Starting capacitors in place on an air conditioning compressor (C) InspectAPedia.comElectric motor start & run capacitor troubleshoot, select, install & repair
Installation of Air Conditioning Compressor Motor & Other Electric Motor Start-Boost or Run Capacitors

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Electric motor start-run capacitor instructions:

Tthis electric motor capacitor article series explains the selection, installation, testing, & use of electric motor starter start and run capacitors used on various electric motors found in or at buildings such as air conditioner compressors, fan motors, some well pumps and some heating equipment. These electric motors use a capacitor to start and run the motor efficiently.

We explain the choice & wiring procedures for a hard start capacitor designed to get a hard-starting air conditioner compressor motor, fan motor, refrigerator, or freezer compressor or other electric motor (such as a well pump) going.

We discuss the following: Use of starting capacitors on electric motors for air conditioning, pumps, etc. Start or Run Capacitor Diagnostic Checks: How to Use a VOM or Multimeter to Test a Motor Starting Capacitor. How to fix or replace a hard-starting air conditioner compressor or other hard starting electric motor. How to inspect, test, or install an air conditioner motor starting capacitor. Warnings about remaining life of hard-starting air conditioner or heat pump compressor motors.

Capacitors are electric devices that get an electric motor running at start-up or that help keep a motor running once it has started. If the capacitor has failed the symptom is that the motor won't start. You may hear it humming or observe that it's getting hot. If you observe this we suggest that to avoid damage you turn off the system while waiting for repairs.

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Air Conditioning Compressor or Other Electric Motor Starting Capacitors

Photograph of  This new compressor was placed directly against
a brick wall. One third of its condenser coil cooling ability was blocked.Capacitors are electric devices that get an electric motor running at start-up by providing a "jolt" of stored electrical energy, or that help keep a motor spinning once it has started.

The starting capacitor helps a motor start spinning by creating a high-torque, rotating, electrical field in the motor. In many electric motors there are actually two capacitors, one boosting the start winding (the start capacitor) and a second that remains in the circuit while the motor continues running (the run capacitor).

If the start capacitor has failed the symptom is that the motor won't start. If either or both start and run capacitors are defective the motor may try to start but will hum and won't keep running. You may hear a compressor or fan motor humming or observe that it's getting hot.

Watch out: If you observe a humming electric motor that is not starting we suggest that to avoid damage you turn off the system while waiting for repairs.

Question: Could replacing a start capacitor cause the compressor to short to ground?

(Aug 16, 2016) Jen said:

We recently had our "Start capacitor" replaced on a 9 year old Trane HVAC. 3 days later, unit stopped cooling and technican says "compressor is shorted to the ground". When asked if the start capacitor replacment could have created any issue with the compressor shortage, he said "absolutely not".

That seems inconsistent. If the starter capacitor is a "energy booster" to the compressor, how could it be completely unrelated? So my question is, could/is it possible for a new start capacitor install create an "energy event' which would cause the compressor to short out? The tech said there was no way they could be related technically, Just need an HVAC genius to provide their insight. I would appreciate it. I'm looking at 2400 to replace compressor or 7K for new unit.

This question was posted originally at AIR CONDITIONING & HEAT PUMP SYSTEMS


Jen, I can't by e-text rule out a wiring error or mistake that could damage an air conditioner compressor motor during service, but most likely the compressor motor was failing in the first place, before the replacement starting capacitor was installed. A common quick and inexpensive "repair" that an HVAC tech will try when a compressor motor is having trouble starting is to simply install a hard-start motor capacitor. If I were doing that I'd probably remember to explain to the customer that this repair might give some added life to the compressor but if it's failing - such as by a motor that's binding - then the unit can fail at any time: be prepared.

Question: smoking start capacitor

2 Sept 2016 Rod said:
Trying to replace this smoking black cannister but I don't know what it is nor the brand of the A/C. It's an ICP heat pump and the A/C logo is a molecule. It's pretty old.


Rod I looked at the photo link you suggested but couldn't understandwhat I was looking at. If it's connected only by wires it may be a start/run capacitor.

Watch out: If equipment is smoking, TURN IT OFF immediately and leave it off as you're risking a fire.

Normal start & run capacitors on compressors, fan motors, other electric motors

Air conditioner compressor motors (and lots of other electrical motors) that run on two-phase (220V) or single phase (120V) electrical power usually include a capacitor in the start circuit to help get the motor spinning; a capacitor can be put into the "run" circuit of the motor as well to increase motor efficiency.

Starting capacitor: the starter capacitor gives extra torque or boost to get a motor spinning in the right direction by providing about double that nominal system voltage.

Run capacitor: once the motor has started, a run capacitor may be used to help the motor retain full power, providing 1.5 x the nominal system voltage and varying as needed depending on the load on the motor.

In some old-school class views adding a run capacitor is similar to making two-phase out of one-phase electricity and is a common practice on air conditioners. What these folks really are saying is that "With capacitive reactive power of about 75% of the nominal power of the motor, the comparison of power is slightly lower than that of a three-phase motor of equal size." [2]

Technical notes: Single phase electric motors such as those used in air conditioner compressors and fan motors contain two different stator windings: an auxiliary starter winding fed by an electric capacitor and a main run winding fed by the principal electrical circuit. The auxiliary starter winding is used to provide an extra boost (and proper direction) to get the motor spinning at start-up. (Three phase electric motors do not use start/run capacitors.)

Electric motors that use start/run capacitors may be PSC (permanent split capacitor) and CSR / CSCR (capacitor start, capacitor run) designs. Unlike a PSC motor, a CSR/CSCR motor must also have a starting relay that will cut the start capacitor out of the electrical circuit once the motor has gotten up to run speed. The run capacitor then remains in the circuit to keep the motor spinning properly.

What are Hard-start start/run capacitors or "booster" capacitors?

Photograph of  This new compressor was placed directly against
a brick wall. One third of its condenser coil cooling ability was blocked.If your air conditioner has stopped running, a possible problem is the failure of the starting capacitor found on the outside compressor/condenser unit. If that unit has electrical power but the compressor and/or its cooling fan are not running, one of the components to check (and that is easy to replace) is the starter.

When an electrical motor is having trouble starting, such as an air conditioning compressor motor, blower motor, a refrigerator motor or a freezer motor, or even a fan motor, the repair technician may install a simple and inexpensive hard-start capacitor.
See HARD STARTING COMPRESSOR MOTORS for complete details about choosing & wiring a hard-start capacitor.

The starting capacitor is a simple electrical device which can give an extra voltage jolt or "boost" to get the hard-starting motor spinning.

The starting capacitor in our page top photo is oval in cross section, but most replacement and many newer air conditioner motor starting capacitors are simply cylindrical in shape (photo at left).

Field report: describes diagnosing & replacing a bad starter capacitor on a heat pump that had stopped working

Start Capacitor Global (C) InspectApediaOur Heat Pump was new in 2002. It is a 3 ton unit ProSeries brand by Service Masters. We just had a big birthday party at our house for my 1 year old daughter. 95 degrees outside. House full of Aunts, Uncles, friends, Great Grandparents.

We got through that before the failure. Next morning it is just real gooey in our house. I can feel the blower fan is on. No cool air at all. I had to go to work so I left the house. Come back home in the mid afternoon to find house is hotter than before and my wife reports that the air hasn’t been cooling at all. It’s around 86-88 degrees in the house.

I check the little screw in fuse on the side of the blower unit. Hoping to find a $2 fuse blown. Nope. Fuse looks fine. Getting a little sense of panic because I remember how expensive our last service call cost for this heat pump. Plus it takes two days to get a technician to come out.

The key that I found on your web pages was the fact that the fan on the top of the condenser unit wasn’t running and the Compressor wasn’t running either. This pointed [possibly] to the start capacitor. In my system it is actually a pair of them, a dual capacitor unit. Aha! Finding the start capacitor took about twenty minutes. I work really slowly when I’m trying to not drop a handful of sheet metal screws in the grass [or worse into an electrical component where left in place a dangerous short circuit could occur].

I could see a little bit of bulging on the capacitor and some rust and grunge on the top.

I found a local HVAC Supply house in the phone book and read them the nomenclature from the side of the old Start Capacitor. The order clerk I spoke to was amazingly patient with me. About an hour and a half later I was plugging the new Start Capacitor in and flipping the circuit breaker back on. I put my hand on the coolant line and felt the copper getting cool fast. - James Oiler

Watch out: remember to turn off electrical power to any equipment before opening or working on it. Otherwise you could be electrocuted. Also, remember that an electrical capacitor can retain a large electrical charge even after electrical power has been turned off. You could still be shocked!


Air Conditioner Motor Starting Capacitor Safety warnings:

When testing a compressor, one must discharge the capacitor first! It'll otherwise have enough power stored on it to be at least very painful. (Author and others have been zapped!) Some systems will automatically discharge the capacitor, but shorting its leads [to ground] with a screwdriver (after verifying that the power's off) is a safe way to ensure that you won't get shocked. Motor starting capacitors can hold a charge for days!

If oil has leaked out of a capacitor: Don't touch any oil that leaked out: old capacitors may contain PCB oils, an extremely carcinogenic (cancer causing) material which require special disposal.

Once the capacitor has been discharged (as described just above), then it can be tested with a multi meter. Either use the meter's built in capacitor test function, or use this trick: Charge the capacitor by using the sense current the meter puts out when set to ohms. You should observe a rapidly rising resistance before the meter indicates over range/infinity. Disconnect the test leads, and switch over to volts. Then, reconnect the test leads. A voltage reading should be observed, approaching zero.

If the capacitor doesn't hold a charge, or the resistance reading never approaches infinity, it probably needs replacement.

Also, the capacitor may be defective if the compressor hums but does not start. Visual inspection may reveal it to be bulged, or have a blown out safety plug.

Examples & Sources of Start / Run Motor Capacitors

The following links to chapters in this motor capacitor article series have been retained for compatibility with cached article data.

Question: the capacitor is leaking oil

(June 15, 2015) Larry said:

I noticed my air conditioner's run capacitor is leaking oil - what does this mean?


The capacitor is shot and should be replaced.

Reader Question: replaced both start & run capacitors on AC compressor but compressor won't turn on

(June 15, 2015) Larry said:

Trane Xl1600 heat pump cools down to set thermostat setting and then will not turn compressor back on and blow hot air in the home. This seams to happen when Phoenix temperatures reach 100 degrees. Replaced both start and run capacitors a couple months ago as issue started last summer was working fine until 100 degrees. Any assistance with this would be greatly appreciated.

This question was posted originally at DIAGNOSTIC GUIDES A/C / HEAT PUMP


Larry if replacing the start/run caps didn't fix the trouble I suspect either a failing control board / relay or a compressor motor that is failing beyond the ability of the starting cap to kick it off. Perhaps your tech will check the current draw of the various motors involved to see if that's going on.


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