How to connect a three-phase motor to a single-phase network. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors
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Three-phase asynchronous electric motors with a squirrel-cage rotor are usually connected to a single-phase network according to the circuit shown in Fig. 1. Calculation shows, and practice confirms, that even with the optimal choice of the capacity of the phase-shifting capacitor C1, the torque on the shaft of a motor connected in this way does not exceed 35% of the nominal one.
This is explained by the fact that the current flowing through winding III of the motor is shifted in phase relative to the currents in windings I and II in such a way that in the total magnetic field of the stator, in addition to the component rotating the rotor in the desired direction, another one is formed, rotating in the other direction . It slows down the rotor, reducing the torque on the shaft and wasting its energy on heating the wires and magnetic circuit of the motor.
By disconnecting winding III (Fig. 2), it is possible to increase the torque to 41% of the nominal one.
It increases even more, to 58%, if you reconnect this winding, changing the direction of the current in it (Fig. 3). The effect is achieved not only by changing the direction of rotation of the “harmful” component of the magnetic field. There is mutual compensation of the component fields created by windings II and III, which coincide in direction with the field of winding I and therefore do not participate in the rotation of the rotor. It has been experimentally established that the use of two phase-shifting capacitors also facilitates engine starting.
The capacitances of capacitors C1 and C2 must be the same. They are calculated using the well-known formula C = 2800*Iph/i, where Iph is the rated phase current of the electric motor, A; U=220 V. Capacitors MBGO, MBGP, MBGT, K42-4 are suitable for a constant operating voltage of at least 600 V or MBGCh, K42-19 for an alternating voltage of at least 250 V. The correct choice of capacitors can be checked by measuring the voltages at each of the three motor windings under load. They should be approximately equal.
The equality of voltages on windings II and III of the electric motor makes it possible to connect them back-to-back, as shown in Fig. 3 dashed line. In this case, capacitors C1 and C2 are replaced with one of double capacity.
Literature
- Biryukov S. Three phases - without power loss. - Radio, 2000, No. 7, p. 37-39.
- Karvovsky G., Okorokov S. Handbook on asynchronous motors and ballasts. - M.: Energy, 1969.
- Bessonov L. Theoretical foundations of electrical engineering. - M.: Higher school, 1964.
Author: V.Kleymenov
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