Analytical Study of the Step Starting and Step Running of a 3-Phase Induction Motor I - Starting Torque with Step Voltage and Step Current

Abstract

A new method called the step starting of induction motors has been analytically developed. Pairs of stator terminals are connected in sequence to a voltage or current pulse from a variable frequency source. The frequency of the voltage or current pulse is zero at standstill, and it is gradually increased to bring the rotor up to required speed. In this paper, it is shown that the starting torque with a rated current pulse is much greater than the starting torque with a voltage pulse, the magnitude of the voltage pulse being that required for steady-state rated peak current in the stator. Part II of the paper discusses the electromechanics of the method with current pulses fed to the stator. It is found that the torque developed is proportional to the square of the magnitude of the current pulse. Variations in torque at any running speed are obtained by varying the magnitude of the current pulse. Any desired speed-torque characteristic of the motor is obtained by properly controlling the current with a current-control mechanism. Low resistance increases the torque developed in starting as well as in running. This analysis holds good for an unsaturated machine.