# PSC Motor question

Discussion in 'General Electronics Chat' started by bobster, Aug 15, 2010.

1. ### bobster Thread Starter New Member

Aug 15, 2010
3
0
I am interested in understanding more about the theory & operation of PSC motors (like those used for HVAC condenser fans & compressors). I think I understand that the purpose of the "run" capacitor is to create a phase shift in the current to the stator's aux or "start" windings, in order to produce a rotating magnetic field / net torque to start the rotor.

My question relates to the design considerations in sizing the run capacitor.

Since I haven't been able to find any "typical" values for the inductance of the start & run windings, I am unable to analytically characterize the typical circuit.

But, qualitatively, if the start winding current needs to be phase shifted 90 degrees relative to the run winding current (for a motor with start & run poles 90 degrees apart), it seems to me that this means that the value of the run capacitor needs to be selected such that the overall impedance of the series circuit (start winding in series with run capacitor) is totally resistive (or even slightly capacitive to compensate for the series resistance of the run winding) at 60 Hz.

That way, the current in the run winding lags the supply voltage by just short of 90 degrees, and the current in the start winding is approximately in-phase with the supply voltage, giving a 90 degree difference in the currents in run & start windings.

Can anyone confirm this or give me a different way to look at it? (I have some follow up questions once I get this resolved)

Thanks!

2. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
202
The motors are designed to use a certain cap value, there are also some where the cap only comes into play during start and others where it's used both in start and run.

There's some stuff in here that gets kind of technical but should provide your answer:
http://en.wikipedia.org/wiki/Induction_motor

3. ### bobster Thread Starter New Member

Aug 15, 2010
3
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Nope.

I'd already looked at that link (and a bunch of others) before I posted the question.

What I'm interested in is a phasor diagram which shows the relative magnitudes and angles of the currents in and voltages across the run & auxiliary windings and the run capacitor. Preferably under full load and no load conditions. I want to move past the generic description of operation and gain a deeper understanding of what's going on.

I may have to rig up a test stand and take some measurements to get what I want, but I was hoping to find someone who'd already done that...

4. ### bobster Thread Starter New Member

Aug 15, 2010
3
0
Found the answer I was looking for in the Transactions of AIEE. P.H. Trickey's paper is entitled "Design of Capacitor Motors for Balanced Operation" and was published in 1932. Discusses in detail how to size the run capacitor (for balanced operation at rated full load).