Rotating Magnetic Field

Rotating Magnetic Field

The rotating magnetic field can be defined as the field or flux having constant amplitude but whose axis is continuously rotating in a plane with a certain speed.

In three phase induction motors such a rotating magnetic field is produced by supplying currents to a set of three phase stationary windings, with the help of three phase a.c. supply.

The current carrying windings produce the magnetic field or flux. The three phase stator winding produces three alternating fluxes which are seperated from each other by 120°.

Due to interaction of three fluxes produced due to three phase supply, resultant flux has a constant magnitude and its axis rotating in space, without physically rotating the windings.

Thus when stationary three phase winding is excited by a three phase supply then,

a) The resultant of the three alternating fluxes, separated from each other by 120° has a constant amplitude of 1.5 ϕ_m where ϕ_m is maximum amplitude of an individual flux due to any phase.

b) The resultant always keeps on rotating with a certain speed in space. Such a magnetic field is called rotating magnetic field.

There exists a fixed relation between frequency f of a.c. supply to the windings, the number of poles P for which winding is wound and speed N r.p.m. of rotating magnetic field. 

For a standard frequency whatever speed of R.M.F. results is called synchronous speed, in case of induction motors. It is denoted as N_s.

N_s = 120f / p =  Speed of R.M.F.

where         

f = Supply frequency in Hz

P = Number of poles for which winding is wound.

By interchanging any two terminals of three phase winding while connecting it to three phase a.c. supply, direction of rotation of R.M.F. gets reversed.

Review Question

1. Explain the concept of rotating magnetic field. How to change the direction of rotation of rotating magnetic field ?