Electrical Drives Starting Methods

STARTING METHODS

INTRODUCTION

1. PRIME PURPOSE (or) NECESSITY OF A STARTER FOR MOTORS

The Current drawn by the armature of motor is given by,

Where,        Ia = ( V- Eb ) / Ra

V       - Supply Voltage

Eb      - Back EMF

Ra      - Armature Resistance

·         When the motor is at rest, there is no back emf developed in the armature. If now full supply voltage is applied across the stationary armature., it will draw a very large current.

·         Because armature resistance is very small.

·         This excessive current will blow out the fuse and damage the motor.

·         To reduce high stating current, a resistance is connected in series with the armature circuit at the time of starting.

·         When the motor speed is increased the back emf is also increased.

·         Then Ia value is decrease.

·         That time external resistance is cut out.

2. PROTECTIVE DEVICES IN A DC/AC MOTOR STARTER

·         Over load Release (O.L.R) or No volt coil

·         Hold on Coil

·         Thermal Relays

·         Fuses (Starting /Running)

·         Over load relay

3. STARTERS FOR DC MOTOR

·         Two point Starter

·         Three point Starter

·         Four point Starter