On 6 Jan 2006 10:27:22 -0800, jim rozen
wrote:

In article , Don Foreman says...

On 4 Jan 2006 10:09:34 -0800, jim rozen
wrote:

In article , Don Foreman says...

The rotor field is always in space quadrature from the stator field.
This is well-established in about any textbook on the subject. That
being the case, the emf it induces in the third leg is necessarily
in quadrature with the emf impressed by the line (and countered by the
stator field) in the other two windings.

I thought it was the rotor *current* that was in quadrature.

It is. Same thing. Ampere's law. Field is proportional to current.

Ah, hang on there. The field in the rotor has to be 180 degrees
out from the stator fields, from Lenz's law, right?

I was vague, sorry. I meant rotor current. Rotor field (the field
the rotor produces) is in phase with rotor current by Ampere's law
relating magnetic field to current.

That field is produced by rotor current resulting from EMF induced in
the rotor by rate of change of stator field linking it -- Lenz's law,
as you say. Rotor current is then this EMF / rotor_impedance.

The rate of change is at slip speed -- stator flux rotates at synch
speed, rotor rotates at slip rpm below synch speed. This is from
the perspective of an observer on the rotor, using the artifice of
rotating fields of constant magnitude to represent sinusoidally
varying fields in space quadrature. The equivalance is mathematically
correct, and a convenient way to look at things. It isn't the only
way to look at things by any means, but I find it easiest to
visualize. Reference: "Electric Machinery", Fitzgerald & Kingsley,
McGraw Hill.

Rotor field and stator field can be dealt with separately and
independently, as when resolving vectors into components. They differ
in phase because induced voltage in the rotor is the time derivative
of stator flux apparently rotating at slip speed. They are usually
very nearly in quadrature because the rotor impedance looks primarily
resistive to rotor emf induced at slip frequency. Slip frequency is
typically about 2.5 Hz at rated speed so rotor self-reactance is
negligable.