# Do back force and back emf come together?

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1. Apr 13, 2015

### 21joanna12

i have heard it said that back force occurs in generators and back emf occurs in motors, however I was considering a motor and I thought that it seems to me that as the coil is rotating and it has the changing flux linkage, and thus a back end is induced in it by Lenz's law, this reduces the overall potential difference in the coil circuit, reducing the circuit current and so the rate of rotation of the coil decreases, effectively acting as if there is a back force on the coil. Do back emf and back force always come together? I'm not quite sure how to apply this to a generator though- the coil rotates in the magnetic field so an emf is induced in it which creates a current so that a back force is experienced on the coil, but I can't see how back emf comes into this...

2. Apr 15, 2015

### Hesch

Sorry, I don't quite understand your question. Anyway consider a dc-motor (simple):

Say the stator induces a magnetic field through the turning rotor. Then an emf is induced in the windings of the rotor, no matter if the acts as a motor or a generator. The direction of this emf is always given only by the direction of rotation.

Say that at some rotational speed, the emf = 200V. If you supply the motor by say 220V, the current through the windings of the rotor will be driven by the voltage:

U = 220V - 200V = +20V ( call that direction: Forward ). The forward current will induce a magnetic field in the rotor, so that the attraction between the stator field and the rotor field will make the motor accelerate (acts as a motor).

Now you lower the supply voltage to say 180V, and the current through the windings of the rotor will be driven by the voltage:

U = 180V - 200V = -20V (direction: backward). The backward current will induce a magnetic field in the rotor, so that the attraction between the stator field and the rotor field will make the motor decelerate (acts as a generator). The locations of north- and south-pole in the rotor have been swapped, as the current in the rotor windings has changed direction.

So if the direction of the current in the motor is the same as the direction of the emf, the motor will act as a generator. If the directions are opposite, the motor will act as a motor.

Please note: AC-motors cannot be controlled by only supply voltage in order to make them act as a motor/generator.

Last edited: Apr 15, 2015