On Jan 22, 11:50 pm, nestork wrote:
;3001034 Wrote:



Not only do I wonder *why* they change rotation, but *how*? I'm
guessing they use a shaded-pole motor (I'm not sure), and as far as I
know, those motors only turn one direction.

Homeowner:

This post is going to be off topic because I believe your question has
been answered.

All electric motors operate on the principle that a magnet will spin if
you put it in a rotating magnetic field. It's producing a rotating
magnetic field that can be a bit of a trick sometimes.

With three phase electric power, producing a rotating magnetic field is
easy. If each phase of your power is 120 degrees apart, you just
arrange the three windings 120 degrees apart around the stator and you
have a near perfect rotating magnetic field.

With two phase power, you can do a similar thing and get good results.

It's when you get to single phase 120 VAC power that you have to get
creative in using that single phase power to produce what appears to be
a rotating magnetic field instead of just an oscillating magnetic field.
ALL of the different kinds of 120 volt electric motors that you hear
about (like split phase motors, shaded pole motors, capacitor start
motors, etc.) are different only because they use a different way of
making the rotor see what appears to be a rotating magnetic field.

The easiest of these to explain is the capacitor start motor.

If you imagine two metal plates in close proximity, if you apply a
voltage to one plate, that applied voltage will repel the electrons in
the second plate, and you'll get a small current flowing out of that
second plate.

Now, if the voltage you apply to the first plate were in the form of a
sinusoidal wave, just like the voltage in your wall outlets, the current
coming out of the second plate would be at a maximum when the RATE OF
CHANGE IN VOLTAGE in the first plate was at a maximum, and that actually
occurs when the applied voltage is going from positive to negative or
negative to positive, or when the applied voltage is actually ZERO for a
very short period of time. That is, by putting a capacitor in a
circuit, you completely change the relationship between the applied
voltage and the resulting current through the circuit.

In a simple circuit with only a single resistor in the circuit, current
through the resistor is maximum when the applied voltage is maximum.
Similarily, current is theoretically zero when the voltage goes from
positive to negative, or negative to positive, or when the applied
voltage is temporarily zero.

If you replace that resistor with a capacitor, the current through the
circuit is a maximum when the applied sinusoidal voltage is changing the
fastest, and that occurs when the voltage goes from negative to
positive, or positive to negative, or when it's actually temporarily
zero volts.

So, one way to use single phase power to create an apparant rotating
magnetic field is to build an electric motor for two phase power (with
the windings 90 degrees apart) and apply the same 120 VAC to both
windings. BUT, if you put a capacitor in series with one of those
windings, the current through that winding will be 90 degrees out of
phase with the current through the other winding.

Since a coil of wire develops it's magnetism as a result of the CURRENT
flowing through the coil and not the voltage applied to it, the magnetic
field of one winding will develop 1/4 of a AC voltage cycle before or
after the other winding, thereby creating much the same thing as the
rotor would see if it were in a two phase motor while the motor is
starting.

In actuality, in a capacitor start motor the winding with the capacitor
in series is cut out of the circuit by a centrifugal switch once the
motor comes up to speed. After the "start" winding is shut off, the
capacitor start motor continues to run on the other "run" winding only.
It's been found that the motor will run smoother and more efficiently
that way, and the explanation of "why" is something I just don't know.

In a "split phase" electric motor, you have very much the same thing
happening as in a capacitor start motor, except that you don't have a
capacitor in series with one of the windings.

Instead, in a split phase motor, one of the windings consists of a lot
of turns of thin wire whereas the other winding consists of only a few
turns of thick wire. This difference causes the two windings to have
different "impedance", and that results in the winding with the thin
wire developing it's magnetic field earlier in the AC voltage cycle than
the winding with the thick wire.

And, that difference in the timing of the magnetic field from each
winding creates the impression of a rotating magnetic field for the
rotor to follow. And, just like in a capacitor start motor, once a
split phase motor gets up to speed, a centrifugal switch cuts out the
start winding and the motor will continue to turn on it's run winding
only. (I can't remember now whether the start winding is the one with
thin or thick wires.)

Now, both a capacitor start motor and a split phase motor will be happy
to turn in the opposite direction if you want them to. All you have to
do is reverse the polarity of ONE of the windings. It can be the start
winding or the run winding; doesn't matter which. If you reverse the
polarity of one of the windings, the motor will turn happily in the
opposite direction. Washing machines use that feature to advantage by
having the timer (or something called a "motor reversing relay") reverse
the polarity of one of the windings on the motor so that the motor turns
in one direction while the washer is in the agitate cycle, and in the
opposite direction when the washer is in the spin cycle. This is
important because during the spin cycle, the motor turns the washer's
pump one way to pump water OUT of the washer, but during the agitate
cycle, the pump turns so as to pump any water that leaks into it back
into the wash basket. Similarily, Maytag top loading washing machines
have a pulley which turns on a threaded shaft. The pulley screws UP the
shaft when the motor turns one way, and down the shaft when the motor
reverses direction, and it's which direction that pulley is pushing that
determines what the transmission does; agitate or spin.

I've been told not to reverse the polarity of BOTH windings on a
capacitor start or split phase motor. Apparantly, doing that will
create a dislocation in the space-time continuum causing you to complete
the reversal procedure before you began, with the necessary result that
the motor will turn in it's original direction. (smirk)

A shaded pole motor is a different kettle of fish. A shaded pole motor
has only two poles in it's stator. But, there will be a thick loop of
copper wire going through the middle of both of those two poles, thereby
separating each pole in half. A current flows in that thick wire as a
result of the magnetism produced at the two poles, and the magnetic
field of the thick wire opposes the magnetic field on one side of the
pole and adds to the magnetic field on the other side of the pole. So,
what the rotor sees is a weak magnetic field on one side of the pole
becoming a strong magnetic field on the other side of the pole, and that
gives the rotor the illusion of the magnetism "sweeping across" the
pole, or something similar to a rotating magnetic field.

The thing to remember here is that there is only ONE kind of three phase
motor and only ONE kind of two phase motor, but several different kinds
of single phase electric motors. That's because each different kind of
120 VAC electric motor uses a different method of getting the stator to
create the appearance of a rotating magnetic field using only single
phase electric power. Two and three phase electric motors don't need to
do that because their stators actually produce a rotating magnetic field
for the rotor to follow.

There, now you know more about electric motors than 99 percent of people
named "Homeowner".

--
nestork

Interesting..... Now, why does the turntable alternate in direction
each
time it is stopped and started...?????
This is a reliable thing, and can easily be verified..... it isn't
random....