variable speed single phase AC capacitor motor

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Sounds kind of wrong. However that's what's in my Breezair evaporative  
Aircon, and it has a short.

It is what appears to be a split capacitor motor, 550W.
Since the auxiliary winding with the capacitor has its own connecting  
wire, it could even be also just a starting capacitor for all I know.  
There is a circuit board that's driving the motor and the rest of the  
A/C. However, having a value of 20uF/440VAC it's more likely permanently  
connected.

The main winding of the motor is driven by a triac circuitry. I measured  
the voltage with my LCD DVM (with graphic display, primitive but kind of  
works) The frequency never changes, only the pulse shape, in 9 stages.

I bought myself a cheap replacement motor that is advertised to have 3  
speeds. I thought it should be doable to get it working. It is a split  
capacitor motor. Also 4 pole like the original and fits in the box. When  
I connect it up on the bench it appears that the 3 speeds are all the  
same. I guess it must be different under load. If the original driver  
circuitry won't work with it I could just drive this motor with a remote  
switch. Three speeds will do me.

But if I actually knew the principle of these beasts I feel I could get  
it going, even variable.

My knowledge of AC motors doesn't go far enough to explain how a  
basically synchronous motor can have different speeds just by changing  
the voltage of the main winding(s). The Aux winding seems to be always  
straight on 240V sinus.

Does anyone know how this works? Does this motor have any specific  
properties, different to a normal simple 1 phase capacitor ac motor with  
a bit of slip?

Thanks

Tony


Re: variable speed single phase AC capacitor motor
Quoted text here. Click to load it

Slip is fairly common with fan motor speed control, unlike a hard mechanical
load like an elevator, compressor, or a saw blade, a fan that's moving air
has reduced torque at reduced speed. So the heating due to slip does
not get out of hand.

--  
  When I tried casting out nines I made a hash of it.

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