Asymmetrical Modes in Polyphase Motors and Drives

The effect of power supply and/or motor asymmetry on the phase-load distribution and the ripple of electromagnetic torque in a polyphase drive is studied by the method of symmetrical components. It is shown that the presence of asymmetry causes the mapping of all harmonics in the supply voltage into all planes of symmetrical components and eliminates the possibility of independent control of harmonics. The dynamic model of a five-phase squirrel-cage induction motor demonstrates that even minor phase or amplitude asymmetry causes asymmetry in the phase load and ripple of the electromagnetic torque. Their level eliminates the possibility of continuous operation of the motor in this mode and requires the use of control systems that provide load balance in the intact phases.