Page 377 - Power Electronics Handbook

Electrical mactune contiil 367
rotor current is such that the torque is always unidirectional. A.C. is
induced in the rotor by the stator flux, the action being identical to that of a
transformer with short-circuited secondary. An equal and opposite current
flows in the stator to balance the rotor ampere turns, this being the power
current.
This machine is called an induction motor. In an actual machine the
stator poles are not physically rotated, but instead they are supplied with
magnetising currents displaced in amplitude and time to produce a rotating
stator flux. The stator will cany this input a.c. magnetising current, with an
input a.c. power current which produces a mechanical motor output.
The rotor of an induction machine may be driven by an external force at
a speed greater than that of the stator field. This is shown in Figure 14.16
where the stator poles are assumed almost stationary. Applying the
principle that the poles induced in the rotor by the stator field will oppose
this stator field and that the rotor ampere turns are balanced by flow of
stator power current, the current directions will be as shown. It is seen that
the power current is reversed, the machine converting mechanical input
into electrical output and is now a generator.
Since current is induced in the rotor due to a changing flux linkage with
the stator field, it is evident that an induction machine must always run
below (motor) or above (generator) the speed of the rotating stator field,
never at the same speed. It was also seen that in a d.c. machine power and
magnetising currents are carried by two separate windings, whereas for an
induction machine they both flow in the same winding, this being the
fundamental difference between the two. It is essential in both systems that
stator poles be maintained during both motor and generator action.
If the rotor of Figure 14.13 is replaced by a magnet, the result is a
synchronous machine. Like the induction machine, the stator field is
rotating which enables the rotor to lock on and follow at the same speed.
Any other speed would cause alternate attraction and repulsion, making
the machine unstable and causing it to stop. For motor action the input at
the stator provides excitation for the revolving field and supplies the power
component necessary to overcome the load torque and cause the rotor to
follow the stator field. In this respect the machine is similar to an induction
motor. However, when generating, no input is required to the stator, since
excitation is provided independently to the rotor. The action is now more

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Figure 14.16 Generator action in an induction machine