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Re: REPLY TO ALL, text misconceptions




Date: Thu, 12 Feb 1998 08:54:11 -0500
From: GARY HEMMINGER <Hemmig@d-e.pvt.k12.nj.us>

... How does an AC induction motor work? The basic principle
seems clear enough (famous last words), but how do they get
started? and why do some AC motors have large capacitiors?

*****************************************
Gary Hemminger

I'll take a shot at this one.
A useful demonstration is a large 3-phase induction motor
with the rotor removed, and driven by a low voltage variable frequency AC
oscillator.
A pivoted compass needle placed in the gap where the rotor normally spins
reveals the presence of a rotating magnetic field.
Examination of a rotor ( of the squirrel cage type) shows that loops of
wire (sometimes a cast-aluminum shroud) fill the slots in the steel stampings.
If the the rotor is initially stationary, the rotating field induces a
large voltage into the low-resistance rotor loops, and the field from the
resulting current reacts with the rotating field so as to set the rotor in
motion, whereon the frequency of the field seen by the rotor is reduced
until the speed of the rotor is sufficiently close to the speed of the
rotating field so as to reduce the magnetic reaction between stator and
rotor to just balance the opposing torque from the load.

For smaller motors, it is convenient to avoid the need for a 3 phase supply
by shifting the phase of the main stator winding through a capacitor into a
second stator winding to provide a (less-perfect) moving field.
For the lowest power motors, it is convenient to shift the phase of the
stator's magnetic field directly by means of one or several shorted loops
around a part of the stator poles. This again imparts some motion to the
magnetic field which the rotor sees.

In connection with the questions concerning the practical arrangements of
many present-day mechanisms, I have found it helpful to refer to a
translation of
Wie Functioniert Das? ( How Does It Work?) Bibliographisches Institut
translated and republished by Granda/Paladin as
How Things Work I & II C. van Amerongen, George Allen & Unwin Ltd.
(A two-volume paperback set).

Sincerely

brian whatcott <inet@intellisys.net>
Altus OK