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This air conditioning repair article discusses the selection, installation, testing, & use of electric motor starter start and run capacitors 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. If you don't see information you want, ask us for it using the comments box on this page.

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Air Conditioning Compressor or Electric Motor Starting Capacitors

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.

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, by creating a high-torque, rotating, electrical field in the motor. In many of these 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). These two capacitors may, however, be hidden within a single physical device, as we'll illustrate below.

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

Air conditioner compressor motors (and lots of other electrical motors) 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.

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.

Once the motor has started, in some designs 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]

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

If your air conditioner has stopped running (see LOST COOLING CAPACITY) , 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 (see HARD STARTING COMPRESSOR MOTORS), blower motor, a refrigerator motor or a freezer motor, or even a fan motor, the repair technician may install a simple and inexpensive starting 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).

How to tell what type of capacitor is installed on your motor: start capacitor vs. start/run capacitor

To spot a Start/Run capacitor that combines both functions, look for three terminals on the capacitor: one will be marked "common" or may be marked with a red dot to indicate that role. Note that especially among replacement starting capacitors, the replacement unit may be designed to provide either just the "start" function or both start and run function, depending on whether or not all of the wires on the capacitor are connected.

How & Where to Find or Locate the Starter Capacitor in or on Electrical Equipment

Look for starter capacitors in your air conditioning equipment in the outside air handler where a starter capacitor may be used to aid compressor motor start-up, and look for a smaller motor starting capacitor on fan motors, both in the outside compressor/condenser and indoors in the air handler unit's blower compartment.

Make These Simple A/C Compressor Checks Before Adding a Hard-Start Capacitor

Most electrical problems in air conditioning systems are in th compressors and their relays or motor overload switches.

In a single phase (common residential A/C) compressor you can verify with an ohmeter whether or not the A/C compressor is bad.

A fractional horsepower electrical motor should show different electrical resistance beween the three terminals (Start, Run, and Common) as we illustrate just below.

Find the two highest resistance terminals. The third one will be the common terminal. Our example is for a Frigidaire compressor motor.

In our capacitor testing and wiring sketch at left, you note we use the letters S, C, and R to identify the usual terminals to which a start/run capacitor is wired. On many systems these terminals may be labelled so that the three leads on a start/run capacitor can be wired correctly:

• S = start wire connector
• R = Run wire connector
• C = common connector

See How to Install and Wire Up an Air Conditioner Compressor, Blower Motor, or Fan Motor Hard Starting Capacitor.

Electric Motor run speed side note: Incidentally while most electric motors are marked with a data tag indicating the motor run speed (in RPMs) it's worth noting that the number of run coils is what determines the run speed of the motor. Two-coils marks a motor that runs at 3450 rpm (3600 rpm "nominal"), while 4 coils marks a 1725 rpm motor. (120V, 60 cycle/sec x 60 sec/min = 3600 rpm).

Causes of Hard-Starting Air Conditioners, Refrigerators, Freezers, and other Electric Motors

A compressor or other motor may have trouble starting for any of a variety of reasons such as:

• Low line voltage supplied to the equipment or excessive power drop on a long circuit wire
• Un-balanced cooling or refrigeration systems - such as a compressor motor that is having trouble starting when the refrigerant pressures are high on one side and low on the other side of the cooling system. This hard starting condition happens when a compressor is turned off in the middle of an "on" run cycle.
  
  You can guess that you have this condition on an air conditioner if the system starts just fine when it's been shut down for a half hour or more (refrigerant pressures are equalized on both sides of the compressor) but starts with difficulty (the compressor motor "hums" and does not start quickly or may even trip the circuit breaker) when trying to restart a few minutes after having been shut off.
• Old, aging compressor motors or other electrical motors that are at or near the end of their life may have trouble starting and may be able to function for some additional time given the "help" provided by a starting capacitor. See HARD STARTING COMPRESSOR MOTORS and also TIGHT or SEIZED AC COMPRESSORS for more details about old or failed compressor motors.
• Troubles with an electric motor such as used on an air conditioner air handler, condensing coil fan, or a well water pump: see ELECTRIC MOTOR OVERLOAD RESET SWITCH
  
• Bad or failed starting capacitor: the air conditioner compressor (and some other electric motors) may already have a starting capacitor installed, but the starting capacitor might have failed, causing the air conditioner compressor to start with difficulty or not at all. A bad starter capacitor can also disable the fan in the outdoor compressor/condenser, or the blower fan in the indoor air handler unit.
  
  We give more detail about failed electric motor starting capacitors, and we explain possible visual diagnosis of a failed starter capacitor (bulged ends) without having to perform electrical testing, at HARD STARTING COMPRESSOR MOTORS. Thanks to reader George Fazio for this air conditioner start-up diagnosis suggestion and for the photo (above left).

Starting capacitors or electric motor starting capacitors (or motor start boosters) are often present on large single phase air conditioning compressors, as found on home air conditioning units, or on occasion on blower motors or even fan motors. Electric motor starting capacitors are only very rarely present on small refrigeration compressors, such as those in refrigerators, and as far as we know, never present on 3-phase power systems.

How to Use a VOM or Multimeter to Test a Motor Starting Capacitor

Use a VOM in ohms setting to check resistance across the capacitor. If the meter does not move (no current flows) the capacitor is "open". If there is zero resistance the capacitor is shorted.

In the partial wiring diagram at left, the compressor (COMP) is at lower right, and the component labeled SC shows the position of the starting capacitor in the air conditioning equipment's wiring schematic. Here is the full wiring diagram for this portable air conditioner.

Watch out: while you might get lucky by finding that just replacing the starter capacitor fixes an air conditioner compressor, a fan motor, or another electrical motor, a hard-starting motor can be an indication that the more expensive A/C compressor or electrical motor is itself beginning to fail.

A service technician can test for a failing motor by measuring the current draw in amps during start and run, and by comparing the result to information on the motor's data tag.

General advice: Electrical Tests to Check HVAC Blower Fan Motor or Outdoor Compressor Fan Motor Winding on Heating or Cooling Equipment or on Other Electrical Motors

see USING DMMs VOMs SAFELY. Example: testing a blower fan motor winding: referring to the electrical diagram for your equipment, unplug electrical connectors at the fan motor. Measure the resistance between each lead wire with a multimeter or VOM. The multimeter should be set in the X1 range. For accuracy, don't measure when the fan motor is hot, allow it to cool off.

When the resistance between each lead wire are those listed in the specifications for your equipment the fan motor should be normal. Zero resistance or infinite resistance are indicators of a problem. More examples of checking wiring: see BURNED-OUT COMPRESSOR. see HARD STARTING COMPRESSOR MOTORS also TIGHT or SEIZED AC COMPRESSORS for more details about old or failed compressor motors. Also see Test a Motor Starting Capacitor

Details of How a Starting Capacitor or Booster for Hard Starting Air Conditioners Works

What makes the Start Capacitor Drop from the Circuit when the Motor has Started

As we explain next at How to Find, Choose, & Buy a Replacement Electric Motor Starting Capacitor, once a starting capacitor has provided the necessary boost to get the electric motor spinning, to avoid possible motor damage the capacitor has to drop from the electrical circuit, leaving the motor to run on normal operating current.

Centrifugual switches: a mechanical centrifigual switch may be used to disconnect the starting capacitor when the motor speed has reached a critical rpm, typically about half of the motor's run speed.[2]

PTC-devices: Positive Temperature Coefficient devices are the traditional means of dropping the capacitor from the motor circuit once the motor has successfully started. PTC devices are basically a tiny electric heater that use the change in electrical resistance of the heating element to open an electrical switch that removes the start winding in the motor from the run circuit. The switch opens in less than a second after the motor has started.

PTC devices have the advantage of being simple and avoiding the need for more complex electrical wiring of a motor starting system. Supco explains that this device is unable to sense whether or not the motor has successfully started, and if the motor does not start, several minutes are needed to let the heater cool down before the motor restart can be attempted again. This cool-down time provides a safety margin that helps protect against burning out the windings of a hard-starting motor.

PRD-devices: Potential Relay Devices use voltage sensing (The Supco method) or current sensing devices (two different approaches) to determine when to release the starting capacitor from the motor run circuit. Supco points out that "The electronic potential relay is inherently more reliable and precise than the older type mechanical potential relay." [1]

Both of these start capacitor control approaches work fine, and typical HVAC or residential appliance motor repairs the technician won't need to consider which method is being used to control the capacitor as long as she/he follows the manufacturer's recommendations on the product for its selection and use.

How to Find, Choose, & Buy a Replacement Electric Motor Starting Capacitor

The best option if you are replacing a starting capacitor or a start/run capacitor is to match the existing device on your system. You can substitute capacitors on a cooling system but the substitute capacitor must be able to handle the voltage. For example you can't sub a 110V-rated capacitor into a 220V system. Depending on the application, the micro-farad range of starting capacitors varies according to the motor size. Run capacitors typically range in micro-farads from 1.5 to 50uf. Start capacitors typically range from 20-30 uf up to 250-300 uf. The example capactor charts (left) are adapted from AFCAP. [2] You can also check a capacitor to compare its peformance with its microfarad rating by using an ohmeter. In a working capacitor ohms will build-up and then fall off (when the capacitor discharges). If you reverse the + and - leads of the DC ohmeter leads it will repeat. If you do not see any resistance in the capacitor then it has an internal short and it's shot - you need a new one. HVAC suppliers sell general-use starting capacitors that are intended for use across a range of electrical motors and motor sizes. But at least some industry sources (the Sealed Unit Parts Company or Supco) make a more careful argument explaining that it's best not to install a significantly oversized starting capacitor on an electric motor. According to Supco, [quoting] If the start capacitor is too large for the application, the capacitor can actually mask the developed voltage in the start windings and keep the start capacitor in the circuit continuously. .... The ... run-start voltage is suppressed below the trigger voltage of the start device. As a result, the start capacitor remains in the circuit as the motor runs. A secondary, fail-safe method is necessary to ensure that the start device is ultimately removed from the circuit. ... A start device that fails to remove the start capacitor from the circuit has the potential to cause premature failure of the start windings in the compressor. ... If the capacitor is never removed from the start windings, premature winding failure could occur. As such, care should be taken when selecting capacitor sizes for an application. Care should also be taken regarding products that tout a "bigger capacitor is better" approach to compressor starting. SUPCO E-Class devices provide a secondary timing safety device to ensure that the start capacitor is dropped from the circuit in a fail-safe mode. [Italics are ours-Ed.].

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

Our 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!

How to Install and Wire Up an Air Conditioner Compressor, Blower Motor, or Fan Motor Hard Starting Capacitor

The instructions that come with a starting capacitor are simple enough that they are often printed right on the capacitor itself.

Watch out: But be careful, failing to turn off electrical power, and failing to discharge a capacitor when working on electrical equipment can result in a nasty or even a fatal shock. See our motor starting capacitor safety warnings just below.

The particular starting capacitor to be purchased is matched to the horsepower range and voltage of the compressor or motor being repaired. Many motor starter capacitors to support a pretty wide range of motors. For example our sample capacitor was rated for use on 115V electric motors rated from 1/12 horsepower to 1/2 horsepower.

Prices for typical air conditioner compressor starter capacitors range from around $10. to $50. U.S.

Simple Relay and Hard Start Capacitor Instructions - Example 1

Simple Relay and Hard Start Capacitor Instructions - Example 2

1. Remove the power supply cord from the electrical outlet - in other words, be absolutely certain that electrical power has been turned off to the equipment being serviced.
2. Remove the old starting relay, leaving the old overload protection in place.
3. Push the wire with the one single pin terminal onto the "start" terminal of the air conditioning compressor. (See the wiring diagram above).
4. Push the other wire with the pin terminal onto the "run" terminal of the air conditioning compressor.
5. Connect the line from the old starting relay to the spade terminal on the "run" wire (insulating sleeve).
6. Restore electrical power

Start / Run Capacitor Mounting Positions

Start/run electric motor capacitors can be mounted in any direction or position. However there are some other capacitor mounting considerations that can affect capacitor life: basically you want to minimize the capacitor's exposure to vibration and heat.

As Afacp points out, ..."the temperature on the surface of the capacitor cannot exceed, even under the worst conditions, the maximum permitted temperature. It is advisable to make an experimental measurement of the temperature reached by the capacitor under the
working conditions in the final application and after the thermal equilibrium has been achieved."[2]

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 grounbd] 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.

Questions & Answers regarding this article

Questions & answers about installing a hard-start capacitor to get an air conditioner motor, fan motor, or other electric motor running.

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