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Three Phase AC, Generators and Motors

It consists of three similar rectangular coils displaced equally from each other, i.e., 120o. Each coil is provided with its own brushes and slips rings.

Three emfs are generated when they are rotated at a constant velocity in a uniform magnetic field. They are of the same frequency and of equal values. Each of the three sources of voltage is called a phase. Each phase voltage lags 120o behind that of the one preceding it.

The instantaneous values of emf in each coil may be written as

E1 = E0 sin ωt, E2 = E0 sin (ωt + 2π/3), E3 = E0 sin (ωt + 4π/3)
It can be used to supply a three-phase system or three single phase circuits.

Advantages of 3-phase system

(1)    In 3-phase alternators the total power does not fluctuate, while a single phase generator the current fluctuates.
(2)    The output power of a 3-phase alternator is always greater than that of a single phase generator of the same size.
(3)    Three phase system is superior for transmission and distribution of electric energy. It involves lot of savings.

Frequency of AC. When a coil is rotated between two poles N and S, one positive half cycle is generated as a N-pole passes and one negative of emf is generated by the passage of one pair of poles. Hence, if there are n pairs of poles on the rotor, n cycles of emf is generated per rotation. It the rotor makes m rotations per second, there will be nm cycles generated per sec. Therefore, frequency of alternating emf is V = nm.

Three Phase AC

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