# Back EMF

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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.

• 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.