On 1/23/2013 12:21 PM, nestork wrote:
'Robert[_18_ Wrote:
;3001703'] Interesting..... Now, why does the turntable alternate in
direction each time it is stopped and started...?????

Capacitor start and split phase motors will always turn in the same
direction because the rotor will follow the direction of the apparant
rotating magnetic field, and that's always going to be the same
direction unless and until something is changed in the wiring of the
motor. That's because the strongest point of the apparant magnetic
field will go from the pole of one winding (start or run) to the pole of
the next winding (start or run, I forget which).

But, a shaded pole motor doesn't have two windings, only one. So the
magnetic rotor will be attracted to the closest winding of the opposite
magnetism. That is, if the north magnetic pole of the rotor is
clockwise of the south magnetic pole when power is applied to the
stator, the motor will start (and continue) turning counter clockwise.
If the north magnetic pole of the rotor is counter clockwise of the
south magnetic pole when power is applied to the stator, the motor will
start (and continue) to turn clockwise.

In a shaded pole motor a small part of each pole has a shorting ring on
it. That delays the magnetic field on the shaded part of the pole and
produces a 'rotating magnetic field' just like motors with start
windings. Shaded pole motors will always start in the same direction.
They are commonly used in fans and dial type clocks, both of which
always rotate in the same direction.

(Clocks are synchronous motors, fans are induction motors. Some clock
motors can be reversed by taking the motor apart and reversing the side
of the pole that has the shading.)


Now, if the turn direction is random, then it's as explained in this and
a previous post, that the direction of rotation depends entirely on
where the rotor is when power is applied to the stator. But, if the
direction of rotation alternates predictibly, then I would expect that
there's some mechanism at work in the microwave that senses the
direction or rotation of the motor and reverses it each time the
microwave is restarted.

I don't know how you get a random direction motor in microwaves.

A shaded pole type motor can be made with shading on each side of the
poles. The shading is wound (not a shorting ring) and the direction is
determined by which pair of shading windings are shorted and which are
left open. Could be a mechanical switch operated by the rotation of the
motor to reverse direction at each start.

If the poles are not shaded at all I don't think the motor would
reliably start.


There are some that use both a start and a run capacitor. Why do they
need both?

What you're saying is absolutely correct. Capacitor start motors
have been around for a long long time, but in the past 15 to 20 years
we've seen capacitor run motors (where the capacitor is on the run
winding instead) and capacitor start/capacitor run motors where there's
a different strength capacitor on the start winding and on the run
winding.

Capacitor-run motors have 2 windings with the "run" capacitor
permanently in series with what would be the start winding. The main
winding connects across the line, as with other induction motors. It is,
in effect, a 2-phase motor. Starting torque is relatively low.

Capacitor-start capacitor-run motors also have a "run" capacitor
permanently in series with the start winding. A second "start" capacitor
is temporarily connected across the run capacitor with the usual
starting switch to start the motor. Far as I know the main advantage of
these motors is higher power factor.