Re: [CAD_CAM_EDM_DRO] Re: spindles powered by REGULAR AC induction motor

Carlos Guillermo wrote:

> Peter -
>
> I regularly take my Bridgeport 1720-rpm motor to 150%, but I have
> no idea what the limit is. I'm also tempted to take a smaller
> motor and crank it slowly up to 600% speed to see what happens.
> (I'll take Jon E's approach and hide under the bench, of course)

While overspeeding a 1725 RPM motor up to 200 % is not likely to
cause an "uncontained disassembly", (as the jet engine manufacturers
call an "engine explosion"), taking any motor to 6 times its rated
RPM sounds pretty dangerous. A 1725 RPM motor would reach
over 10,000 RPM and a 3450 would go over 20,000 if they could
follow the frequency. Since most inverters go to constant voltage
about 100%, the motors would lose rotor excitation and level off,
or possibly spin down and just stop when driven much above 200%
but only 100% voltage. If you had an arrangement (like a transformer)
to supply 6 x voltage at 6 x frequency, then the motor would either
have an insulation breakdown, or the dreaded uncontained disassembly.
The latter, you DON'T want! I'm not joking, a bench won't save
you, unless it is more like an armored bank vault! Bursting the
rotor of a modest 60 Hz 1/2 Hp or so motor at 10 - 20 K RPM
would be something like setting off an artillery shell inside your house.

Oh yeah, there was a story in the paper about two years ago. An
aircraft instrument technician in the St. Louis area was killed by an
aircraft gyroscope that blew up! I'm guessing he fired up a 12 V
gyro on 28 V by mistake, and it overspeeded! Now, this thing has
a rotor weighing about a pound, and it was presumably only going
twice its rated speed!

Jon