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Calculation of Electromotive Force in a Generator

Author: E.E. Kimberly

Whether an armature winding is lap or wave, the active conductors lie in two or more parallel paths. By active conductors are meant those which are not under commutation but are actively cutting flux and so contributing to the useful emf of the generator. The conductors in every path are in series, and the sum of the electromotive forces generated in them determines the emf appearing at the generator terminals at no load.

Let ϕ = the total lines of flux per pole;

Z = the total number of active conductors.

Then the flux cut per second by one conductor is


The average emf generated in one conductor is


The number of active conductors per path is Z divided by the number of parallel paths, and the total emf generated by the machine is


Example 10-1. - An 8-pole, lap-wound generator with 800 active conductors is excited to 8 X 106 lines of flux per pole and is driven at 600 rpm. Calculate its generated emf.

Solution. - This generator, being lap wound, has 8 parallel paths in its armature. Hence, the total emf generated is


In a generator the conductors lie in slots which present a high reluctance to the main flux, and the main flux appears almost entirely in the teeth. It might seem then that the conductors shielded from flux by the teeth would cut flux at a low or ineffective rate. One theory proposed is that the flux lines "snap" across the slot from tooth to tooth as the armature rotates. It is properly more logical to consider the turn in the coil rather than a single conductor of that turn. When the turn has its sides in the neutral zones between poles, it encloses all of the flux of a pole and generates no voltage because there is no rate of change of flux. When the turn has its sides under the middles of adjacent poles, it encloses no flux but the rate of change of flux is high and the voltage generated is


Last Update: 2010-10-06