Re: Determining Motor HP

["PETER VANDERWAL" <peterv@peoplepc.com>]

> Does a PM motor not have brushes?  I ask this because I opened up a
> non-working motor today on a very large industrial fan to check the
brushes,
> and there were none.  Instead of "normal" fields mounted on the case,
there
> is what looks very much like the same layout as the rotor.

A Large industrial motor, as in an AC motor?  If that's the case then it's
probably not a PM (Permanent Magnet) motor, it's most likely an Induction
motor.

Induction motors do not have brushes, the AC (Alternating Current) being fed
into the field coils on the motors case creates a rotating magnetic field.
This rotating field creates a current in the rotator coils, the current in
the coils creates a magnetic field that gets sucked around in the rotating
field created by the field coils. (Sounds complicated don't it?)

There is a bit more to it than that, but that's the basics.

FWIW:  There is a type of PM motor that doesn't have brushes, it's called a
Brushless DC motor (BLDC).  These have the Permanent Magnets mounted on the
motors shaft instead of the case.  They use field coils on the case (like an
Induction motor) that are energized with a DC square wave.  Usually they
have three sets of coils that are actuated in sequence.