Application of Faraday's Law and Lenz's Law

Author: E.E. Kimberly

When a generator is driven, but no current is delivered by its conductors, a voltage appears at its terminals in accordance with Faraday's Law, When a load current is produced by the generator, that current flowing in the armature conductors immersed in the main magnetic field produces a reactive force on the conductors in accordance with Lenz's Law, and this force accounts for the increase in driving torque when the current output increases. Thus, in a loaded generator these laws apply simultaneously.

When a motor is operated at no load, it follows from Lenz's Law that just sufficient current flows in the armature conductors to produce the torque required to maintain its no-load speed. Simultaneously, according to Faraday's Law, the movement of the armature conductors through the field generates an electromotive force opposite in sense to the applied voltage. When a mechanical load is demanded at the motor shaft, the no-load torque is not sufficient to maintain the speed, and the speed drops.

When the speed drops, the generated voltage also drops in proportion, and more current can then flow to supply the new demand for torque. Thus, in a motor both Faraday's Law and Lenz's Law always apply, regardless of the amount of the load; whereas, in the generator both laws apply only when load current is being supplied by the generator,

There is no essential difference in construction between the direct-current generator and the direct-current motor. There are, however, two minor differences. First, the motor shunt-field coils are somewhat smaller; secondly, the series-field coils (if there are any) and the interpole windings are inter-connected with the armature and the shunt field differently in the two machines.