BACK EMF CONCEPTS
Back emf
Now, as the first two animations show, DC motors and generators may be the
same thing. For example, the motors of trains become generators when the train
is slowing down: they convert kinetic energy into electrical energy and put
power back into the grid. Recently, a few manufacturers have begun making motor cars
rationally. In such cars, the electric motors used to drive the car are also
used to charge the batteries when the car is stopped - it is called regenerative
braking.
So here is an interesting corollary. Every motor is a generator. This is
true, in a sense, even when it functions as a motor. The emf that a motor
generates is called the back emf. The back emf increases with
the
speed, because of Faraday's law. So, if the motor has no load, it
turns very
quickly and speeds up until the back emf, plus the voltage drop due
to losses,
equal the supply voltage. The back emf can be thought of as a
'regulator':
it stops the motor turning infinitely quickly (thereby saving
physicists some embarrassment). When the motor is loaded, then the
phase of the voltage becomes closer to that of the current (it
starts to
look resistive) and this apparent resistance gives a voltage. So the
back
emf required is smaller, and the motor turns more slowly. (To add
the back
emf, which is inductive, to the resistive component, you need to add
voltages
that are out of phase. See AC
circuits.)
