Single-phase operation of a three-phase induction motor with the Smith connection

Abstract

This paper presents a systematic analysis of the Smith connection that enables perfect phase balance to be achieved in a three-phase induction motor when operating on a single-phase supply. The Smith connection is essentially an asymmetrical connection, but by suitable choice of terminal capacitances, balanced currents can be injected into the motor winding for a given speed and load. A single-phase induction motor drive with three-phase machine efficiency is thus possible. 'Inspection' equations are established and solved using the method of symmetrical components to give the motor performance. Using a phasor diagram approach, the values of the capacitances to give perfect phase balance and the resulting line power factor can be computed. The theoretical analysis is verified by experiments performed on a 2.2 kW induction machine.