On Sat, 09 Jan 2010 10:59:21 -0500, Bob Engelhardt
wrote:

Bruce L. Bergman wrote:
...
They usually set those limits because the test motor started to come
apart at 3600 but held long-term at 2750, and they left a cushion in
the max speed ratings.

I doubt it. 3600 rpm motors are usually max'ed at 5400 & it's unlikely
that 1750 motors are much differently constructed. I don't think that
I'd run a 1750 at 5400, but I would & do run 1750 at 3600 (on DP with VFD).


I agree, Bob. The construction of a squirrel cage rotor lends itself
to high speed operation. If you look into high RPM motors most of them
are 2-pole induction motors running on high frequency supplies. It's
much easier to keep the rotor bars in place in an induction motor than
the brittle magnets in a DC or brushless motor. Even small brushless
motors top out around 7000 RPM, while 20KRPM induction motors are not
difficult to source.

This is not to say that a vanilla 1800 RPM motor is safe at 400Hz, but
I wouldn't worry about the rotor coming unglued at 3600 RPM.

--
Ned Simmons