A device for starting a single-phase electric motor with a starting winding. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors
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Asynchronous single-phase electric motors with a starting winding are widely used in electric drives of various household appliances (washing machines, refrigerator compressor units), they are used by radio amateurs for their needs.
Having well-known advantages, such electric motors require the use of an additional device that provides automatic connection of the starting winding when turned on, as well as when the operation is stopped in cases of excessive short-term increase in load. In most cases, such a device, called a starter in everyday life, is a current relay, the winding of which is connected in series with the working winding of the motor. The high current of the working winding with a stationary rotor ensures the operation of the relay and the connection of the starting winding in specified cases. After the motor rotor accelerates, the current of the working winding decreases to the nominal value, the relay turns off the starting winding.
Known shortcomings of the relay (burning, closing of contacts, jamming of moving parts) cause its insufficient reliability and can lead to failure of the electric motor.
The author proposed an electronic device (patent UA273C1) that performs the same functions (Fig. 1).
Ris.1
The device contains only two elements: a triac VS, which ensures the inclusion of the starting winding of the motor, and a current transformer with a saturable magnetic circuit TR, which controls the triac.
The principle of operation of the device is illustrated by graphs in Fig.2.
Ris.2
The cross section of the magnetic circuit and the number of turns of the primary winding of the current transformer are chosen such that at a high current I, corresponding to the start mode, the magnetic flux F is limited as a result of the saturation of the magnetic circuit. In this case, the voltage U on its secondary winding will be a sequence of bipolar pulses coinciding in time with the beginning of the current half-cycles in the primary winding. The number of turns of the secondary winding of the current transformer and the order of its connection must be such that the amplitude of the pulses is sufficient to alternately unlock the triac, respectively, in the positive and negative half-cycles of the current.
As the motor rotor accelerates and reaches the nominal speed, the current I decreases, the current transformer magnetic circuit does not saturate, the voltage on the secondary winding U becomes sinusoidal, and its amplitude is insufficient to trigger the triac. The starting winding is switched off.
The experiments have shown that for the manufacture of a current transformer, it is possible to choose a magnetic core of any type from electrical steel with a cross-sectional area of only 510 mm2. In this case, the primary winding is carried out with PEL-0,7 wire and contains 5-7 turns, and the secondary winding - 80 ... 100 turns of PEL-0,1 wire. Triacs KU208G, TS106-4 or two counter-parallel thyristors KU202L,M are used in the manufactured and tested samples of the trigger device. In the latter case, the transformer has two identical secondary windings.
Author: N.A.Fisyun
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