Back EMF

The coil(s) in an electric motor will experience a retarding emf due to the change in flux as they move though the magnetic field. This phenomenon is known as back emf and is an application of the Lenz’s law.

Magnetic flux over time for a simple electric motor
  • The flux through the coil changes over time (see above)
  • Back emf is produced while the motor coil is rotating
  • It always opposes the supply emf, but can never exceed it

Effect on Current

  • Net emf is the difference between the supply and back emf, and is directly proportional to the current in the rotor coil
  • Back emf is directly proportional to the rotational speed of the rotor coil (motor speed), i.e. \text E_{back} = -\frac{\Delta \Phi}{\Delta t}\propto v

The effect of back emf on current and torque is that:

  • There is no back emf when a motor starts (no rotation), so total emf is at maximum. Torque is also at maximum.
  • As the coil rotates faster, the back emf rises, reducing the total current and thus the torque.
  • When rotating at max speed, back emf is at a maximum and total current flowing is at a minimum. Hence the torque is at minimum.