The B field does not come from rotation. It takes current to produce
a B field. Since there are no permanent magnets present, rotation
alone cannot produce a B field.

Given that there is a B field, it takes energy to keep it rotating if
it induces voltage (hence current) in a winding somewhere that
therefore produces a counter field and counter torque.

Calling it a "rotary transformer" doesn't clear anything up. What is a
rotary transformer and how does it work?

Transformers work by inducing voltage from a varying B field.
Induction motors (and generators) work with a rotating B field of
constant magnitude, the rotation causing the field linking a
stationary coil to vary with time. One method of analyzing
single-phase motors and unbalanced polyphase motors is by the method
of symmetrical components, which is a collection of constant B fields
rotating in one direction or the other but all at the same speed,
whose resultant sums to the actual field.

A complete electrical understanding requires a description, by
whatever method, of how the field (or contrived fields as in
symmetrical component analysis) vary with time at the location of
each winding and in the rotor. Once this is known, the exchange of
electrical power and mechanical power (instantaneous torque * speed)
can be described by the interactions of the fields with currents.

This has been done. Fitch found it and sent it to me some years ago.
It is a mathematical nightmare!





On 31 Aug 2004 21:55:20 -0700, jim rozen
wrote:

In article , Don Foreman says...


Sure it does - it takes energy to provide the rotating B field
that spins with the rotor's squirrel cage. The energy is in
the kinetic energy of the rotor. That seems to be pretty consistent
with the 'rotary transformer' approach to converter operation.

Jim