Back emf in electric motors

When something like a refrigerator or an air conditioner (anything with a motor) first turns on in your house, the lights often dim momentarily. To understand this, realize that a spinning motor also acts like a generator. A motor has coils turning inside magnetic fields, and a coil turning inside a magnetic field induces an emf. This emf, known as the back emf, acts against the applied voltage that's causing the motor to spin in the first place, and reduces the current flowing through the coils of the motor.

At the motor's operating speed, enough current flows to overcome any losses due to friction and other sources and to provide the necessary energy required for the motor to do work. This is generally much less current than is required to get the motor spinning in the first place.

If the applied voltage is DV, then the initial current flowing through a motor with coils of resistance R is:
I =
For example, I =
120 V
6 W
= 20 A

A device drawing that much current reduces the voltage and current provided to other electrical equipment in your house, causing lights to dim.

When the motor is spinning and generating a back emf e, the current is reduced to:
I =
(DV - e)

If the back emf is e = 108 V, we get:
I =
(120 - 108)
= 2 A

It takes very little time for the motor to reach operating speed and for the current to drop from its high initial value. This is why the lights dim only briefly.