MOTORS

MOTORS To convert electrical energy into

mechanical energy in order to accelerate the motor and load to operating speed – Starting Function

To convert electrical energy into productive work output from the machine – Work Function

MOTOR PARTS STATOR-The stator consists of magnetic poles

and stator windings within the frame of the motor. By variation of winding configuration and the contour of the stator laminations , the full load characteristics are determined

The motor speed is determined by the number of poles

ROTOR- The rotor consists of a cylindrical short-circuited winding around iron laminations The rotor design affects starting performance.

The shape, position and material of the rotor bars affect the current drawn and torque produced during motor starting.

CHARACTERISTICS Full load characteristics are well understood with

factors such as motor speed,torque and efficiency being the typical selection criteria.

A motor‘s start performance characteristics are usually the least understood but set the limits of what can be achieved with either a full voltage or reduced voltage starter.

It is especially important to consider motor start characteristics when seeking to:- Minimise start current

- Maximise start torque

TORQUE-SPEED CHARACTERISTICS

Starting Torque (LRT) is produced by a motor when it is initially turned on. Starting torque is the amount required to overcome the inertia from standstill.

Pull-up Torque is the minimum torque generated by the motor as it accelerates from standstill to operating speed. If the motor’s pull-up torque is less than that required by its application load , the motor will overheat and eventually stall.

FULL VOLTAGE STARTING

Current rises instantaneously to LRC levels. This causes a current transient that can have undesirable effects on the supply.

Current gradually falls as motor speed increases.

Motor loading affects only the time taken for acceleration, not the magnitude of current which is always LRC.

FULL VOLTAGE STARTING LIMITATIONS Current transient Current magnitude Torque transient Torque magnitude

Reduced voltage starting attempts to overcome these limitations by applying the voltage gradually.