Operation of three-phase induction motors connected to one-phasesupply

The behavior of a three-phase induction motor is analyzed operating according to the Steinmetz connection, conceived to allow operation of small rating motors connected to a one-phase supply and with a small degree of voltage unbalance. The conditions for canceling the negative sequence component of the stator winding terminal voltages and currents are also established. The increase in electromagnetic torque deriving from balancing with capacitors and the overvoltages closely related to the changes in amplitude and phase of positive sequence motor impedance verified during rotor acceleration are emphasized. Results presented by considering a capacitor in parallel to one of those phases connected to the motor terminal switched off from the three-phase supply define the requirements for correct capacitor selection and switching off in order to obtain proper motor behavior in the whole slip range from starting to the final operating point. A careful evaluation of the amplitude and phase of the positive sequence impedance should be taken into account for capacitor application to be considered in reducing the degree of unbalance during motor starting and even in normal operating slip range. This will be very useful for reliable capacitor specification and for properly defining rotor speeds at the instant of capacitor switching off