Electric motor puzzle
 

I recently had a problem with my compressor starting (it wasn't getting
up to full speed before the centrifugal switch opened). The caps tested
OK and it would start fine if the tube between the pump and tank was
disconnected. So as a work-around I plumbed a solenoid valve into the
line between the pump and tank and connected it to the centrifugal
switch. The idea was that the valve would be opened when the motor
started and would stay open until the motor was up to speed and the
cent. switch opened. This would unload the motor until it was up to speed.

Well, I was surprised and puzzled by what happened. Do you know?

See my reply for the answer.

The valve did open when the motor started, but it STAYED open even after
the motor was up to speed! Yes, the switch was open. WTF??!!

When I drew a schematic to visualize the situation, the problem was
obvious. The start winding was disconnected from the line but still
connected to the solenoid. It was acting like the secondary of a
transformer and generating enough voltage to hold the solenoid open!

If anybody figured this out without having experienced it, I am
impressed! It's something that I never would have anticipated.

Bob

BTW - "fixed" this problem with a relay that separated the winding and
the solenoid, but the motor still didn't start (reliably)! So I
connected it to 220 and it seems fine.

Bob Engelhardt wrote:
I recently had a problem with my compressor starting (it wasn't getting
up to full speed before the centrifugal switch opened). The caps tested
OK and it would start fine if the tube between the pump and tank was
disconnected. So as a work-around I plumbed a solenoid valve into the
line between the pump and tank and connected it to the centrifugal
switch. The idea was that the valve would be opened when the motor
started and would stay open until the motor was up to speed and the
cent. switch opened. This would unload the motor until it was up to
speed.

Well, I was surprised and puzzled by what happened. Do you know?

See my reply for the answer.


Don't most reasonably sized air compressors have an "unloader" valve
designed in specifically to avoid the starting problems you encountered?

Is it possibly your compressor has one of those and it was faulty?

Just my .02

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"

Bob Engelhardt wrote:
The valve did open when the motor started, but it STAYED open even after
the motor was up to speed! Yes, the switch was open. WTF??!!

When I drew a schematic to visualize the situation, the problem was
obvious. The start winding was disconnected from the line but still
connected to the solenoid. It was acting like the secondary of a
transformer and generating enough voltage to hold the solenoid open!

If anybody figured this out without having experienced it, I am
impressed! It's something that I never would have anticipated.
Of course. It is not a transformer, but a two-phase motor. You said
something in the first message about the start capacitors. SO, the
starting switch just cuts in a larger capacitor to provide proper
phase shift during starting.

But, even a real single-phase motor with a resistance start winding will
produce an induced voltage from the start winding. This is how
potential relays are used to control the starting when you can't have
sparking contacts in the motor, like a refrigeration compressor.

To be precise about the mechanism, the start winding is displaced
angularly from the run winding (usually 90 degrees) and the rotating
induced field flux from the rotor passes by the start winding and
induces a voltage in it.

Jon