Induction Motor

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Induction Motor

Rotating Magnetic field (R.M.F.)

Induction Motor

    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. So if the arrangement is made to rotate a permanent magnet, then the resulting field is a rotating magnetic field. But ion this method, it is necessary to rotate a magnet physically to produce rotating magnetic field.

    But in three phase induction motors such a rotating magnetic field is produced by supplying currents to a set of stationary windings, with the help of three phase a.c. supply. The current carrying windings produce the magnetic field or flux. And due to interaction of three phase fluxes produced due to three phase supply, resultant flux has a constant magnitude and its axis rotating in space, without physically rotating the windings. This type of field is nothing but rotating magnetic field. Let us study how it happens.

Introduction to Control Valves

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Production of R.M.F.

A three phase induction motor consists of three phase winding as its stationary part called stator. The three phase stator winding is connected in star or delta. The three phase windings are displaced from each other by 120o. The windings are supplied by a balanced three phase a.c. supply. The three phase windings are denoted as R-R' , Y-Y' and B-B'.

The three phase currents flow simultaneously through the windings and are displaced from each other by 120 electrical. Each alternating phase current produces its own flux w hich is sinusoidal. So all three fluxes are sinusoidal and are separated from each other by 120o. If the phase sequence of the windings is R -Y-B, then mathematical equations for the instantaneous values of the three fluxes ΦR , ΦY and ΦB can be written as,

ΦR = Φm sin(ωt) = Φm sin θ ...........(1)

ΦY = sin (ωt - 120) = Φm sin (θ - 120) ............(2)

ΦB = Φm sin (ωt - 240) = Φm sin (θ - 240) .............(3)

As winding are identical and supply is balanced, the magnitude of each flux is Φ m. Due to phase sequence R-Y-B, flux lags behind ΦR by 120 and ΦB lags ΦY by 120. So ΦBultimately lags ΦR by 240. The flux ΦR is taken as reference while writing the equations.

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