InspectAPedia ^TM

Installation Guide to Air Conditioning Compressor Motor Start-Boost Capacitors AirCondAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• Use of air conditioner compressor starting capacitors
• Air conditioner compressor defect diagnosis

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. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

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 which can give an extra voltage jolt or "boost" to get the hard-starting motor spinning.

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 and also TIGHT or SEIZED AC COMPRESSORS for more details about old or failed compressor motors.

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).

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.

References and More Reading:
"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.
Federal Pacific Electric (FPE) Website Stab-Lok Circuit Breaker Panel Hazards Latent fire hazards
Zinsco Electrical Circuit Breaker Failures: overheating, failure to trip, burn-ups involving Zinsco and certain Sylvania electrical panel components

AIR CONDITIONING SYSTEMS Chapter Index

To continue reading this air conditioning inspection guide, use links to the document chapters at left or below. Links shown in green font indicate where you are in this document.

Technical Reviewers

• Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator.
• 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/
• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and for providing a copy of Carson Dunlop Weldon & Associates Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).

More Information on Building Diagnostic Inspections and Repairs

InspectAPedia ^TM Home & Site Map - Building Inspection, Diagnosis, & Repair, Environmental Inspection & Testing - Research Website

The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems

Home Inspection Construction Consulting Services & advice for home buyers

Use this simple, economical mold test kit by following our instructions on how to collect and mail mold samples to our lab

Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.

Contact Daniel Friedman for website content suggestions or for fee-paid consulting

	Enter your search termsSubmit search form
	Search This Website - InspectAPedia.com