Heat pump run capacitor

[RalphVa]

Does anyone have a circuit diagram that shows what a run capacitor on a heat pump does?

I can understand a start capacitor but have no concept of what the run capacitor does.

Ralph

 

[techman]

The motor design is called PSC=permanent split (phase) capacitor. Sometimes also called a "capacitor run motor". The capacitor is used to produce a phase shift on the starting winding. This guarantees proper rotation and higher starting torque than some other designs. Some heat pumps and ACs incorporate a "hard start kit" which adds a starting cap and (usually) relay (also known as a "capacitor start-capacitor run").

Once the motor is running the cap adds little to performance. When the cap goes bad, you lose starting torque and the motor will not start and draw high current.

Paul

 

[RalphVa]

Still hard to understand why it's a "run" capacitor. The unit wouldn't come on with it blown out.

On this unit, I'd had a start capacitor installed before installing our generator, to reduce some load on the generator for starting it. So, it also has a "start" capacitor.

Ralph

 

[Mendonsy]

In some cases these motors use two capacitors, one value for starting and a second value for running. With this type of motor there is normally a mechanical switch built into the motor to select the correct value when the motor reaches about 50% of normal run speed.

 

[Bird]

In some cases these motors use two capacitors, one value for starting and a second value for running. With this type of motor there is normally a mechanical switch built into the motor to select the correct value when the motor reaches about 50% of normal run speed.

I sure don't understand much of this, but at the recommendation of my service tech, I had an "optional" start capacitor added to my heat pump when it was about 2 years old (3 years ago); did make a difference in the sound when it starts. Then in March of this year, on the bi-annual checkup, the tech said the run capacitor was weak, so it was replaced. Would have cost $219 but was under the 10 year warranty I paid for when the unit was installed 5 years ago. But yes, I have two capacitors; one of which the manufacturer of the unit lists as "optional".

 

[techman]

It is called a "RUN" capacitor, but its main purpose is to start the motor. A simple single phase induction motor needs something to start it. If you spin the shaft, it would start, in either direction. Adding a capacitor to a winding that is physically displaced in rotation to the main winding, causes a phase shift of the AC voltage, on that additional winding. The result is a pull in one specific direction. Thus starting. Other methods of starting use a centrifugal switch (split phase motor) or an additional copper winding on the stator (shaded pole) or a capacitor plus switch (capacitor start)

Paul

 

[s219]

It is called a "RUN" capacitor, but its main purpose is to start the motor. A simple single phase induction motor needs something to start it. If you spin the shaft, it would start, in either direction. Adding a capacitor to a winding that is physically displaced in rotation to the main winding, causes a phase shift of the AC voltage, on that additional winding. The result is a pull in one specific direction. Thus starting. Other methods of starting use a centrifugal switch (split phase motor) or an additional copper winding on the stator (shaded pole) or a capacitor plus switch (capacitor start)

Paul

That is a very good explanation.

When the run capacitor on our AC's outside unit failed a few years ago at our old house, I was able to diagnose the problem by manually starting the fan with a push (used a pencil) and determining that I could do that in either direction. So that suggested the run capacitor had failed. However, the compressor also wasn't kicking on, so that pointed to the unit having a "dual" capacitor that served both the compressor and fan, and when those fail both motors will fail to start. With that little bit of troubleshooting info, I was able to get an HVAC tech out that night since they knew right away what the problem was based on my description over the phone. They swapped out the cap and all was good.

Years earlier, the run capacitor on the air handler blower had failed, but replacing it didn't solve the problem -- ended up having to install a new motor (luckily there was an Emerson motor that was almost a direct match and I was able to get it next day shipping). Not quite sure if the failing capacitor caused the motor to die.

I am kind of tempted to open up the hood on our geo unit and make a note of the capacitors used for the blower/compressor, so I can have extras on hand. The compressor has a soft start kit, and I haven't studied the wiring to know if/how it operates with or replaces a run capacitor.

 

[kenmac]

It's always good to keep xtra caps. on hand.. Try to buy the ones made in the us... The chinese ones are cheap

 

[techman]

It's always good to keep xtra caps. on hand.. Try to buy the ones made in the us... The chinese ones are cheap

At the supply house, there are no US made caps. The "good" ones are made in Mexico !

paul

 

[charlz]

I can usually find what I want on Ebay... a lot cheaper to have a couple on hand then pay those crazy service fees for a $20 cap and 15 minutes or less of work...

When the capacitor on my RV air conditioner went out I put in one of those 'hard start' kits, sure made a big difference on how much the compressor drew down the generator on start up.