The field coils of a 6-pole DC generator each having 500 turns are connected in series. When the field is excited, there is a magnetic flux of 0.02 Wb/pole. If the field circuit is opened in 0.02 s and the residual magnetism is 0.002 Wb/pole, calculate the average voltage induced across the field terminals.

CONCEPT:Faraday's Laws of Electromagnetic InductionFaraday summed up the facts of Electromagnetic INDUCTION into two laws. They areFirst LAW:Whenever the magnetic field linked with a circuit changes, an EMF is always induced in it. orWhenever a conductor CUTS a magnetic field EMF is induced in it. Second law:The magnitude of induced EMF is equal to the rate of change of flux linkages.Induced EMF equation:From both the laws,Induced EMF (E) = N\({dϕ \over dt}\)where N = Number of turns of a coil.dϕ = Change of flux linkages in the coil.dt= Change in time.Calculation:GivenP = Number of poles = 6N = Total turns = 6 × 500 = 3000 (since 500 turns PER pole in series)Total initial flux = 6 × 0.02 = 0.12 Wb. (since number of poles is 6)Total residual flux = 6 × 0.002 = 0.012 WbChange in flux, dϕ = 0.12 - 0.012 = 0.108 WbTime of opening of circuit, dt = 0.02 secondFrom concept,Induced EMF (E) = \(3000\times{0.108\over0.02 }\) = 16,200 V