Back emf

Definition

Back electromotive force (back emf) is the voltage generated by an electric motor or generator that opposes the applied voltage. It is a consequence of electromagnetic induction and acts to limit the current in the circuit.

5 Must Know Facts For Your Next Test

1. Back emf is proportional to the speed of the rotating machinery.
2. The presence of back emf reduces the net voltage across the motor or generator's windings.
3. In a DC motor, back emf increases with rotor speed until it nearly balances the applied voltage at maximum speed.
4. Back emf plays a critical role in protecting electrical components from excessive currents by providing natural feedback control.
5. The mathematical expression for back emf in a motor is $E_{back} = k \cdot \phi \cdot \omega$, where $k$ is a constant, $\phi$ is magnetic flux, and $\omega$ is angular velocity.

Review Questions

• How does back emf affect the current in an electric motor?
• What factors determine the magnitude of back emf in a generator?
• Why is back emf important for protecting electrical components?

Related terms

Faraday's Law of Induction: States that a change in magnetic flux through a coil induces an electromotive force (emf) in the coil.

Lenz's Law: Indicates that induced current flows in such a direction as to oppose the change in magnetic flux that produced it.

Electric Generator: A device that converts mechanical energy into electrical energy by electromagnetic induction.