PSC Motor capacitor wiring

[Brannigan]

Hello All:

I understand the basics of a PSC motor but I have done a fair bit of Googling and I have not been comfortable with the answers. I've learned a lot (here), but no cigar.

If I have a motor with the recommended capacitance listed on the rating plate, AND the windings are symmetrical, everything is good, it runs well and is reversible.

But, I have a number of nice sub-horsepower gearmotors that have asymmetrical windings and I can't find an/answer explanation to how it should be wired. One motor tech told me "whichever way is quietest and coolest". This did not give me the warm fuzzies.

For example, I have a 1/6 hp PSC gear motor with a 22 ohm winding and a 66 ohm winding. The start torque is much higher when I directly connect the 22 ohm winding, which makes sense, and the smaller winding provides the start direction? I'm not seeing any abnormal heating but it is not loaded.

So, are there any hard and fast rules? I am assuming that a motor with asymmetrical windings is designed to only go in one direction, but frankly, I'm guessing.

Also, I'm guessing that a symmetrical motor is acting like a two phase motor, but an asymmetrical motor has the smaller winding *kinda* acting like a start winding to ensure it always goes in the same direction. Am I close, or out to lunch?

Much appreciated if anyone can clear this up or point me to a good tutorial.

Cheers,

Bob

 

[waross]

1. Are you sure that the motors are all PSC? If there is a centrifugal switch then it is not a PSC.
2. Symmetrical winding PSC motors are easy to reverse. The windings are connected together at one end and a lead is brought out.
Call this lead Common.
The capacitor is connected between the other ends of the windings. A lead is brought out from each winding end. Call these leads Forward and Reverse.
If power is applied between Common and Forward the motor will run in one direction.
If power is applied between Common and Reverse the motor will run in the other direction.
Asymmetrical windings and reversing. If either winding is reversed in relation to the other winding the motor will run in the other direction. This requires four wires brought back to the control panel while the motor with symmetrical windings only requires three wires to be brought back.
Which winding is the starting winding? I'll leave that question for one of the motor gurus.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Brannigan]

Thanks for the reply.

Yes, I am sure it is a PSC.

The rest of what you say is all true, but it doesn't address my question.

If I have a PSC with asymmetrical windings, how do I ensure it is connected as the manufacturer intended? If the "reverse" winding is used, obviously the characteristics of the motor will change as well as direction, and likely not for the better.

It would seem to me that if the motor is wired in reverse, a different capacitor value would (or should) be used, but this would change the circulating current and the temperature rise, in both windings. Calculating this is above my pay grade.

To put it a simpler way, if the motor has asymmetrical windings, does one always connect the lower resistance windings directly, and the higher resistance winding via the capacitor?

 

[ScottyUK]

To put it a simpler way, if the motor has asymmetrical windings, does one always connect the lower resistance windings directly, and the higher resistance winding via the capacitor?

In my experience, yes that is the case.

However, the winding configuration you're describing is also common on split phase machines without a capacitor which rely upon the phase displacement between the currents in a high resistance, low inductance winding and a low resistance, high inductance winding to to develop torque. The phase displacement is considerably less than that achieved with a start capacitor - perhaps 30° or 40° rather than nearly 90° - and the starting torque is much reduced. It is possible from the information presented that your motors aren't PSC types, as Bill previously alluded to, unless you have definite evidence that it is - such as a capacitor bolted to the frame or something else that we can't see.

 

[Brannigan]

Thank you Scotty, that all makes sense.

Just to clarify, I seem to collect these motors, either as new old stock or in very good shape, as motors are always handy things to have around. For me to classify the motor as PSC there needs to be a capacitance on the rating plate or a capacitor present AND no centrifugal switch. The range of the capacitance is also a good indication whether it is Start or Run type, depending on the motor size.

Unfortunately, usually by the time I get them, the remaining, or non-existent wiring/capacitor/data sheet leaves the connections ambiguous, at least to me.

More ambiguity: If I can't get an exact replacement for the capacitor, what sort of range can I get away with, and what performance changes can I expect, depending on whether I am over or under?

My guess is; Higher capacitance=higher circulating current=higher starting torque but also higher running current and higher heat, but I'm still learning.

FWIW, during my searches, I came across a few small (<= 1hp) 3 ph VFDs out there that can be configured as 2 phase outputs 90 degrees apart, so a PSC can be used in a VFD, minus the capacitor of course. I may give one a try. Thoughts anyone?

 

[waross]

Capacitors are pretty forgiving. The tolerances on motor starting caps may be plus or minus 20%
Generally more capacitance means more starting torque.
Most of my small motor experience has been with capacitor start motors. I had an application for two very heavy rolling garage doors. The openers were ordered as 3/4 HP but when they arrived in the third world they were 1/2 HP.
They didn't have enough breakaway torque to start the doors rolling.
If they were given a little help starting they moved the doors well. I doubled the starting capacitance on each motor and they have been working well for years. The last time I heard they had been working for about 10 years. That was 10 years ago and I haven't heard any more since.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

The larger the capacitance the more closely the phase displacement will approach 90°, but it's a rapidly diminishing return once you get past a certain size. The manufacturers will provide a capacitor which is large enough to do the job but no more because it adds cost and size. I've never known a problem from going 'too big'.

Make sure they're motor-rated capacitors and not capacitors intended for lighting ballasts (although these are getting less common now LED's dominate the market). Choosing the wrong type might let the magic smoke out.

 

[waross]

I had forgotten to mention that Scotty.
Also use motor run capacitors, not motor start capacitors.
The cap doesn't have to show a rating for "motor running" but if it is listed as "motor starting" it may overheat on continuous use.

Bill
--------------------
"Why not the best?"
Jimmy Carter