On 03/08/2011 10:39 PM, Ed Huntress wrote:

A motor that has both a wound rotor and a wound stator is not, as far as I
know, an AC induction motor. Induction motors have a rotor that's almost
always made up of very short conductors.

There is a class of AC induction motor, now rarely seen, that DOES have
a wound rotor and a commutator much like on a DC motor. One difference
is they often have a "bracelet" in the commutator that flies out and
shorts all the segments together when the motor reaches a certain speed.

A variable resistor can be put in series with the brushes to regulate
torque when starting. We have an ancient merry-go-round here that has
such a motor, it takes about 3 minutes to come up to synchronous speed,
which would fry a standard induction motor. The operator slowly cranks
out the resistor as the motor speeds up.

These were also used on streetcars, and a variety of other things that
took a long time to start moving. Now, of course, a VFD would be used.

Jon