage of the soft starter rating. Motor protection parameter.

Current limit- current limitation during start

Soft Stop- soft stop time. 0 - motor overrun (power off, coasting)

Motor trip class- Class of thermal protection of the motor. The higher the value, the slower the motor thermal overload protection will trip.

AUX relay, Phase rotatoin- internal relay function, protection against phase change from incorrect rotation

Excess Start Time- Exceeding the start time. The engine could not reach its rated speed during this time. An increase in the current limiting level is required.

By control contacts.

C1, C2 - motor thermistor connection terminals. If there is no thermistor, a jumper is installed.

R33 ... R44 - functional relay outputs

02, 01 - connection of control buttons

A2, A1, A3 - outputs for power supply of control circuits and control circuits of the soft starter circuit.

Protection

Since the Soft Starter is an electronic power device, fast-acting fuses are required to protect the input. In extreme cases - high-speed circuit breakers with characteristic B. I spread a lot about this in the article about solid-state relays,.

On the other hand (at the output of the Soft Starter), it is necessary to protect the starter and the motor from prolonged overload. It is determined protection actuation class... The tripping class of the protection determines the starting time at a given motor current before the protection trips. There are several classes of protection - 10, 20, 30. The higher the class, the greater the inertia of the protection system.

Soft starter (SCP) is a special "mechanism", the main purpose of which is smooth start and the same smooth stopping of the electric motor. The motor soft starter can be mechanical, electromechanical or electronic.

SCP characteristics

In the modes of starting and stopping electric drives operating with asynchronous motors, the instantaneous current is 5-8 times higher than its nominal values. It is logical that such an increased current creates a large load on the power supply network, and as a result, this can lead to a short circuit or overheating and, as a result, to rapid wear of the starter windings. At the same time, the rotor torque is almost doubled, which leads to the appearance of dynamic shocks and rapid wear of the elements of the electric motor.
You can avoid the occurrence of the above problems by using a soft starter for asynchronous motors. Such a device, when starting the engine, linearly increases the voltage supply to it - from 30% to the nominal value. Also, the motor soft starter limits the upper bar of the starting current, allowing it to increase only within 3-5 times of the nominal value.

The motor soft starter can be installed in drive systems:

✓ fans;
✓compressors;
✓pomp and pumps;
✓ conveyor and conveyor lines;
✓centrifuges, mills, crushers (systems with high inertia);
✓ in combination with chain, belt and reversible gear drives.

The operation of such "soft" starters is based on the interaction of power-type thyristors connected in opposite directions. The variability of these devices is due to differences in voltage change methods, which may depend on the load on the electric motor, as well as on service functions and control schemes.
Regulation schemes are due to the direct connection of the soft starter to the power grid:

✓ Single-phase - to mitigate mechanical shock loads. No soft braking and no starting current limitation. Such soft starters can only be used for electric motors up to 11 kW.
✓ Two-phase - for starting an electric drive with a capacity of up to 250 kW in light modes.
✓ Three-phase - universal type starters suitable for frequent starts and stops. Such devices are capable of providing accurate user performance.

Additional service functions significantly expand their scope. So, SCP can be additionally used for:

✓ torque control (an important function for the system to operate at a steady speed);
✓ protection of devices from mechanical overload;
✓alarm the arisen imbalance or phase failure;
✓ thermal protection;
✓pseudo-frequency regulation (reducing the speed of the electric motor only for a certain period of time);
✓dynamic braking;
✓transferring the mechanism to the jogging mode (only for mechanisms with a high inertial mass).

The setting of the outgoing parameters for the soft starter operation (initial deceleration, motor start and deceleration times) is done manually. As for external control, it can be both analog and digital. Models with analog control are regulated by special potentiometers or by means of additional external devices. Digital devices control outgoing parameters through microprocessor controllers. It is also worth noting that digital soft starters have great functionality and a wide range of settings. A large number of high-quality digital SCPs are produced under the following brands:

✓ schneider soft starter;
✓ Altistart soft starter;
✓ soft starter abb;
✓ Schneider electric soft starter.

The choice of a suitable soft starter should be based on its overload capacity, as well as taking into account the requirements for the full and starting current of the electric motor and the estimated number of required starts per hour. The rated current of the electric motor must be less than the current of the soft starter. The connection diagram of such a device is very complicated, therefore, in some cases, if it is necessary to install a soft starter, you should contact the professionals who will prompt and correctly select the required equipment.
In our online store you will find a wide range of various electrical equipment, among which you can easily choose a soft starter. It is quite simple to buy an UPP, you just need to choose the appropriate model and fill out a simple order form. In the assortment of our online market you will find both foreign and domestic soft starters. The price of this equipment is different and depends on the technical characteristics, as well as the manufacturer's brand.

A characteristic of any electric motor during the start-up process is a multiple excess of the current and mechanical load on the equipment being driven. In this case, overloads of the supply network also occur, creating a voltage drop and degrading the quality of electricity. In many cases, a soft starter (soft starter) is required.

The need for a smooth start of electric motors

The stator winding is an inductor consisting of resistance and reactance. The value of the latter depends on the frequency of the applied voltage. When the engine is started, the reactance changes from zero, and the starting current has a large value, many times higher than the nominal one. The torque is also high and can destroy the equipment being propelled. In the braking mode, current surges also appear, leading to an increase in the temperature of the stator windings. In an emergency situation associated with engine overheating, repair is possible, but the parameters of the transformer steel change and the rated power is reduced by 30%. Therefore, a soft start is required.

Starting the electric motor by switching the windings

The stator windings can be star or delta connected. When all the ends of the windings are removed from the motor, it is possible to switch the "star" and "delta" circuits from the outside.

The motor soft starter is assembled from 3 contactors, load relay and time relay.

The motor is star-started when contacts K1 and K3 are closed. After an interval set by the time relay, K3 is disconnected and the "delta" circuit is connected by the K2 contactor. In this case, the engine reaches full speed. When it accelerates to rated speed, the starting currents are not so high.

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

Frequency regulation of rotation speed

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

A frequency converter may be included in the soft starter for asynchronous motors.

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 ripple voltage direct current.

In the intermediate circuit, the ripple is smoothed out.

In the inverter, the output signal is converted into a variable with a specified 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.

The principle of operation of the soft starter

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

a gradual increase in load;
decrease in voltage drop;
control of starting and braking at certain points in time;
reduction of interference;
protection against voltage surges, phase failure, etc.;
increasing the reliability of the electric drive.

The motor soft starter limits the amount of voltage supplied when starting. It is regulated 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 overheating of the triacs after it is connected after the motor is cranked up. Switching options are one-, two- and three-phase. Each circuit is functionally different and has a different cost. The most perfect is three-phase regulation. It is the most functional.

Disadvantages of soft starters on triacs:

simple circuits are used only with light loads or with an idle start;
prolonged start-up leads to overheating of the windings and semiconductor elements;
the shaft torque is reduced and the motor may not start.

Types of SCP

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

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

Selection of SCP

Most soft starters are voltage regulators on triacs, differing in functions, control circuits and voltage change algorithms. In modern models of soft starters, phase control methods of electric drives with any starting modes are used. Electric circuits can be with thyristor modules for a different number of phases.

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

Two-phase regulation also mitigates mechanical shocks, but does not limit current loads. The permitted motor power is 250 kW. Both methods are applied based on the calculation of reasonable prices and the characteristics of specific mechanisms.

The multifunctional soft starter with three-phase regulation has the best specifications... This enables dynamic braking and its optimization. As disadvantages, only high prices and dimensions can be noted.

Take the Altistart soft starter as an example. You can choose models for starting asynchronous motors, the power of which reaches 400 kW.

The device is selected for its power rating and mode of operation (normal or heavy).

Selection of SCP

The main parameters by which soft starters are selected are:

the limiting current strength of the soft starter and the motor must be correctly selected and correspond to each other;
the parameter of the number of starts per hour is set as the characteristic of the soft starter and should not be exceeded during engine operation;
the set voltage of the device must not be less than the mains voltage.

Soft starter for pumps

The soft starter for the pump is designed primarily to reduce water hammer in pipelines. The Advanced Control soft starter is suitable for working with pump drives. The devices almost completely eliminate water hammer when the pipelines are filled, thus increasing the equipment resource.

Smooth start of power tools

The power tool is characterized by high dynamic loads and high revs. Its visual representative is the angular Sander(LBM). Significant forces of inertia act on the working disc at the beginning of the gearbox rotation. Large overcurrents occur not only at start-up, but also every time the tool is fed.

The soft starter of the power tool is used only for expensive models. Do-it-yourself installation is an economical solution. This can be a ready-made block that fits inside the tool body. But many users assemble a simple circuit on their own and connect it to the break in the supply cable.

When the engine circuit is closed, voltage is applied to the KR1182PM1 phase regulator and capacitor C2 begins to charge. Due to this, the VS1 triac turns on with a delay that gradually decreases. The motor current increases smoothly and the speed is gained gradually. The engine accelerates in about 2 seconds. The power delivered 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. The manufacturer's specifications can be found in the documentation supplied with the equipment. There should be no mistake when choosing, since the functioning of the device will be disrupted. Consideration of the speed range is important in order to select the best combination of inverter and motor.

The asynchronous electric motor has the ability to start independently due to the interaction between the rotating flow magnetic field and the flux of the rotor winding, 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 the rated current, which can lead to overheating of the motor and its damage. To prevent this, you need a soft starter for the electric motor (SCP).

The principle of operation of the starter

It consists in the fact that the device regulates the voltage applied to the motor during starting, controlling the characteristics of the current. For induction motors, the starting torque is approximately proportional to the square of the starting current. It is proportional to the applied voltage. The torque can also be considered 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 the configuration as shown, the soft starter can regulate the voltage supplied to the motor at start-up from 0 volts to the rated line voltage. Soft start of the electric motor can be carried out in three ways:

Direct start using full load voltage.
Applying gradually downgraded.
Application of starting a partial winding using an autotransformer starter.

SCP can be of two types:

Open control: the starting voltage is applied with a time delay regardless of the motor current or speed. For each phase, the two SCRs are first delayed 180 degrees for the respective 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 stress system. This method does not actually control the acceleration of the engine.
Closed loop monitoring: Monitors any characteristics of the motor output such as current or speed. The starting 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 start

Solid state soft starters use semiconductor devices to temporarily depress parameters at the motor terminals. This allows the motor current to be monitored to reduce the torque of the motor limit value. The control is based on the voltage control of the motor terminals on two or three phases.

Several reasons why this method is preferred over others:

Increased efficiency: The performance of a soft-state soft starter system is mainly due to the low voltage state.
Controlled launch: The starting parameters can be controlled by easily changing them, which ensures that it starts without any jerks.
Controlled acceleration: The acceleration of the engine is controlled smoothly.
Low cost and size: This is done 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.

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

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

SCR basics

SCR (Silicon Controlled Rectifier) is a high power DC controlled power regulator. SCR induction motor soft starters are PNPN four-layer silicon semiconductor devices. It has three external terminals and uses alternate symbols in Figure 2 (a) and has a transistor equivalent circuit in Figure 2 (b).

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

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

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

Only a short shutter pulse is required to turn on the SCR. Once the SCR is fixed, it can be turned off again by briefly reducing its anode current below a certain value, usually a few milliamps; in AC applications, shutdown occurs automatically at the zero crossing point in every half cycle.

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

An internal capacitance (a few pF) exists between the anode and the gate of the SCR, and the surging voltage appearing at the anode can cause enough gate breakthrough to turn on the SCR. This “speed effect” can be caused by transients on the supply line, etc. The problems with the speed effect can be overcome by running a CR smoothing network between the anode and cathode to limit the rate of rise to a safe value.

The AC mains voltage (Fig. 5) is rectified using a passive diode bridge. This means that the diodes are triggered 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 moves from one diode to the next. This is typical when it is used in the DC link of a drive and some load is present. Inverters use wide pulse modulation to generate output signals. The triangle signal is generated at the carrier frequency with which the IGBT inverter will switch.

This waveform is compared to a sinusoidal waveform at the fundamental frequency that must be driven 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 limits of the inverter and / or the mechanical limits of the motor. Note that the drive always runs within the motor slip rating.

Start control process

SCR timing is the key to controlling the voltage output for a soft starter. During start-up, 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 for a while after that. This is a well-known process called "gradual recovery" of the SCR. The SCR switch point is set or programmed so that the starting torque, starting current, or current limiting is strictly controlled.

The SCR phase recovery result is a non-sinusoidal undervoltage on the motor terminals, which is shown in the figures. Since the motor is inductive and the current lags behind the voltage, the SCR remains on and conducts until the current reaches zero. This happens after the voltage has turned negative. Individual SCR voltage output.

Compared to the total voltage waveform, it can be seen that the peak voltage is the same as the total waveform 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 linear frequency, the current frequency is also the same. SCRs gradually transition to admittance, the current gaps are filled in until the waveform looks the same as the motor.

Such a smooth 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 start-up, 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 evaluating is engine torque. Standard motors produce approximately 180% of full load torque when starting. Therefore, a 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 wiring diagrams

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

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

Inside the triangle, there is another circuit according to which the starter is connected, called the internal delta circuit. In this diagram, two cables that connect to one of the motors are 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.

Soft starters (soft starters) (Soft starters) are a mechanism that ensures a smooth increase in the starting characteristics of electric motors. It softens the process of starting and stopping work .

Functions and capabilities of the soft starter

For motors that are directly started, the characteristics are significantly higher than the nominal values. Increased values of starting currents and starting torque are sources of damage, these are mechanical jerks, damage to the insulation of the winding, overheating, hard start and other problems with the electric motor. But with the help of a soft start, all unwanted malfunctions can be prevented, therefore electric motors need a soft starter (soft starter).

Main functions of SCP:

Smooth acceleration and stopping.
Reducing the starting current.
Matching the load torque with the motor torque.

In the soft starter, the voltage on the motor windings gradually increases, providing current limitation. Due to this, the parameters of the electric machine at start-up are kept within safe limits.

SCP device

Soft starters are available in different modifications and may differ in the principle of operation. But all softstarters have the same main parts.

The main components of the SCP:

Thyristors... These elements regulate the voltage that is supplied to the electric motor.
PCB assembly ... This part of the softstarters controls the thyristors.
Radiators, fans ... These appliances are essential for heat dissipation.
Current transformer ... Thanks to this component, the current is measured.
Frame .

Some soft starters are equipped with a keypad and display. Also, depending on the type of softstarter, the device can be equipped with a built-in overload relay, which eliminates the need for an external relay.

The principle of operation of the soft starter

Adjustment of starting characteristics is carried out according to two principles:

Mechanical.
Electric.

Mechanical soft starters:

A simple way to smoothly start the engine is to force the accelerating rotation speed to be forcibly used with brake pads, fluid clutches, and other elements.

This method has significant disadvantages:

Decreasing the voltage reduces the torque on the shaft.
Prolonged starting of the engine increases the risk of engine overheating.
Prolonged starting can lead to overheating of the semiconductor components of the soft starter, after which they can fail.

Also, mechanical start control is carried out exclusively at light loads or when the engine is idle.

Electric soft starters are considered more advanced, they are divided into two types according to the specifics of the work:

Amplitude... Soft starters of this type provide engine start in idle mode or with moderate load. These devices gradually increase the voltage at the motor terminals to their limits.
Frequency (phase) ... These soft starters control the frequency characteristics of the phase current without depressing the voltage. Thanks to this, the motor can be started even under heavy load.

Phase soft starters provide the following benefits:

The ability to carry out a measured addition of the rotational frequency in the operating mode.
They guarantee the stability of the high power of the motor even when changing the shaft speed.

Cons of phase soft starters:

Complexity of installation.
Complex setup.

Electrical devices for a soft start process do not have such drawbacks that could lead to a malfunction of the device itself or the motor. They always pay off in operation, but they are much more expensive than mechanical controllers.

Types of SCP

SCPs are divided into the following types:

R Voltage regulators with feedback function ... These are advanced soft starter models that control the phase shift between winding current and voltage.
Voltage regulators without feedback function ... The devices are widely used in comparison to other starters. They can be controlled in two or three phases exclusively according to the previously indicated parameters.
Starting torque regulators a... These devices can only coordinate one phase of the electric motor. And this allows you to control the starting torque of the engine and very slightly reduce the starting current. We can say that these regulators do not control the current, its decrease is hardly noticeable, so it is practically the same as with a direct start. If such a current flows through the motor windings longer than usual with direct starting, then the motor may overheat. Therefore, this type of soft starter is not used for devices requiring reduced inrush currents. But they can be used to smoothly start single-phase asynchronous motors.
Closed loop current regulators ... These are the most advanced soft starters. They have direct control over the current, which allows for more precise control of the start. Dominated by simple setup as well as starter programming. Most of the parameters are set automatically.

Voltage control devices without feedback are the most common type of soft starter. They are two- and three-phase. These soft starters can monitor the voltage in two and three phases of the motor at once. Regulation is carried out exclusively according to a previously set program, which includes indicators of the initial starting voltage and the exact time during which the voltage must rise to the nominal value. Some models of these starters are capable of limiting the starting current, but most often this limitation is associated with a decrease in voltage when starting the motor. They can also control the deceleration process by slowly reducing the voltage to stop.

The electrical and mechanical characteristics of these devices meet all the standard requirements for a soft starter. But a more advanced version of these soft starters are regulators that have feedback.

The closed-loop voltage regulators receive information about the motor current and, using this information, suspend the voltage rise during start-up. The regulators begin to reduce the voltage rise when the current reaches the limit values that are indicated in advance. These soft starters allow starting with a minimum current value and a satisfactory torque value. And the data they receive is used to organize protection against phase imbalance, overload, etc.

Application of SCP

SCPs are used in all areas of industry and Agriculture... They can be used wherever there is an electric motor. But soft starters are selected based on the engine load, as well as the frequency of starts.

For light loads and infrequent starts, use open loop controllers or starting torque controllers. These soft starters are suitable for grinding machines, some types of fans, vacuum pumps and other low-load equipment.

For frequent inertial starts and high loads, closed loop controllers are recommended. It is advisable to use them in a centrifuge, band saw, vertical conveyor, spray, etc.

Advantages and disadvantages

The use of a soft starter reduces the likelihood of motor overheating. Thus, we can highlight the main advantages of using SCP:

Increase the service life of electric motors and other actuators in contact with the electric motor.
Reduces energy consumption.
Reduce machine operating costs.
Adjusts the duration of acceleration and deceleration of the electric motor.
Reduces the strength of electromagnetic interference.
It is installed and operated without any particular difficulties.

Disadvantages:

Do not reverse the direction of rotation.
Do not control the engine speed in steady state.
Reduce the starting current to the lower values required at the time of start to rotate the rotor.

Motor soft starters are considered to be common devices that solve direct starting problems.