INDUCTION MOTORS

Dr.P S  Srinivasan used to teach us  Induction Motors  at  NIT Calicut  in 1985 and used to do a splendid job and this write-up is for him.At  8:00 AM  in the morning in the top floor of the Departmental Building he used to teach this subject almost every day.

There might be people around the world who would like to know something about this subject. In Induction Motors unlike DC motors there is no conductive link between the stator and rotor ..the link is inductive.

Note that there are two kinds of Induction Motors classified based on the type of input power namely  three phase induction motors and single phase motors.

In three phase Induction Motors the Stator is wound with three phase winding which is displaced from each other by 120 degrees.This is space displaced winding. When three phase supply which is also time displaced by 120 degrees passes through these coils the stator creates Rotating Magnetic Field.

Note that there are two kinds of rotors associated with three phase induction motors  namely wound rotor motors and cage motors also called squirell cage  motors. When the rotating magnetic field travels over the rotor it creates rotary motion. Squirell cage motors produce low starting torque whereas Slip ring motors or wound rotor motors have higher starting torque. In Slip Ring induction motors speed control is possible through brushes on the slip ring that varies rotor resistance. Squirell cage motors have good running torque.

The rotating magnetic field mentioned earlier  is given by  F = P N / 120  F is frequency of input power .....N is speed of the field..P is number of poles it creates. Change in input frequency changes speed of the magnetic field.

Rotor copper loss and rotor iron loss is  linked to slip....slip  is  N as calculated above minus n the rotor speed divided by N..,, namely  (N-n)/N.

Kindly refer to the specific formulae. Slip times stator frequency is rotor frequency..slip times rotor power input is rotor copper loss.Rotor power input is Stator power input  minus stator copper and iron losses. There is a similar formula for rotor iron looses..please check.

Single phase Induction motors  use a Capacitor for operation and uses a space split winding of 90 degrees. One of these wingdings have a capacitor which creates 90 degree phase shift from a single phase power supply thus creating rotating magnetic field.

Speed control is one of the disadvantages of Induction motors. Devices of Power Electronics is used to vary the input frequency there-by achieving speed control.