Features of the Single-Phase Mode of Operation of a Three-Phase Asynchronous Engine

There is sometimes a need to connect a three-phase asynchronous electric motor with a squirrel-cage rotor to a single-phase network in order to obtain mechanical energy for the working part of various devices. The quality operation of an automated electric drive is based on the conditions of operating the electric motor at the most rigid electromechanical characteristics, which need to be maintained under different operating conditions, especially concerning energy saving. It should be noted that in [1-10], during such connections, the useful power of the motor immediately decreases, sometimes up to 50% of the rated value. Sometimes a caution is given that not all three-phase electric motors can operate under such conditions, but the most important thing is that none of them explains why this happens, what physical processes occur in the stator winding of the electric motor, and how this can be prevented. It is known that when a two-phase motor or a three-phase motor is powered from a single-phase supply, it lacks a starting torque. In this case, the electric motor represents an asymmetrical machine in the form of a two-phase winding, which is located on the stator and spatially shifted by a certain angle α. If they are powered with a sinusoidal voltage, sinusoidal currents will flow through them, generating magnetomotive forces. The total magnetomotive force of the oppositely rotating fields is not equal, and as a result, we obtain not a pulsating magnetic field, but the hodograph of the resulting spatial magnetomotive force vector in the form of an ellipse. A series of studies were conducted on the connection of a three-phase electric motor, whose stator winding is connected in a star, to a single-phase network. After conducting the experiments, the following conclusions were made: doubling the capacitance from the recommended value significantly improves the current waveform, but its magnitude equals the nominal value, even in no-load mode. In addition, the currents in the capacitor winding also increase significantly, close to the nominal value, while in the auxiliary winding it exceeds twice the nominal, which is acceptable for prolonged operation; it is necessary to analytically confirm the reason for such a harmonic composition when the working capacitance is included in the circuit; it is also advisable to measure the motor's operating characteristics under load and compare the results with those at no-load.