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This chapter of "How to Inspect the Central Air Conditioning or Cooling System" discusses the use of air conditioning motor capacitors to get a hard-starting air conditioner, refrigerator, or freezer compressor motor going. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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Air Conditioning Compressor Motor Capacitors

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 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). Where to Find the Starter Capacitor in 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.

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.
• 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 a Starting Capacitor or Booster for Hard Starting Air Conditioners Works

The starting capacitor, by "accumulating" a large electrical charge inside the capacitor. Traditionally capacitors were also called "accumulators" for this reason. The capacitor's electrical charge is released at motor start-up time, gives the compressor motor or other electrical motor a boost for starting.

This "boost" can be particularly needed if an air conditioner is suddenly switched off and back on when it has been operating. Suddenly switching off an air conditioning compressor leaves a high "head pressure" inside the compressor which can provide extra mechanical resistance when the motor is attempting to re-start.

How to Install an Air Conditioner Compressor, Blower Motor, or Fan Motor Starting Capacitor

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

Here are some sample capacitor installation instructions for adding a motor starter capacitor to an air conditioning compressor motor - taken from the product package for a relay and start capacitor intended for use on a refrigerator or freezer. Similar starter capacitors are available for air conditioning compressors.

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

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 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.

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AIR CONDITIONING & HEAT PUMP SYSTEMS
AGE of AIR CONDITIONERS & HEAT PUMPS
AIR CONDITIONER BTU CHART
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR CONDITIONER NOT WORKING
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER UNIT
BACKUP HEAT for HEAT PUMPS
BLOWER DOORS & AIR INFILTRATION
BOOKSTORE - Air Conditioning "How To" Books
CLEANING & Legionella BACTERIA
COMPRESSOR & CONDENSING COIL
  BURNED-OUT COMPRESSOR
  CAPACITORS for HARD STARTING MOTORS
  CONDENSING COIL FINS, SUPPORT PADS
  HARD STARTING COMPRESSOR MOTORS
  INSTALLATION ERRORS, COMPRESSORS
  MOTOR OVERLOAD RESET SWITCH
  NOISY COMPRESSOR UNITS
  PRESSURE READINGS, COMPRESSOR
  REPLACING A COMPRESSOR
  TIGHT or SEIZED AC COMPRESSORS
CONDENSATE HANDLING
CONTROLS & SWITCHES
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
COOLING COIL or EVAPORATOR COIL
DATA TAGS on AIR CONDITIONERS
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DUCT SYSTEMS
DUCTS - Asbestos
DUCT SYSTEM DEFECTS
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCTS, Asbestos Transite Pipe
DUST FROM HVAC?
ENERGY SAVINGS in BUILDINGS
EVAPORATIVE COOLING SYSTEMS
FAN AUTO ON Thermostat Switch
HEAT LOSS (or GAIN) in BUILDINGS
HUMIDITY LEVEL TARGET
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS
LOST COOLING CAPACITY
  What to check first
  Compressor failure diagnosis
  Duct & Air Handler diagnosis
MOTOR OVERLOAD RESET SWITCH
OPERATING COST
OPERATING DEFECTS
OPERATING TEMPERATURES
  Air Conditioning System Temperatures
  Instruments Used to Measure A/C Temperatures
  Procedures for Making Temperature Measurements
REPAIR GUIDE for AIR CONDITIONERS
REPAIR & DIAGNOSTIC FAQs for A/C
REFRIGERANTS
SEER RATINGS & OTHER DEFINITIONS
SYSTEM OPERATION
SWAMP COOLERS
THERMOSTATS
THERMOSTATIC EXPANSION VALVES
CRITICAL DEFECTS
Air Conditioning "How To" Books

• Thanks to Sealed Unit Parts Company, Allenwood, New Jersey, 08720, for the compressor starting capacitor and packaging information (purchased by the author from an air conditioning parts supplier in New York) - our example uses a Sealed Unit Parts Company Solid State part No. RSC 10 115V starting capacitor which was designed for installation on refrigerators and freezers. See www.supco.com/
• George Fazio, reader, contributed comments on failed starter capacitor diagnosis by noting the bulged capacitor ends. 09/25/2009
• Troubleshooting Compressor Problems," Henry Puzio, Fuel Oil & Oil Heat with Air Conditioning Magazine, June 1993, p. 39
  Tom Morris, Engineer, capacitor discussion and correction to the original data. Email to D Friedman 5/29/2006 - Thanks Tom for critical editing. The text above explaining about capacitors was suggested by Mr. Morris. The original text of the 1993 compressor diagnosis article had the resistance explanation backwards.

Air Conditioning How-To Books on Air Conditioning Inspection, Diagnosis, & Repair

• Books, Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.
• Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
• Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
• "Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
• Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
• ALUMINUM WIRING HAZARDS - aluminum electrical wiring often burns-up at high-amperage electrical connections such as air conditioner wiring.
• Federal Pacific Electric (FPE) FPE Stab-Lok Circuit Breakers can increase the risk of a fire if these breakers are used on air conditioning or heat pump equipment
• Zinsco Electrical Circuit Breakerss: overheating, failure to trip, burn-ups involving Zinsco and certain Sylvania electrical panel components. These components can also increase the risk of a fire if these breakers are used on air conditioning or heat pump equipment
• ...

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