Cydrome Leader wrote:

Leo Lichtman wrote:

"John Hall" wrote: (clip) 1/8 HP, 2.5 A, 1 phase, 110 v. 1725 RPM (clip)
^^^^^^^^^^^^^^^^^
1725 is the "slip speed" of an induction motor whose field is going 1800
RPM. If you try to slow the motor down by reducing the voltage it will slow
down a little, maybe stall, overheat and probably burn out.


Explain this slip speed for us.

Basic motor theory*: One magnet wants to turn to align to another.
Presto! A motor -- at least for slightly less than 90 degrees, anyway.

Slightly more advanced motor theory: A stator produces a spinning
magnetic field which the magnet in the rotor locks to. AKA a
"synchronous machine" (if it's big-ass and connected straight to 440V
3-phase) or "brushless DC motor" (if it's little and connected to some
fancy electronics). Great machine, but how do you get it up to speed,
and what happens when it falls out of sync (bad things).

Even more (slightly) advanced motor theory: Turn the thing inside out,
have a stator with a fixed magnetic field, and switch the electricity to
the rotor so that _it's_ magnetic field is always crossways to the
stator's. If you do this with a commutator then you have a traditional
DC motor (or generator).

More advanced motor theory: Go back to the stator with a spinning
magnetic field. Stick a copper squirrel cage inside, attached to the
rotor. The field rotating in relation to the squirrel cage induces
current, which _just happens_, through the magic of physics, to generate
a magnetic field at right angles to the stator. So a torque is induced,
the rotor tries to follow the spinning magnetic field, and everyone is
happy.

But if the rotor is stopped then the spinning magnetic field can't
induced much current in the rotor because of the rotor's self
inductance. And if the rotor is going as fast as the spinning magnetic
field then it doesn't see a _changing_ magnetic field, which is
necessary for inducing a current. The difference between the rotation
rate of the spinning magnetic field and the rotor is called 'slip' (see,
I didn't forget the question). There will be some magic value of slip
that results in the most torque, and another one that results in the
best efficiency. These numbers are usually a few 10s of RPM to a few
hundreds.

* I don't _care_ about those dang variable reluctance motors, go _away_.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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