Does back EMF only occur in DC motors? (1 Viewer)

[VenomP]

?

Also, take this scenario:

A drill works because of the motor effect. A drill begins drilling into a hard piece of wood, and slows considerably. This can cause the motor to overheat.

Explain why the drill may overheat in this situation.

 

[youngminii]

I haven't done AC motors (and I don't plan to, until my trials) but I know that back EMF occurs in DC motors.

As the drill spins, the current flowing (supply emf) is regulated by the back emf. Now when the drill begins to feel resistance against the motion, the back emf is reduced as there is less relative motion between external magnetic field and the coil (part of the drill). As such, there is more current flowing because there is less back emf and so the drill begins to overheat and will short circuit if the drill is not pulled out of the wood (unless there is a circuit breaker).
Hope that helps, might need a bit more detail as to why the drill overheats.

 

[Nuendo]

just to add, in an ideal motor (where there is nothing attached to slow it down) the speed of the armature coil increases to a point where the back emf = supply emf. thus, there will be no voltage across the coil -> no current flow -> no net force acting -> armature coil rotates at a constant rate.

when there is a load on the motor (the resistance acting on the drill) the coil will rotate at a slower rate -> back emf is reduced -> voltage across armature coil -> current flows through -> net force used to do work

now, the drill overheats because the resistance acting on it is too high -> slows rotation rate -> reduces back emf -> net voltage across circuit remains high (net voltage = supply emf - back emf) -> produces a high current -> burn motor

 

[JasonNg1025]

Yeah, the back emf produced is directly proportional to the change in flux according to Faraday's law. When the load increases, the speed of the motor reduces. The change in flux reduces, and so the back emf is less. The supply voltage remains constant, and as Nuendo said, this makes the net current high which would burn the motor.

Pretty much what they said xD