What is rotating magnetic field (RMF)?

A magnetic field having constant amplitude whose axis continuously rotates in a plane with a certain speed is Rotating Magnetic Field.

Why 3-phase and not 1-phase?

Owing to the advantages of 3-phase power over 1-phase, like cheaper, can be used for providing both industrial and domestic power, better voltage regulation, easier load balancing, and easier minimization of harmonic currents, we prefer 3-phase widely for generation, transmission and distribution of electricity.

A question must arise in us, can a 1-phase machine produce a rotating magnetic field. The answer is not.

To be clear, in 1-phase, the amplitude varies sinusoidally and the axis is fixed. In 3-phase, as we already stated the axis will rotate (axis is pole producing magnets) and the amplitude is constant. In 3-phase, between the phases, there is a 120-degree phase shift.

What about 2-phase?

The answer is, yes!! Actually, more than 1 phase is enough to produce rotating magnetic field.

Then what is the necessary condition for producing RMF?

“The time-angle displacement(wt) between the currents and the space angle displacement between the winding axes must be equal.”

The picture below shows the time-angle displacement between the currents (balanced current) as 120-degree.

Creation of RMF:

Consider a 3-phase, 2-pole machine, having 3 phases (red R, yellow Y, blue B) Assume the current flow at the very instant as shown in the respective color of the phases. (the arrow mark shows the current flowing direction).

The dotted lines show 3 instants namely 1, 2 and 3

Applications:

It applies to most AC motors.

Both synchronous and Induction types.