Capictor Short in Aux. Winding of PSC Motor

[fifty60]

What would I expect to see the capicitor in the startup winding of a PSC motor was short circuited? This would effectively remove the capicitor right? Would it be the same as simply removing the capacitor? How would I test this?

 

[kwired]

What would I expect to see the capicitor in the startup winding of a PSC motor was short circuited? This would effectively remove the capicitor right? Would it be the same as simply removing the capacitor? How would I test this?

Removing the capacitor altogether leaves an open circuit as the capacitor is going to be in series with the aux winding. A shorted capacitor will be the same thing as jumping the terminals of the capacitor, it is not in the series anymore but does not leave open circuit either. In that case the winding will draw current but there likely is not enough phase shift from the main winding being developed to create sufficient torque to start the motor. Some instances it still could possibly start with no load on the output shaft though, but would be a longer acceleration time since there is less torque.

That said I can't recall ever seeing a shorted capacitor, they usually fail open. Now if they do start to short, they likely end up progressing to the point that they end up open as most run capacitors have an internal cut out that will open before it gets to the rupture point. If the case is bulging it is a sure sign that this is what has happened and it will be open circuited no matter what actually happened to start the failure process.

 

[fifty60]

What I meant by remove was "remove and replace with short", sorry for that confusion. Thanks for the answer. I could just expect the single phase motor to lose efficiency but still operate if the motor was already started. If the motor was not started, it would not begin to spin. When this happens, the motor does not begin to spin, does the motor still draw current like a Locked Rotor condition?

 

[kwired]

What I meant by remove was "remove and replace with short", sorry for that confusion. Thanks for the answer. I could just expect the single phase motor to lose efficiency but still operate if the motor was already started. If the motor was not started, it would not begin to spin. When this happens, the motor does not begin to spin, does the motor still draw current like a Locked Rotor condition?

What to expect also depends on if it is a capacitor start only or a capacitor start - capacitor run motor. If there is a capacitor in the circuit during run time then it is depending on that capacitor to provide some phase shift between main and aux winding. Without that shift torque is greatly diminished at the very least, and exactly how the motor responds is likely to depend on how much load it is trying to drive.

 

[StarCat]

HVACR Applications

HVACR Applications

Kwired as always has good points to make.
I will add that in HVAC Troubleshooting, one will deal with a large number of capacitors on PSC motors.
Mostly they do fail open, but every now and then you will see one fail in some kind of unusual apparent shorted manner.
Memory tells me that those motors typically will not start, but the other thing is they will self destruct pretty quick if the condition is left unchecked.
Same thing with an open fault.
The " open failure " will sometimes allow said motor to still start, but it will draw high current and go off on thermal in most cases.
This is especially true with respect to Single Phase compressors having start and run capacitors.
The common failure being run capacitor open and start gear still operable.
High current and off on thermal after minimal run time.
Its rare but not unheard of that condenser fan motors have gone out with incorrect rated capacitors from the factory and the result is the same.
This is especially true when dealing with fractional horsepower motors utilizing capacitors in the range of 2.5-7.5 mfd as is does not take much at all to be over the 10% rule on tolerance.

 

[kwired]

Kwired as always has good points to make.
I will add that in HVAC Troubleshooting, one will deal with a large number of capacitors on PSC motors.
Mostly they do fail open, but every now and then you will see one fail in some kind of unusual apparent shorted manner.
Memory tells me that those motors typically will not start, but the other thing is they will self destruct pretty quick if the condition is left unchecked.
Same thing with an open fault.
The " open failure " will sometimes allow said motor to still start, but it will draw high current and go off on thermal in most cases.
This is especially true with respect to Single Phase compressors having start and run capacitors.
The common failure being run capacitor open and start gear still operable.
High current and off on thermal after minimal run time.
Its rare but not unheard of that condenser fan motors have gone out with incorrect rated capacitors from the factory and the result is the same.
This is especially true when dealing with fractional horsepower motors utilizing capacitors in the range of 2.5-7.5 mfd as is does not take much at all to be over the 10% rule on tolerance.

Yes you will run into mostly PSC motors in the HVAC field. Compressors in particular simply will not have enough torque to start if they don't have a good capacitor in the aux winding circuit. Condensor fan motors wouldn't require a very high level of starting torque and may start rotating, but as the speed increases the load will increase and then they may not have enough torque output to deliver the required demand - which would cause increased current and heating in the main winding of those motors.