Back emf and back torque

Many ways of producing an emf

• Emf can be generated in a circuit in several different ways, and it is worth listing them to see where induction fits in.
• Battery: a chemical force gives rise to the emf.
• Piezoelectric crystal: mechanical pressure generates the emf.
• Thermocouple: a temperature gradient across the junctions of two metallic wires produces the emf.
• Photoelectric cell: incident light above a certain frequency produces the emf.
• Van de Graaff generator: electrons are literally loaded onto a conveyor belt and swept along to create a potential difference.
• Generator: the movement of a wire through a magnetic field produces a motional emf and hence a current.

A motor is also a generator

• The physical construction of a DC generator and of a DC motor is practically the same. If a DC generator is connected to a battery, it will run as a motor; if a motor is turned by any external means, it will behave as a generator.
• Consequently, whenever a motor is running, its generator action cannot be turned off.

Back emf

• By Lenz's law, the emf induced by that unavoidable generator action must oppose the change which causes it. In a running motor the 'cause' is the current through the armature, so the induced emf tends to reduce the armature current.
• The induced emf which is unavoidable due to the generator action in a motor is called the back emf. The corresponding opposing torque is the back torque.
• Initially, when a motor is just starting up, its armature is not turning and hence it produces no back emf, so the armature current is limited only by the winding resistance and is large.
• As the motor speeds up, the back emf increases and the armature current decreases.
• This explains why the current through a motor is much larger at the beginning than when the motor is running at full speed — the standard board question on this section.

An application: hybrid and electric vehicles

• The limitation of electric and hybrid vehicles is battery life. To increase the charge in the battery, the car is made to act as a generator whenever the brakes are applied.
• During braking, the frictional force between the tyres and the ground provides the torque that turns the magnets inside the generator. The car thus takes advantage of the back emf to charge the battery and get a longer drive.

Standard results

• Back emf is the emf induced in a running motor by its own unavoidable generator action; it opposes the applied voltage and hence the armature current.
• Starting current of a motor is large because the back emf is zero at start-up and grows as the motor gains speed.