Induction motor gone bipolar?

[mike]

Wild_Bill wrote:

(snippage of good stuff)

Thanks for the detailed reply, I have a candidate to give that a try
on, when I can find the time.

Cheers,
WB
.............


Mike

 

[David Farber]

"David Farber" <farberbear.unspam@aol.com> wrote in message
news:O788m.51973$9P.42991@newsfe08.iad...

Just to update what I found out in the past few days: The label on
the motor
says 1/3 hp. I called some repair shops and asked if they knew about
the start capacitance value. One tech told me that the GE 5KH series
was a split
phase motor and did not require any start capacitor. I finally found
a local
guy who was able to come to my shop and call me back with an
estimate. He said the aluminum start windings were bad and that he
would replace both the
start and run windings with copper wires. As to the question of
starting capacitance, he said adding a capacitor wouldn't hurt
especially if it was running that way for a long time. I'll be
getting the repaired motor back tomorrow and hopefully it will be as
good as new.

Wild_Bill wrote:

I'm glad you found a solution, David. From what I've been hearing in
recent years, shops that are interested in repairing small motors are
becoming scarce.

I realized that I'd mis-stated the HP rating you stated from the
motor label after the reply was sent. Still, that value of
capacitance is excessively high for a small motor.

The guy that said a split-phase motor doesn't require a start
capacitor is correct, although when S-P motors are manufactured with
a start capacitor, they're capable of being used in applications that
require higher starting torques, hence the CS,S-P naming.
Many heavy duty motor applications, such as industrial-duty air
compressors require motors with both start and run capacitors..
CS,CR,S-P type.
Because of the high gear ratio to attain the 6 RPM output, the
starting torque requirement wouldn't be very high. The ratio is over
250:1 for a 1700 RPM motor.

I got a reply from Regal Beloit regarding my wiring inquiry. I've uploaded
the diagram here:
http://members.dslextreme.com/users/farberbear/GE_motor/Motor-wiring.jpg

Interestingly enough, in the reply from the support representative, he said
that this motor does not require a capacitor nor is it likely it will
operate if one is used. So why is there a capacitor drawn in the diagram?

By the way, I reinstalled the repaired motor exactly the way it was
previously installed, with the two capacitors, and it is working fine.

--
David Farber
David Farber's Service Center
L.A., CA

 

[David Lesher]

"Wild_Bill" <wb_wildbill@XSPAMyahoo.com> writes:

In properly operating capacitor start split-phase motor, when power is
applied, the start winding spins up the rotor, the weights of the
centrifugal switch overcome the tension of the springs associated with the
switch, and the switch opens, allowing the run winding to take over.

I'll take exception to one part of Bill's otherwise great wording....

As you can see in the diagram linked in this thread; the running winding
is always on [...when the motor is powered up, obviously...] but the
starting winding is soon turned off.

The starting winding is on only for the ~~~100-200 ms it takes the motor to
spin up. (I measured the time once in Machines Lab. Hi Prof Klingshirn!)

(You can work all this out with copious amounts of math but the following
will do.)

What makes it all work is phase shift, both electrical and physical.
The starting winding jerks the motor shaft slightly, then 90 degrees
later [electrically] the running winding [offset physically around the
path of the rotor] follows up with a second one. That one-two punch is
what starts the motor in the correct direction. If you want to go the
other way, flip the running winding around so the running jerk is the
opposite side of the starting peak.

The capacitor also limits the starting current to some finite value;
more capacitance in parallel is more current. Since it's used for only
200 ms, the starting winding can get away with much higher current
than the running one. But hold the motor stalled; and smoke will soon
arise...unless the breaker trips.

Now a shaded pole motor fudges in the phase shift without a cap, but
gives far less torque. If you recall analog electric clocks; that's what
they use.

Three phase makes it all trivial. You get 3 peaks for the three windings,
each peak offset by 120 degrees [electrically] and the winding offset by
120 degrees physically; so you get ABCABCABCABC and the motor needs no
starting winding nonsense at all....it just goes.
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