When conductors move relative to magnetic flux, their flux linkage changes and an electromotive force can be induced. In a rotating electrical machine this induced voltage appears in both generator and motor operation.

Generated EMF

In generator operation, mechanical rotation changes the flux linking the windings and produces an electrical EMF.

For many machine models, the generated magnitude has the proportional form

$$E=k\Phi\omega,$$

where $\Phi$ represents the relevant magnetic flux, $\omega$ the mechanical angular speed and $k$ a constant determined by machine geometry and winding arrangement.

Increasing flux or speed therefore increases the generated voltage.

Back EMF in a motor

In motor operation, the rotating conductors also generate an EMF. By Lenz's law its polarity opposes the electrical cause driving the motion, so it is called back EMF.

As motor speed rises, back EMF rises. The difference between the applied voltage and the back EMF therefore becomes smaller, which reduces the current that would otherwise be limited mainly by winding resistance.

One physical effect, two operating directions

Generated EMF and back EMF are not separate phenomena. Both are electromagnetic induction caused by motion through magnetic flux; the name reflects the direction of power flow and the role the induced voltage plays in the circuit.