Electromagnetic induction in motors

[Josielle Abdilla]

why does an electrical motor produce an induced back emf which is equal to V-It when terminal speed of electrica, fans or electric drills is reached

 

[Merlin3189]

Or, why does it stop accelerating when the back emf is equal to V-It. (I'm not sure what t is here. But simple DC motors have an equation of that form, where t=R)

So a) it produces a back emf because wires are moving through a magnetic field, or magnetic flux is varying through a coil.
b) the back emf increases with speed of movement or rate of change of flux (caused by speed of movement)

c) the current causes torque, because there is a force on a wire carrying current in a magnetic field.
d) torque causes angular acceleration and the motor speeds up.

e) but when it speeds up and the back emf increases, there is less forward emf (from the assumed constant voltage supply) to drive current through the resistance of the wires.
f) so less current flows, torque drops, accn. drops, eventually to zero when the speed is fast enough.
g) then just enough current flows for torque to balance the load (maybe just friction). All the applied emf is just balancing the back emf + the PD required to make that (maybe small) current flow through the Ohmic resistance of the windings, brushes & whatever.

That is said assuming a simple DC permanent magnet motor. You are talking about a universal AC/DC motor and an induction motor. Things get more complicated there, but IF there is a terminal speed for the motor, then the general idea is similar. (Series wound motors can be unstable and terminal speed may be when they fall apart!)

 

[Josielle Abdilla]

Thanks a lot!

 

[essenmein]

why does an electrical motor produce an induced back emf which is equal to V-It when terminal speed of electrica, fans or electric drills is reached

@Merlin3189 described the mechanism for BEMF generation.

From a definition perspective you are looking at this backwards, terminal speed is when the BEMF reaches the supply voltage, ie maximum speed possible at a given voltage. So why does BEMF equal V-It when terminal speed is reached? Because that is the definition of terminal speed.

In other words this is a machine specification. This can be given in as a constant V/krpm, or Speed-torque curves at different voltages etc. Eg if you want to do 2000rpm with 12V, you need a machine with a BEMF constant less than 6V/krpm.