A simple soft starter for motors up to 4 kW. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple soft starter for motors up to 4 kW. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electric motors

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The device considered in the article allows shockless starting and braking of a three-phase asynchronous electric motor, which increases the service life of the equipment and reduces the load on the power grid. Soft start is achieved by changing the effective value of the voltage on the motor windings with the help of trinistors.

Soft starters (hereinafter referred to as soft starters) are widely used in industry, transport, utilities and agriculture. The basis of three-phase. UPP - three pairs of anti-parallel SCRs installed in the breaks of each of the phase wires. Soft start is achieved by gradually increasing the voltage applied to the motor windings from a certain initial value to the nominal value. For this, for some time, called the start time, the conduction angle of the trinistors gradually increases from the minimum value to the maximum. Usually, the initial voltage is small, so the torque on the motor shaft during start-up is much less than in the rated mode. In this case, the drive belts are smoothly tensioned, the gear wheels of the gearbox are engaged. As a result, dynamic loads on drive parts are reduced, which helps to extend the service life of mechanical equipment and increase the overhaul period.

Application. The soft starter also allows to reduce the peak load on the power grid, since the starting current of the electric motor in this case exceeds the rated current by only 2 ... 4 times, and not 5 ... 7 times, as with direct starting. This can be important when powering an electric drive from sources of limited power, for example, diesel generator sets, uninterruptible power supplies, low-power transformer substations (especially in rural areas). Reducing the starting current prolongs the life of electrical equipment. On fig. 1 shows a diagram. Soft starter designed for electric motors powered by a three-phase network 380 V, 50 Hz (phases A, B, C), the windings of which, connected by a "star", are connected to the circuits L1-L3. The common point of the "star" is connected to the network neutral (N).

The maximum engine power is 4 kW.

Simple motor soft starter up to 4 kW

SCRs VS1-VS6 are inexpensive 40TPS12 in the TO-247 package, allowing direct current up to 35 A. Damping RC circuits R8C11, R9C12, R10C13 are connected in parallel to the SCRs. preventing their false inclusions, as well as varistors RU1-RU3, absorbing switching impulses with an amplitude of more than 500 V.

Each pair of anti-parallel trinistors is controlled by the KR1182PM1 (DA1 - DA3) phase regulator chip, well known to radio amateurs. Capacitors C5-C10 provide the formation of a sawtooth voltage inside the microcircuits, synchronized with the mains. Comparing the sawtooth voltage with the current voltage between pins 3 and 6, each microcircuit generates turn-on signals for the corresponding trinistors.

A step-down transformer T1, a rectifier on a diode bridge VD1 with a smoothing capacitor C4 and an integral stabilizer DA4 provide the voltage of 12 V necessary for the operation of the K1-KZ relay.

After the three-phase voltage is supplied by the power switch Q1 with the SA1 switch open, the motor shaft remains stationary, since the conclusions 3 and 6 of the DA1 - DA3 microcircuits are shunted by the resistors R1 - R3 through normally closed relay contacts, the voltage between these terminals is small, the pulses that open the trinistors do not are being formed. In this state, the HL1 LED is on, indicating readiness. UPP to work.

When the contacts of the switch SA1 are closed, a voltage of 12 V is supplied to the relay windings, their contacts open and the charging of capacitors C1-C3 begins with the current generated inside the microcircuits. Trinistors start to open. As the voltage on the capacitors C1-C3 increases, the on-state angle of the trinistors gradually increases and after a while reaches a maximum. After this time, the accelerated engine runs at full power. Turning on the engine is signaled by the HL2 LED.

When the SA1 switch is open, the relay contacts will return to their original closed state and the capacitors C1-C3 will be discharged almost to zero in a few seconds, after which the opening SCR pulses will stop. The engine will slowly slow down and stop.

Since the currents through the motor windings are non-sinusoidal during start-up, there is no complete compensation of the phase currents in the neutral wire. At certain moments, the current in this wire can turn out to be significant, and in steady state it is much less, since it is due only to phase “skew” and non-identity of the motor windings, and usually does not exceed 10% of the rated phase current.

Transformer T1 - TPG-2 with a secondary voltage of 15 V, relay K1-KZ - TRU-12VDC-SB-CL, capacitors C11-C13 - film K73-17. As a switch SA1, you can use a button with latching in the pressed state.

Drawing of a double-sided printed circuit board. SCP is shown in fig. 2. It is placed in a suitable housing, the LEDs HL1, HL2 and the SA1 switch are installed on its front panel.

The cross section of the wires connecting. Soft starter with switch Q1 and with motor must match the capacity of the latter. The cross section of the neutral wire must be the same as that of the phase wires.

When working with a motor with a power of up to 1,5 kW and a frequency of starts of no more than 10-15 per hour, insignificant power is dissipated on the VS1-VS6 trinistors, so it is not required to remove heat from them. With more frequent starts or a more powerful engine, the trinistors must be equipped with heat sinks from an aluminum strip. If the heat sink is common, the trinistors must be reliably isolated from it with appropriate gaskets. To improve heat transfer, KPT-8 paste can be used.

The assembled device, before connecting it to the electric motor, must be checked by connecting three identical incandescent lamps to the outputs. During testing, it may be found that the lamps do not light up and go out at the same time. This is due to the spread of the characteristics of the DA1-DA3 microcircuits and the capacitance of the capacitors C1-C3. The turn-off time also depends on the resistance of the resistors R1-R3. Time mismatch of more than 30% must be eliminated by a selection of the capacitors and resistors mentioned above.

Due to the spread in the capacitance of the capacitors C5-C10, which are included in the sawtooth voltage formation circuits, a constant current component may appear in the phase wires, causing undesirable bias of the magnetic circuits of the motor and the power transformer supplying it. Practice has shown that such an impact is small, and no measures are required to eliminate this component.

Author: A. Sitnikov, pos. Stulovo, Kirov region

See other articles Section Electric motors.

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