Dynamic phase shifter - starter for an asynchronous motor. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Dynamic phase shifter - starter for an asynchronous motor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electric motors

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In E 7/2002, p.3, a circuit for contactless shutdown of the starting winding of a single-phase asynchronous motor with a squirrel-cage rotor at the end of the process of starting it is published, which can also be used to start three-phase asynchronous motors in single-phase mode. If there is no need for emergency braking of the working mechanism, the starting circuit can be greatly simplified by eliminating the wires going to terminals 5 and 6 of the two-pole switch SA1, and replacing it with a conventional two-pole or single-pole switch, which, for example, can be used as thermal relay contacts, if the device be used to start the refrigerator motor.

I propose another option for non-contact disconnection of the starting winding of an electric motor (EM) or a starting capacitor using a dynamic phase shifter.

The figure shows a schematic diagram of the device, which provides more reliable operation of the EM compared to the relay circuit for separating the starting winding with the possibility of using it for a capacitor EM. The proposed technical solution is protected by copyright [1].

Dynamic phase shifter - starter for asynchronous motor

The device contains a single-pole switch SA1 with two positions, with the help of which the working winding P and the starting winding P are connected to the network through the starting capacitor C and a non-contact switching unit 1, consisting of two back-to-back uncontrolled valves - diodes VD1 and VD2. Moreover, in series with the diode VD2, an electrolytic capacitor C1 is connected, shunted by a resistor R.

In the initial pre-start position, the capacitor C1 is connected through the contacts 2-3 of the SA1 switch to the terminals of the working winding P and is in a discharged state.

Switching unit 1 and capacitor C provide: 1) the duration of the connection of the starting winding P to the network at start-up, which corresponds to the charge of capacitor C1; 2) the braking torque on the EM shaft and its zero readiness when disconnected from the network (discharge of the capacitor C1 on the working winding P); 3) the readiness of the device to start in the event of a power failure (discharge of the capacitor C1 on the resistor R).

The device works as follows. When the ED is turned on using switch SA1, the current flows through the working winding P and the starting winding P through the capacitor C and the switching unit 1, while the positive half-wave of the winding current P passes through the diode VD1, and the negative half through the diode VD2.

The asynchronous motor starts. After a period of time determined by the capacitance of the capacitor C1, the diode VD2 is blocked by the capacitor C1, the switching unit 1 passes only the positive half-wave of the alternating current through the diode VD1, while the capacitor C blocks the diode VD1. As a result, the flow of current through the switching unit 1, and consequently, the starting winding P of the engine, stops. The start of the asynchronous motor is over. During operation of the ED, capacitors C and C1 are always in a charged state.

When the ED is disconnected from the network, the capacitor C1 is connected through the contacts 2-3 of the SA1 switch to the terminals of the working winding P and is discharged to this winding, while creating a braking torque on the shaft and thereby simultaneously preparing the ED for restarting, i.e. ensuring zero readiness of the latter. Capacitor C is discharged to the starting winding P through diode VD2. Restart is possible immediately after stopping the asynchronous motor.

In the event of a power failure in the supply network during EM operation and the SA1 switch is on (contacts 1-3 are closed), the circuit returns to its original pre-start position automatically by discharging capacitor C1 to resistor R. As a result, the circuit is ready to restart the EM, which ensures its self-starting when the mains voltage is restored. The value of the resistor R is chosen large enough (on the order of several tens of kiloohms) to reliably turn off the starting winding P. If there is no need to brake the working mechanism and the circuit is not ready for restart, the device can be simplified by replacing switch SA1 with a single-pole switch. This excludes the wire going to terminal 2 of switch SA1.

Details. As a switch SA1 use any suitable current and voltage. Diodes VD1 and VD2 for micromachines (up to 600 W) of the KD203G, KD203D types for a voltage of 700 V and a current of 5, 10 A, respectively. It is possible to use, but with less reliability, diodes of the D248B type for a voltage of 600 V and a current of 5 A or KD202R for a voltage of 600 V and a current of 3 A. At a mains voltage of 220 V and a frequency of 50 Hz, the starting capacitance, μF, C \u132d 1Rn, where Рn - rated motor power, kW. The capacitance of the capacitor of the switching unit 1 is C2=(3...2)C and is specified, if necessary, during commissioning. Capacitors of the MBGO-400, KBG-MN or MBGCH types for a voltage of at least 2 V. Resistor R of the MLT-50 type with a resistance of 100 ... XNUMX kOhm.

The device during operation of the ED does not consume electricity and practically does not need to be adjusted.

References:

Author's certificate of the USSR No. 731537, class. H02R 1/42, 1980.

Author: K.V. Kolomoitsev

See other articles Section Electric motors.

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