Basics of Power Generation

PRINCIPLE OF OPERATION:

Generators, motors form a part of an electro-mechanical energy network. They convert one form of energy to another as governed by some laws. A generator converts mechanical energy to electrical energy as defined by Faraday?s law of induction.

Faraday's Law of Induction :

According to Faraday's law, in any closed linear path in space, when the magnetic flux surrounded by the path, varies with time, a voltage is induced around the path equal to the negative of rate of change of flux in Webers/sec.

V = - dF / dT volts.

where, V - EMF in volts & F - Flux in webers

A generator achieves the relative motion by providing a rotating, rotor mounted field against the stationary armature coils.

When a DC field current If flows in the rotor field winding, a MMF (magneto motive force) is set up which causes the formation of a rotor based field flux. In a salient - pole generator the pole faces are tapered, resulting in a maximum flux density along the ?d? axis diminishing to zero in the ?q? direction. By proper tapering one creates, in actuality, a sinusoidal flux distribution along the air gap periphery.

In case of a turbogenerator the magnetic flux distribution has a staircase look, where each step is due to added MMF contributed by the field current in the discrete rotor slots. However the fundamental sinusoidal space wave is dominating over the harmonics.

If the rotor spins at a constant synchronous speed, the stator conductors will experience a travelling "flux wave".

When a magnetic flux of density B cuts a perpendicular conductor at a relative speed s an EMF (electromotive force) is induced in the conductor, the instantaneous magnitude E of which follows the formula.

E = B.s V/m
Where, B = magnetic flux in Tesla , S = relative speed in metres/sec.

As the flux distribution is assumed sinusoidal so will be the EMF distribution. The stator will thus experience an "EMF wave" or " E wave" of the same speed as the flux wave.
The two waves consist of P/2 full cycles around the full periphery. Thus if the rotor speed is n rev./min., each stator conductor will experience an AC EMF of frequency

F = (P / 2) . (n / 60)= (P. n / 120) Hz
where, P = no. of poles, n = speed, F=Frequency

For e.g. a two-pole machine, when rotating at 3000 rev./min. will generate a 50 Hz EMF. When the rotor turns one mechanical degree, the stator EMF completes P/2 electrical degrees. In a two-pole machine electrical & mechanical degrees are identical. Due to their winding locations the EMFs' induced in phase b & c will lag that in phase a by 120 & 240 electrical degrees respectively.