Application of Schmitt Trigger Circuit for Controlled Braking of Slip Ring Induction Motor using Plugging

Abstract

The main objective of this paper is to apply the principle and technology of controlled braking of
slip-ring Induction motor using Schmitt-trigger circuit. Slip-ring Induction motors is widely used
in industries for machines which require high starting torque. Invariably, these machines have
high inertia, and while stopping, electrical braking is applied to quickly decelerate the load. The
commonly used method of braking AC motors is "Plugging", if the control system fails to detect
zero speed and reversing torque continues, the machine may start operating in reverse which is
undesirable. Many machines have mechanical holding brakes which are applied at near-zero
speed. If the mechanical brakes operate when the motor speed is high, it will lead to the brakes
wearing out quickly and cause maintenance problems. Many machines use electro-mechanical
speed switches, these electro-mechanical switches are prone to failure due to many moving
parts, springs and coupling with motor shaft, which results in over-travel of the machine. To
solve these reliability issues, it is proposed to develop an alternative system which detects zero
speed of the motor by an electronic circuit.