Friday, 20 January 2017

SINGLE PHASE INDUCTION MOTOR - CONSTRUCTION OF ELECTRIC MOTOR

Actual 1-phase induction motors are,
  1. Ceiling fans
  2. Other single phase induction motor: 


Construction

Construction is similar as that of 3-phase squirrel induction motor
A single phase induction motor has two main parts

Stator

Similar to 3-phase induction motor
consisting of a steel frame that supports a hollow, Laminated cylindrical core core, constructed from stacked laminations (why?), having a number of evenly spaced slots, providing the space for the stator winding.

Rotor

Squirrel cage rotor is used
aluminum bus bars shorted together at the ends by two aluminum rings.

Shaft:

Should withstand maximum breaking strength

Bearing:

Roller or ball bearing

It is used in vacuum cleaner, fans, washing machine, centrifugal pump, blowers, washing machine, small toys etc.

Principle:

When stator winding is fed from a single phase supply, an alternating (pulsating) field is produced.
Due to this alternating flux an alternating EMF (current) is induced in the rotor conductors
Now when a current carrying conductor is place in magnetic filed it experiences a force.
But after each half cycle the direction of induced current is change and hence the direction of force (torque) is changed after each half cycle. So pulsating flux acting on stationary squirrel cage rotor can’t produce rotation and therefore 1-phase induction motor is not self starting.
However, if the rotor o such machine is given initial start by hand or otherwise in either direction, then motor starts rotating in that direction. Above peculiar behavior can be explained by

Double filed revolving theory

To make it self-starting, it can be temporarily converted into a two-phase motor while starting. This can be achieved by introducing an additional 'starting winding' also called as auxillary winding.
Hence, stator of a single phase motor has two windings: (i) Main winding and (ii) Starting winding (auxiliary winding). These two windings are connected in parallel across a single phase supply and are spaced 90 electrical degrees apart. Phase difference of 90 degree can be achieved by connecting a capacitor in series with the starting winding.

Hence the motor behaves like a two-phase motor and the stator produces revolving magnetic field which causes rotor to run. Once motor gathers speed, say upto 80 or 90% of its normal speed, the starting winding gets disconnected form the circuit by means of a centrifugal switch, and the motor runs only on main winding.

Type of single phase induction motors

The  single phase induction motors are made self starting by providing an additional flux by some additional means( Additional winding ). Now depending upon these additional means the single phase induction motors  are classified as:
  • Resistance start induction motor
  • Capacitor start induction motor
  • Capacitor start capacitor run induction motor
  • Permanent capacitor motor
  • Shaded pole induction motor.

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