Permanent Split Capacitor Motor Theory

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mbrooke

mbrooke

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Its been over a decade since I studied motor theory, but I am curious about these motors. Are they simply a 2 phase motor with the capacitor providing phase displacement or is there more into it? For that matter what is a 2 phase motor? How are PSC motors wound and what type of operational characteristics do they exhibit?
 
170510-9720 EDT

mbrooke:

Yes it is a 2 phase motor, and has a continuous rotating magnetic field from the stator.

Good power factor. No centrifugal switch, unless there is also a starting coil or additional capacitor. Without a larger starting capacitor the starting torque is likely less.

Should be less torque ripple.

.
 
170510-0749 EDT

Any true single phase motor will not self start without some assistance. So all self starting "single phase" motors are really a multiphase motor for starting.

Once a single phase motor is started and running and not overloaded, then it receives energy pulses twice per cycle. Thus, this type of motor has torque ripple.

In a multiphase motor there are stator windings (could be on the rotor) that are spatially located around the motor that are fed from electrical signals that are different in phase relationship to each other. With this arrangement a motor can be built that has a magnetic vector that rotates in space. Put a fix magnet in this field and it will rotate in sync with the rotating field, a synchronous motor. Put a closed coil in this field and it will receive an induced current that in turn produces a magnetic field that interacts with the rotating stator field. But this induced field has a frequency that is different than the inducing field because of slip and therefore also is the cause of slip.

To the extent that the rotating magnetic can be made to be a constant amplitude magnetic vector will determine how much torque ripple there will be. Note, a DC brush type motor has low torque ripple.

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mbrooke said:
Its been over a decade since I studied motor theory, but I am curious about these motors. Are they simply a 2 phase motor with the capacitor providing phase displacement or is there more into it? For that matter what is a 2 phase motor? How are PSC motors wound and what type of operational characteristics do they exhibit?
Click to expand...

Current in one winding is out of phase with current in the other winding. The capacitor along with differences in winding impedance will determine the phase difference.

You need enough difference to create enough torque to turn the load. Capacitor start/capacitor run motor is essentially the same thing as a PSC except it introduces the start capacitor to create more phase differential a low speed to help provide more starting torque.

PSC are common for fans as they don't normally require high starting torque. A lot of hermetic compressors are PSC. Low starting torque is alright as long as refrigerant pressure is equalized when starting.
 
kwired said:
Current in one winding is out of phase with current in the other winding. The capacitor along with differences in winding impedance will determine the phase difference.

You need enough difference to create enough torque to turn the load. Capacitor start/capacitor run motor is essentially the same thing as a PSC except it introduces the start capacitor to create more phase differential a low speed to help provide more starting torque.

PSC are common for fans as they don't normally require high starting torque. A lot of hermetic compressors are PSC. Low starting torque is alright as long as refrigerant pressure is equalized when starting.
Click to expand...

I've always assumed compressors were high starting torque devices. At least one would think from the typical inrush.
 
mbrooke said:
I've always assumed compressors were high starting torque devices. At least one would think from the typical inrush.
Click to expand...
How many have you seen with a start capacitor?

Nearly all household refrigerators and air conditioner compressors only have a run capacitor AFAIK. That is why they must wait before restarting for the pressure to equalize, they don't have enough starting torque to overcome that pressure. One with a start capacitor or even a three phase unit very well may be able to start under pressure, or at least will be able to produce enough starting torque before pressure is completely equalized.

Some may have longer starting time because they have lower starting torque, which will result in high current for longer duration at startup. After the initial inrush of current, once the magnetic fields are established the lower torque PSC will operate closer to LRC for longer time as it will accelerate slower then same motor driving same load with an additional start capacitor in the circuit during starting.
 
kwired said:
How many have you seen with a start capacitor?
Click to expand...

Several :p

https://www.grainger.com/product/4E...!!g!104635265877!&ef_id=U7BD:20170510140013:s


Nearly all household refrigerators and air conditioner compressors only have a run capacitor AFAIK. That is why they must wait before restarting for the pressure to equalize, they don't have enough starting torque to overcome that pressure. One with a start capacitor or even a three phase unit very well may be able to start under pressure, or at least will be able to produce enough starting torque before pressure is completely equalized.
Click to expand...


I beg to differ- I have seen many with a start "resistor" in addition to the run capacitor.


https://www.google.com/search?q=ref...UIBygC&biw=1120&bih=566#imgrc=Vl2w_CkJ3VYT_M:


Some may have longer starting time because they have lower starting torque, which will result in high current for longer duration at startup. After the initial inrush of current, once the magnetic fields are established the lower torque PSC will operate closer to LRC for longer time as it will accelerate slower then same motor driving same load with an additional start capacitor in the circuit during starting.
Click to expand...

Is there a possibility compressor motors are over sized? In general a compressor had a much higher LRA relative to it RLA when compared to say a blower... at least in my observations.
 
mbrooke said:
....Is there a possibility compressor motors are over sized? In general a compressor had a much higher LRA relative to it RLA when compared to say a blower... at least in my observations.
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I would say there is zero chance of that. There is nothing on a A/C unit or refrigerator that is oversized, everything on them is as cheap as possible.

PSC motors have lower starting torque than a cap start motor but are more efficient. I see a lot of cap start/cap run motors on A/C units, the newest ones have an "all in one" capacitor.
 
ActionDave said:
I would say there is zero chance of that. There is nothing on a A/C unit or refrigerator that is oversized, everything on them is as cheap as possible.

PSC motors have lower starting torque than a cap start motor but are more efficient. I see a lot of cap start/cap run motors on A/C units, the newest ones have an "all in one" capacitor.
Click to expand...

Which would explain the NTC resistor. It effectively turns the PSC into a centrifugal start motor (so to speak) maximizing it. But outdoor AC units are simply a cap- which leads me to think the motor could be oversized for added start torque?
 
mbrooke said:
Several :p

https://www.grainger.com/product/4E...!!g!104635265877!&ef_id=U7BD:20170510140013:s





I beg to differ- I have seen many with a start "resistor" in addition to the run capacitor.


https://www.google.com/search?q=ref...UIBygC&biw=1120&bih=566#imgrc=Vl2w_CkJ3VYT_M:




Is there a possibility compressor motors are over sized? In general a compressor had a much higher LRA relative to it RLA when compared to say a blower... at least in my observations.
Click to expand...
I think that "resistor" is somewhat newer thing you are seeing on smaller compressors, like for refrigerators, dehumidifiers, etc. Don't think you will find them on a 2-5 ton HVAC compressor.

Seems to be more common to see start capacitors on commercial refrigeration that uses single phase compressors, they are usually controlled with a potential relay, but I can't ever recall seeing a start capacitor on a typical central system residential grade air conditioner/heat pump unit. UNLESS IT WAS ADDED AS A HARD START KIT.:p
 
kwired said:
I think that "resistor" is somewhat newer thing you are seeing on smaller compressors, like for refrigerators, dehumidifiers, etc. Don't think you will find them on a 2-5 ton HVAC compressor.

Seems to be more common to see start capacitors on commercial refrigeration that uses single phase compressors, they are usually controlled with a potential relay, but I can't ever recall seeing a start capacitor on a typical central system residential grade air conditioner/heat pump unit. UNLESS IT WAS ADDED AS A HARD START KIT.:p
Click to expand...

I know, tends to come out when the voltage is below 200 volts.


But for the record, any idea on what the starting torque is for PSC? I know centrifugal start motors are around 200%. 3 phase 400%. Though I could be wrong.
 
mbrooke said:
I know, tends to come out when the voltage is below 200 volts.


But for the record, any idea on what the starting torque is for PSC? I know centrifugal start motors are around 200%. 3 phase 400%. Though I could be wrong.
Click to expand...


Don't know, but can tell you that voltage drop because of high current will lessen the torque even more, so what is supplying the motor does have some impact as well.
 
170510-1032 EDT

mbrooke:

A good qualitive, and more technical discussion on AC capacitor motors is in Chapter 17 page 304 of "Alternating-Current Machinery", by Bailey and Gault, McGraw-Hill, 1951. I believe still under copyright, and thus Google Books is of little value. I never knew Bailey or Gault, but Gault's wife worked as a secretary in our Electronics Defense Group in the early 50s. I believe her husband had recently died. Gault was one of the early researchers on electrolytic capacitors.

High starting torque is achieved with a much larger capacitor than the optimum capacitance for good runnig characteristics.

Why should there be confusing information about capacitor motors on the Internet (other than the Internet is a poor source of information at times) when these motors have been generally understood since about 1930?

.

.
 
Ok- been doing some reading and learning lots. But still have a question: how are these motors wound ? And how do they reverse direction in terms of those windings?
 
Ie, here is a 3 phase motor:


2009-02-05_094542_cdrom_wiring.gif
 
They are wound like a single phase motor with a starting winding and a running winding, but there is no starting switch so the starting winding stays in. A cap start motor or a split phase the starting winding drops out after a half second.

I once got my butt chewed here for saying that the running cap limits the current in the starting winding so it doesn't burn up, but to me that's what it does and no one convinced me otherwise.

The motor reverses by send power through the start winding in the opposite direction.
 
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mbrooke said:
Ok- been doing some reading and learning lots. But still have a question: how are these motors wound ? And how do they reverse direction in terms of those windings?
Click to expand...
About same thing as a 2 phase motor, because that is essentially what it is. 4 pole motor would alternate main and aux winding every 45 degrees
 
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