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From: borowski@spk.hp.com (Don Borowski)
Subject: Re: How does a brushless DC motor work?
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Date: Mon, 16 Jan 1995 19:50:30 GMT
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Phil Knaack (flipk@iastate.edu) wrote:

: Yes, but there is a difference .. AC induction motors (which just use
: difference densities of metal in the rotor to produce lags in the field
: and therefore apply force) are only efficient at their designed
: speeds. A washing-machine motor will have very little relative torque at
: near-zero RPMs because the `lag' in the core of the rotor is moving too
: fast in the 60Hz field. They solve this with `starters' which are simply
: an extra set of coils placed around the frame which are only engaged
: when the motor is below a certain speed. It produces very high starting
: torque at the expense of very high starting currents (on the order of
: perhaps 2x in some cases, maybe more). 
 
Sorry, you are off the mark here.

First of all, the rotor of an AC induction motor uses a copper "squirrel
cage".  It is essentially a rotating, shorted transformer secondary, with
the primary being the stator.

Induction motors are most efficient at just below their synchonous speed.
In a three phase motor, a rotating magnetic field is generated by the
stator.  In a two pole motor, this field has one "north" and one "south",
rotating at 3600 rpm.  For a 4 pole motor, there is a north, a south, 
another north, and another south.  This field pattern rotates at 1800
rpm (half of 3600).   These figures are for operation at 60 Hz power.

For most two pole motors, their speed at rated horsepower will be about
3500 rpm.  There is a 100 rpm slip between the magnetic field pattern and
the rotor.  This slip is needed to make up for the losses in the rotor
(an induction motor with a superconducting rotor would rotate at 3600
rpm).

Now, if you reduce the AC power frequency, the magnetic fields also slow
their rotation.  So if you take the two pole motor to Europe, and connect
it their 50 Hz power, the magnetic field pattern would rotate at 3000
rpm, and the motor would run at about 2900 rpm when loaded.  And the motor
is still quite efficient.  (an aside:  the voltage applied to the motor
needs to be reduced by a factor for 50/60 for this example).

If you have a variable frequency, variable voltage AC drive, you can get
an AC induction motor to run over a wide range to speeds at quite high
efficiency.

The key to efficiency is to keep the slip - the difference between the
rotation of the magnetic field and the rotation of the rotor - under
control.  Too large of a slip causes loss of torque and efficiency.

The motors used in home washing machines are single phase motors.  The
magnetic field pattern has to rotational component, simply a
reciprocation pattern.  The problem is starting the motor.  The problem
is very similar to trying to start a single piston steam engine when
the piston is exactly at top dead center - there is no rotational force
on the crank, and so the engine cannot start.  A little shove in one
direction or the other, and the engine will start to run (assuming the
valving is set up to run either way).

Similarly, a single phase induction motor needs a shove in one direction
or another to get started.  What is needed is a rotational component in
the magnetic field.  This is done with a second winding and a phase shift
in the current through the winding (compared to the phase of the current
in the main winding).  The phase shift comes either from
having a capacitor in series with the winding, or by making the winding
with more turns on it, thus raising the inductance.

The starting winding is typically disconnected when the motor approaches
its running speed, though there are some "capacitor run" type motors
which run with the second winding all the time, though much beefier
capacitors are needed compared to capacitor start type motors.

Donald Borowski    WA6OMI    Hewlett-Packard, Spokane Division
"Angels are able to fly because they take themselves so lightly."
                                       -G.K. Chesterton

