The use of a soft starter. Motor soft starter. How it works. Soft start for single-phase motor

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

Soft starter characteristics

In the start and stop modes of 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 almost doubles, which leads to the appearance of dynamic shocks and rapid wear of the electric motor elements.
You can avoid 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 limit of the starting current, allowing its increase only within 3-5 times of the nominal value.

Motor soft starter can be installed in drive systems:

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

The operation of such "soft" starters is based on the interaction of back-to-back thyristors of the power type. The variability of these devices is due to differences in the methods of voltage change, 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 inclusion of the soft starter in the power grid:

• ✓Single-phase - to mitigate shock mechanical loads. Soft braking is not carried out and the starting current is not limited. Such soft starters can only be used for electric motors up to 11 kW.
• ✓Two-phase - for starting an electric drive with a power of up to 250 kW in light modes.
• ✓Three-phase - universal type starters suitable for frequent starts and stops. Such devices are capable of accurately maintaining user performance.

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

• ✓Torque control (important function for running the system at a steady speed);
• ✓protection of devices from mechanical overloads;
• ✓signaling the resulting 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;
• ✓transfer mechanism to jog mode (only for mechanisms with high inertial mass).

The output parameters for the soft starter operation (initial deceleration, motor start and deceleration times) are set 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 soft starters are produced under the following brands:

• ✓Schneider soft starter;
• ✓altistart soft starter;
• ✓soft starter abb;
• ✓Schneider electric soft starter.

The selection 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 expected number of starts required per hour. The motor rated current must be less than the soft starter current. The scheme for switching on such a device is very complicated, therefore, in some cases, if you need to install a soft starter, you should contact 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. Buying a soft starter is quite simple, you just need to choose the right 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 varies and depends on specifications as well as the manufacturer's brand.

Soft starter- an electrical device used in asynchronous electric motors, which allows during start-up to keep the motor parameters (current, voltage, etc.) within safe limits. Its use reduces starting currents, reduces the likelihood of motor overheating, eliminates jerks in mechanical drives, which ultimately increases the life of the electric motor.

Purpose

Controlling the process of starting, running and stopping electric motors. The main problems of asynchronous electric motors are:

• the impossibility of matching the engine torque with the load torque,
• high starting current.

During start-up, the torque often reaches 150-200% in a fraction of a second, which can lead to failure of the drive kinematic chain. In this case, the starting current can be 6-8 times higher than the rated current, causing problems with power stability. A soft starter avoids these problems by making the acceleration and deceleration of the motor slower. This allows to reduce starting currents and avoid jerks in the mechanical part of the drive or hydraulic shocks in pipes and valves at the time of starting and stopping the engines.

The principle of operation of the soft starter

The main problem with induction motors is that the moment of force developed by the motor is proportional to the square of the voltage applied to it, which creates sharp jerks of the rotor when starting and stopping the motor, which, in turn, cause a large induction current.

Softstarters can be either mechanical or electric, or a combination of both.

Mechanical devices directly counteract the sudden increase in engine speed by limiting the torque. They can be brake pads, fluid couplings, magnetic interlocks, counterweights with shot, and so on.

These electrical devices allow you to gradually increase the current or voltage from the initial low level (reference voltage) to the maximum in order to smoothly start and accelerate the motor to its rated speed. Such soft starters usually use amplitude control methods and therefore cope with starting equipment in idle or lightly loaded mode. A more modern generation of soft starters (for example, Energy Saver devices) use phase control methods and therefore are able to start electric drives that are characterized by heavy starting conditions "nominal to nominal". These soft starters allow for more frequent starts and have built-in power saving and power factor correction modes.

Soft starter selection

When an asynchronous motor is turned on, a short-circuit current appears in its rotor for a short time, the strength of which, after a set of revolutions, decreases to a nominal value corresponding to the power consumed by the electric machine. This phenomenon is exacerbated by the fact that at the moment of acceleration, the torque on the shaft also increases abruptly. As a result, the protective circuit breakers may trip, and if they are not installed, then the failure of other electrical devices connected to the same line. And in any case, even if the accident did not occur, when starting the electric motors, there is an increased consumption of electricity. To compensate or completely eliminate this phenomenon, soft starters (SCDs) are used.

How soft start is implemented

To smoothly start the electric motor and prevent inrush current, two methods are used:

1. Limit the current in the rotor winding. To do this, it is made consisting of three coils connected according to the "star" scheme. Their free ends lead to slip rings (collectors) attached to the shaft shank. A rheostat is connected to the collector, the resistance of which at the time of start-up is maximum. As it decreases, the rotor current increases and the motor spins up. Such machines are called slip-ring motors. They are used in crane equipment and as traction motors for trolleybuses and trams.
2. Reduce the voltage and currents supplied to the stator. In turn, this is implemented using:

a) autotransformer or rheostat;

b) key circuits based on thyristors or triacs.

It is the key circuits that are the basis for the construction of electrical devices, which are usually called soft starters or soft starters. Please note that frequency converters also allow you to smoothly start the motor, but they only compensate for a sharp increase in torque without limiting the starting current.

The principle of operation of the key circuit is based on the fact that the thyristors are unlocked for a certain time at the moment the sinusoid passes zero. Usually in that part of the phase when the voltage rises. Less often - when it falls. As a result, a pulsating voltage is recorded at the soft starter output, the shape of which is only approximately similar to a sinusoid. The amplitude of this curve increases as the time interval increases when the thyristor is turned on.

Soft starter selection criteria

In order of decreasing importance, the device selection criteria are arranged in the following sequence:

• Power.
• Number of controlled phases.
• Feedback.
• Functionality.
• Control method.
• Additional features.

Power

The main parameter of the soft starter is the value of I nom - the current strength for which the thyristors are designed. It should be several times greater than the value of the current passing through the motor winding, which has reached the rated speed. The multiplicity depends on the severity of the launch. If it is light - metal-cutting machines, fans, pumps, then the starting current is three times higher than the nominal one. Heavy starting is typical for drives with a significant moment of inertia. These are, for example, vertical conveyors, sawmills, presses. The current is five times higher than the rated current. There is also a particularly difficult start-up, which accompanies the operation of piston pumps, centrifuges, band saws ... Then I nom of the soft starter should be 8-10 times larger.

The severity of the launch also affects the time it takes to complete. It can last from ten to forty seconds. During this time, the thyristors become very hot, as they dissipate part of the electrical power. To repeat, they need to cool down, and this takes as much as the working cycle. Therefore, if the technological process requires frequent switching on and off, then choose a soft starter as for a hard start. Even if your device is not loaded and easily picks up speed.

Number of phases

One, two or three phases can be controlled. In the first case, the device softens the growth of the starting torque to a greater extent than the current. The most commonly used two-phase starters. And for cases of heavy and especially heavy start-up - three-phase.

Feedback

SCP can work according to a given program - increase the voltage to the nominal value for the specified time. This is the simplest and most common solution. The presence of feedback makes the control process more flexible. The parameters for it are the comparison of voltage and torque or the phase shift between the rotor and stator currents.

Functionality

Ability to work on acceleration or braking. The presence of an additional contactor that shunts the key circuit and allows it to cool down, and also eliminates phase asymmetry due to a violation of the shape of the sinusoid, which leads to overheating of the windings.

Control method

It can be analog, by rotating the potentiometers on the panel, and digital, using a digital microcontroller.

Additional functions

All types of protection, energy saving mode, the ability to start with a jerk, work at reduced speed (pseudo-frequency regulation).

Properly selected soft starter doubles the working life of electric motors, savesup to 30 percent electricity.

Why you need a soft starter (softstarter)

Increasingly, when starting the electric drives of pumps, fans, a soft starter (soft starter) is used. What is it connected with? In our article we will try to highlight this issue.

Induction motors have been in use for over a hundred years, and relatively little has changed in how they function. The launch of these devices and the problems associated with it are well known to their owners. Starting currents lead to voltage drops and wiring overloads, as a result of which:

some electrical equipment may spontaneously turn off;

possible hardware failure, etc.

A timely installed, purchased and connected softstarter allows you to avoid unnecessary spending of money and headaches.

What is starting current

The principle of operation of asynchronous motors is based on the phenomenon electromagnetic induction. The buildup of reverse electromotive force (e.f. s), which is created by applying a varying magnetic field during engine start, leads to transients in the electrical system. This transient may affect the power system and other equipment connected to it.

During starting, the motor accelerates to full speed. The duration of the initial transients depends on the design of the unit and the characteristics of the load. The starting torque should be the largest and the starting currents should be the smallest. The latter entail detrimental consequences for the unit itself, the power supply system and the equipment connected to it.

During the initial period, the starting current can reach five to eight times the full load current. During motor start, the cables are forced to carry more current than during the steady state period. The voltage drop in the system will also be much greater at start than during stable operation- this becomes especially evident when starting a powerful unit or a large number electric motors at the same time.

Motor protection methods

As the use of electric motors became widespread, overcoming problems with starting them became a challenge. Over the years, several methods have been developed to solve these problems, each with its own advantages and limitations.

V Lately significant advances have been made in the use of electronics in the regulation of electric power for motors. Increasingly, when starting electric drives of pumps and fans, soft starters are used. The thing is that the device has a number of features.

A feature of the starter is that it smoothly applies voltage to the motor windings from zero to the nominal value, allowing the motor to smoothly accelerate to maximum speed. The mechanical moment developed by the electric motor is proportional to the square of the voltage applied to it.

During the start process, the soft starter gradually increases the applied voltage, and the electric motor accelerates to the rated speed without high torque and peak current surges.

Types of soft starters

To date, three types of soft starters are used for the smooth start of equipment: with one, two, and with all controlled phases.

The first type is applied to a single-phase motor to provide reliable protection against overload, overheating and reduce the effect of electromagnetic interference.

As a rule, the second type circuit, in addition to the semiconductor control board, includes a bypass contactor. Once the motor has run up to rated speed, the bypass contactor is activated and provides direct voltage to the motor.

The three-phase type is the most optimal and technically advanced solution. It provides current and magnetic field strength limitation without phase distortions.

Why do you need a soft starter?

Due to the relatively low price, the popularity of soft starters is gaining momentum in the modern market of industrial and household appliances. Soft starter for an asynchronous electric motor is necessary to extend its service life. The big advantage of the softstarter is that the start-up is carried out with smooth acceleration, without jerks.

An asynchronous motor has the ability to self-start due to the interaction between the rotating flux of the 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 heating of the motor and damage to it. To prevent this, a soft motor starter (SCD) is needed.

The principle of operation of the starter

It consists in the fact that the device regulates the voltage applied to the motor during start-up by 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. Torque can also be considered approximately proportional to the applied voltage, so by adjusting the voltage during start-up, the current drawn by the machine and its torque are controlled by the device and can be reduced.

By using six SCRs in a configuration as shown in the figure, the soft starter can regulate the voltage applied to the motor at startup from 0 volts to rated line voltage. Soft start of the electric motor can be carried out in three ways:

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

SCPs can be of two types:

1. open management: The start voltage is applied with a time delay, regardless of the current or motor speed. For each phase, two SCRs are conducted first, delayed by 180 degrees, for the corresponding half-wave cycles (for which each SCR is performed). This delay gradually decreases with time until the applied voltage reaches the nominal value. It is also known as the temporary tension system. This method does not actually control the acceleration of the motor.
2. Closed loop control: Any characteristic of the motor output is monitored, such as current or speed. The starting voltage is changed 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.

Soft start benefits

Solid state soft starters use solid state devices to temporarily derate the motor terminals. This provides motor current control to reduce the motor limit torque. The control is based on motor terminal voltage control on two or three phases.

A few reasons why this method is preferred over 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 A: The starting parameters can be controlled and easily changed, which ensures that it starts without any jerks.
3. Controlled acceleration: Motor acceleration is controlled steplessly.
4. Low cost and size: This is provided using solid state switches.

Solid State Device Components

Power switches, such as SCRs, which are subject to phase control 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, a control logic 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 gate voltage to the SCR on each phase.

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

Basics of SCR

SCR (Silicon Controlled Rectifier) ​​is a controlled power regulator direct current with high power. The SCR induction motor soft starter is a four-layer PNPN silicon semiconductor device. 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 to cathode controlled at the moment the machine is started.

The main characteristics of the SCR can be understood with the help of these diagrams. A motor soft starter can be turned on and made to act as a silicon forward bias rectifier by momentarily applying gate current to it through S2. The SCR quickly (within a few microseconds) automatically latches on and remains on even when the shutter 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 driver is removed. A saturation potential of 1V or so is created between the anode and cathode.

It only takes a short shutter pulse to turn on the SCR. Once the SCR is latched, it can be turned off again by briefly reducing its anode current below a certain value, typically a few milliamps; in AC applications, the turn-off occurs automatically at the zero-crossing point every half cycle.

Significant gain is available between the SCR's gate and anode, and low gate currents (typically a few mA or less) can control high anode currents (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)).

Internal capacitance (a few pF) exists between the anode and gate of the SCR, and the rapidly increasing voltage appearing across 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 rate of rise to a safe value.

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 drive DC link and some load is present. Inverters use pulse wide modulation to create output signals. The triangular signal is generated at the carrier frequency at which the IGBT inverter will switch.

This waveform is compared to a sinusoidal waveform at fundamental frequency, which is to 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 inverter and/or mechanical motor limits. Note that the drive always operates within the slip rating of the motor.

Start control process

The timing of the SCR on 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 referred to as the "gradual recovery" of the SCR. The trigger point of the SCR is set or programmed so that the starting torque, starting current or current limit is tightly controlled.

The result of the stepwise SCR recovery is a non-sinusoidal undervoltage on the motor terminals, which is shown in the figures. Because the motor is inductive and the current lags the voltage, the SCR stays on and conducts until the current reaches zero. This happens after the voltage has gone negative. Voltage output of an individual SCR.

When compared with the full voltage waveform, it can be seen 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 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 step into full conduction, gaps in current being filled until the waveform looks the same as the motor.

Such a soft start of an asynchronous electric motor, unlike 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 the evaluation is the engine torque. Standard motors produce approximately 180% of full load torque at startup. Therefore, a 25% derating will equal the full load torque. If the motor draws 600% of the full load current at start, then the current in this circuit will reduce the start current from 600% to 450% load.

Starter connection diagrams

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

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 scheme, the 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.

Electric motors are widely used in all areas of human activity. However, when starting the electric motor, a sevenfold current consumption occurs, causing not only an overload of the power supply, but also heating of the stator windings, as well as failure of mechanical parts. To eliminate this undesirable effect, radio amateurs are advised to use soft starters for an electric motor.

Soft start of the engine

The stator of an electric motor is an inductor, therefore, there are active and reactive components of resistance (R). The value of the reactive component depends on the frequency characteristics of the power supply and during start-up it ranges from 0 to the calculated value (when the tool is running). In addition, the current, called the starting current, changes.

The starting current exceeds 7 times the nominal value. During this process, the windings of the stator coil are heated and, if the wire that makes up the winding is old, then an interturn short circuit is possible (with a decrease in the value of R, the current reaches its maximum value). Overheating will shorten tool life. To prevent this problem, there are several options for using soft starters.

By switching the windings, the motor soft starter (SCD) consists of the following main components: 2 types of relays (on-time and load control), three contactors (Figure 1).

Figure 1 - General diagram of a soft starter for asynchronous motors (soft start).

Figure 1 shows an asynchronous motor. Its windings are connected in a star connection. Starting is carried out with closed contactors K1 and K3. After a certain time interval (set using a time relay), contactor K3 opens its contact (disconnection occurs) and contact K2 is switched on. The scheme in Figure 1 is also applicable to soft starters for motors of various types.

The main disadvantage is the formation of short-circuit currents with the simultaneous inclusion of 2 machines. This problem is corrected by introducing a knife switch instead of contactors into the circuit. However, the stator windings continue to heat up.

With electronic control of the frequency of starting the electric motor, the principle of frequency change of the supply voltage is used. The main element of these converters is frequency converter, including:

1. The rectifier is assembled on high-power semiconductor diodes (a thyristor version is possible). It converts the mains voltage into a pulsating direct current.
2. The intermediate circuit smooths out noise and ripple.
3. An inverter is needed to convert the signal received at the output of the intermediate circuit into a signal with variable amplitude and frequency characteristics.
4. The electronic control circuit generates signals for all converter nodes.

Principle of operation, types and choice

During an increase in the rotor torque and Ip by 7 times, in order to extend the service life, it is necessary to use a soft starter, which meets the following requirements:

1. Uniform and smooth increase in all indicators.
2. Management of electric braking and engine start at certain time intervals.
3. Protection against power surges, loss of any phase (for a 3-phase electric motor) and various kinds of interference.
4. Increased wear resistance.

The principle of operation of the triac soft starter: limiting the voltage value due to a change in the opening angle of the triac semiconductors (triacs) when connected to the stator coils of the electric motor (Figure 2).

Figure 2 - Scheme of soft start of an electric motor on triacs.

Thanks to the use of triacs, it becomes possible to reduce the starting currents by a factor of 2 or more, and the presence of a contactor makes it possible to avoid overheating of the triacs (in Figure 2: Bypass). The main disadvantages of triac starters:

1. The use of simple circuits is possible only at light loads or idle start. Otherwise, the scheme becomes more complicated.
2. Overheating of the windings and semiconductor devices occurs during a long start.
3. The motor sometimes does not start (leads to significant overheating of the windings).
4. With an electric brake of an electric motor, overheating of the windings is possible.

Soft starters are widely used with regulators in which there is no feedback (for 1 or 3 phases). In models of this type, it is necessary to set the start time of the electric motor and the voltage immediately before starting. The disadvantage of devices is the inability to adjust the torque of moving mechanical parts according to the load. To eliminate this problem, it is necessary to apply a device to reduce Ip, protection against various phase differences (occurs during phase imbalance) and mechanical overloads.

More expensive soft starter models include the ability to monitor the parameters of the motor in continuous mode.

In devices containing electric motors, soft starters on triacs are provided. They differ in the scheme and method of regulating the mains voltage. The simplest circuits are circuits with single-phase regulation. They are executed on one triac and allow to soften the load on the mechanical part, and are used for electric motors with a power of less than 12 kV. The enterprises use 3-phase voltage regulation for electric motors with a power of up to 260 kW. When choosing the type of SCP the following parameters must be followed:

1. Device power.
2. Operating mode.
3. Equality Ip of the engine and soft starter.
4. The number of launches in a given time.

To protect pumps, soft starters are suitable that protect against impacts from the hydraulic component of the pipe (Advanced Control). Soft starters for tools are selected based on loads and high speeds. In expensive models, this type of protection in the form of a soft starter is present, but for budget ones it is necessary to make it yourself. It is applied in chemical laboratories for smooth start of the fan cooling liquid.

Reasons for using in Bulgarian

Due to the design features at the start of the angular grinder there are high dynamic loads on the tool parts. At the initial rotation of the disk, the axis of the gearbox is subject to the action of inertia forces:

1. An inertial jerk can snatch the grinder from the hands. There is a threat to life and health, as this tool is very dangerous and requires strict adherence to safety regulations.
2. When starting, an overcurrent occurs (Istart = 7*Inom). There is premature wear of the brushes, overheating of the windings.
3. The gearbox wears out.
4. Destruction of the cutting disc.

An untuned tool becomes very dangerous, because there is a possibility of harm to health and life. Therefore, it is necessary to secure it. For this, do-it-yourself soft starters are assembled for power tools.

DIY creation

For budget models of an angle grinder and other tools, you need to assemble your own soft starter. It is not difficult to do this, because thanks to the Internet, you can find a huge number of schemes. The simplest and, at the same time, effective is the universal SCP circuit on a triac and a microcircuit.

When you turn on the grinder or other tool, the windings and gearbox of the tool associated with a sharp start are damaged. Radio amateurs found a way out of this situation and offered a simple soft start for a do-it-yourself power tool (Scheme 1), assembled in a separate block (there is very little space in the case).

Scheme 1 - Scheme of soft start of a power tool.

Do-it-yourself soft starter is implemented on the basis of KR118PM1 (phase regulation) and the power part on triacs. The main highlight of the device is its versatility, because it can be connected to any power tool. It is not only easy to install, but also does not require pre-configuration. Basically, connecting the system to the tool is not difficult and is installed in the break of the power cable.

Features of the soft starter module

When the grinder is turned on, voltage is applied to the KR118PM1 and a smooth increase in voltage occurs on the control capacitor (C2) as the charge increases. The thyristors located in the microcircuit open gradually with a certain delay. The triac opens with a pause equal to the delay of the thyristors. For each successive voltage period, the delay is gradually reduced and the tool starts smoothly.

The revs time depends on the capacitance C2 (at 47 microns, the start time is 2 seconds). This delay is optimal, although it can be changed by increasing the capacitance of C2. After turning off the angle grinder (angle grinder), the capacitor C2 is discharged due to the resistor R1 (the discharge time is approximately 3 seconds at 68k).

This circuit for adjusting the speed of the electric motor can be upgraded by replacing R1 with a variable resistor. When the resistance value of the variable resistor changes, the power of the electric motor changes. Resistor R2 performs the function of controlling the magnitude of the current that flows through the input of the triac VS1 (it is desirable to provide for cooling by a fan), which is the control one. Capacitors C1 and C3 serve to protect and control the microcircuit.

The triac is selected with the following characteristics: the maximum direct voltage is up to 400–500 V and the minimum current passing through the transitions must be at least 25 A. When manufacturing the soft starter according to this scheme, the power margin can vary from 2 kW to 5 kW.

Thus, in order to increase the service life of tools and motors, it is necessary to start them smoothly. It's connected with design feature electric motors of asynchronous and collector types. When starting, there is a rapid consumption of current, due to which there is wear on the electrical and mechanical parts. The use of the soft starter allows you to secure the power tool, thanks to the observance of safety regulations. When upgrading the tool, it is possible to purchase ready-made models, as well as assemble a simple and reliable universal device that not only differs, but even surpasses some factory soft starters.

Soft start of the electric motor has recently been used more and more often. Its fields of application are varied and numerous. These are industry, electric transport, utilities and Agriculture. The use of such devices can significantly reduce the 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 the operating mode. This leads to a “drawdown” of voltage 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 the gradual destruction of their insulation. This contributes to premature failure of the electric motor.

Soft starters can significantly reduce the starting loads on the electric motor and the power grid, which is especially important in rural areas or when the engine is powered from an autonomous power plant.

Actuator overloads

At the time of starting the engine, the torque on its shaft is very unstable and exceeds the nominal value by more than five times. Therefore, the starting loads of the actuators are also increased compared to steady state operation and can reach up to 500 percent. Torque instability during start-up leads to shock loads on the gear teeth, shearing of the keys, and sometimes even twisting of the shafts.

Soft starters of the electric motor significantly reduce the starting loads on the mechanism: the gaps between the gear teeth are smoothly selected, which prevents their breakage. In belt drives, the drive belts are also smoothly tensioned, which reduces wear on the mechanisms.

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

Industrial soft starters

Currently produced by many companies, such as Siemens, Danfoss, Schneider Electric. Such devices have many functions that are programmed by the user. These are acceleration time, deceleration time, overload protection and many other additional functions.

With all the advantages, branded devices have one drawback - a rather high price. However, you can create such a device yourself. Its cost will be small.

Soft starter on the chip KR1182PM1

In it was told about specialized chip KR1182PM1 representing the phase power controller. Typical schemes for its inclusion, soft start devices for incandescent lamps and simply power regulators in the load were considered. Based on this microcircuit, it is possible to create a fairly simple device for a three-phase electric motor soft starter. The device diagram is shown in Figure 1.

Figure 1. Scheme of the motor soft starter.

Soft start is carried out by gradually increasing the voltage on the motor windings from zero to nominal. This is achieved by increasing the opening angle of the thyristor switches in a time called start time.

Circuit Description

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

The output keys are made on thyristors connected back to back - in parallel. The design uses imported thyristors of the 40TPS12 type. At a low cost, they have a fairly large current - up to 35 A, and their reverse voltage is 1200 V. In addition to them, there are several more elements in the keys. Their purpose is as follows: damping RC circuits, connected in parallel with 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 is absorbed, the amplitude of which exceeds 500 V.

DA1…DA3 microcircuits of KR1182PM1 type are used as control units for output keys. These microcircuits were discussed in sufficient detail in. Capacitors C5 ... C10 inside the microcircuit form a sawtooth voltage, which is synchronized by the mains. The thyristor control signals in the microcircuit are formed by comparing the sawtooth voltage with the voltage between microcircuit pins 3 and 6.

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

Description of the operation of the soft starter for electric motors

Mains voltage is applied to the circuit when the circuit breaker Q1 is closed. However, the engine does not start yet. This is because the relay windings K1 ... K3 are still de-energized, and their normally closed contacts shunt pins 3 and 6 of the DA1 ... DA3 microcircuits through resistors R1 ... R3. This circumstance does not allow capacitors C1 ... C3 to charge, so the microcircuits do not generate control pulses.

Putting the device into operation

When the toggle switch SA1 is closed, a voltage of 12 V turns on the relay K1 ... K3. Their normally closed contacts open, which makes it possible to charge capacitors C1 ... C3 from internal current generators. Together with an increase in the voltage across these capacitors, the opening angle of the thyristors also increases. Thus, a smooth increase in voltage on the motor windings is achieved. When the capacitors are fully charged, the turn-on angle of the thyristors will reach its maximum value, and the motor speed will reach the nominal value.

Engine shutdown, soft braking

To turn off the engine, open the switch SA1, This will turn off the relay K1 ... K3. Their normally - closed contacts will close, which will lead to the discharge of capacitors C1 ... C3 through resistors R1 ... R3. The discharge of capacitors will last a few seconds, during the same time the engine will stop.

When starting the engine, significant currents can flow in the neutral wire. This is due to the fact that in the process of smooth acceleration, the currents in the motor windings are non-sinusoidal, but you should not be particularly afraid of this: the starting process is quite short-term. In the steady state, this current will be much less (no more than ten percent of the phase current in the nominal mode), which is due only to the technological spread of the winding parameters and the “skew” of the phases. It is no longer possible to get rid of these phenomena.

Details and design

The following parts are required to assemble the device:

Transformer with a power of not more than 15 W, with an output winding voltage of 15 ... 17 V.

As relays K1 ... K3, any with a coil voltage of 12 V, having a normally closed or changeover contact, for example TRU-12VDC-SB-SL, are suitable.

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 the SA1 switch and the HL1 and HL2 LEDs should be placed.

Motor connection

The connection of the switch Q1 and the motor is carried out by wires, the section of which corresponds to the power of the latter. The neutral wire is made with the same wire as the phase wires. With the ratings of parts indicated in the diagram, it is possible to connect motors with a power of up to four kilowatts.

If it is supposed to use an engine with a power of not more than one and a half kilowatts, and the frequency of starts will not exceed 10 ... 15 per hour, then the power dissipated on thyristor switches is negligible, so radiators can be omitted.

If it is supposed to use a more powerful engine or starts will be more frequent, it will be necessary 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 gaskets. To improve cooling conditions, you can use the heat-conducting paste KPT-8.

Checking and adjusting the device

Before turning 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 nevertheless this circuit is powered by the network, and jokes are bad with it. And even after the specified check, it is still too early to connect the engine.

First, instead of the engine, connect three identical incandescent lamps with a power of 60 ... 100 W. When testing, it should be ensured that the lamps "ignite" evenly.

The uneven turn-on time is due to the spread of the capacitances of the capacitors C1 ... C3, which have a significant capacitance tolerance. Therefore, it is better to immediately pick them up with the help of the device before installation, at least with an accuracy of up to ten percent.

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

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

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 is impossible to repeat all the functions of branded devices in such a home-made device, but it will probably be possible to copy one after all.

It's about the so-called. Its purpose is as follows: after the engine has reached the nominal speed, the contactor simply bridges the thyristor keys with its contacts. 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.

Boris Aladyshkin