Starting capacitors in place on an air conditioning compressor (C) InspectAPedia.comElectric Motor Start / Run Capacitor Operation
Installation Guide to Air Conditioning Compressor Motor & Other Electric Motor Start-Boost or Run Capacitors

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

How does an electric motor capacitor work? This article gives a short simple explanation of how a motor starting capacitor or motor run capacitor actually works to get a motor spinning or to keep it spinning efficiently.

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 most common symptom is that the motor won't start.

Green links show where you are. © Copyright 2015, All Rights Reserved.

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

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.

Photograph of  This new compressor was placed directly against
a brick wall. One third of its condenser coil cooling ability was blocked.

[Click to enlarge any image]

The starting capacitor helps a motor start spinning by creating a high-torque, rotating, electrical field in the motor.

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. t.

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.

Air conditioning basic wiring circuit (C) D FriedmanOur little sketch of a basic refrigeration circuit shown at left illustrates where and how a starting capacitor fits into a refrigeration or air conditioning electrical circuit.

The starting capacitor works by "accumulating" a large electrical charge inside the capacitor. During compressor or other motor startup, the start capacitor releases its charge to give a voltage "boost" to get the electric motor spinning.

During an electric motor start-up (such as an air conditioner compressor motor and some fan motors) where a starting capacitor is included in the circuit, with the added charge stored in the capacitor, run-start and start-common voltages increase to a maximum value to start the motor spinning.

The total supply subsequently current drops back to normal run conditions when the start device is dropped from the circuit - the motor continues to run.

This "electrical starting booster charge" 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.

Traditionally electrical 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.

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.

Centrifugal switches: a mechanical centrifugal 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.

Start & Run & Dual Capacitors

  • Essex, Brown: "Motor Repair Supplies" (Catalog), Essex Group, Inc., 1601 Wall St., Fort Wayne, Indiana 46801, Tel: 219-461-4633, Website:, retrieved 6/20/14, original source: - see pp. 86-89.
  • Sealed Unit Parts Co., Inc., PO Box 21, 2230 Landmark Place, Allenwood NJ 08720, USA, Tel: 732-223-6644, Website:, Email: info@supcocom, Supco Catalog, retrieved 6/20/14, original source: - see pp. 2-6.
  • van Roon, Tony, "Capacitors", [online article], retrieved 6/20/14, original source:, gives a very detailed history of the invention and history of electrical capcitors beginning with van Musschenbroek's Leyden jar in 1745. This article includes
    "Capacitor Nomenclature" by Dean Huster.
  • Kaiser, Cletus J., The Capacitor Handbook: A Comprehensive Guide For Correct Component Selection In All Circuit Applications. Know What To Use When And Where, 2d Ed., [at] C.J. Publishing (2011), ISBN-10: 0962852538, ISBN-13: 978-0962852534 - product description
    This book provides practical guidance and application information when using capacitors in electronics and electrical circuit design. This easy-to-use book covers the following capacitor types: Ceramic, Plastic Film, Aluminum Electrolytic, Tantalum, Glass, Mica, and others. This book also has a very comprehensive Glossary and Index. The Selection Guidelines and the Symbols and Equations sections have the answers to all of your daily application questions. This book is one in a series of component handbooks.

Article Series Contents


Continue reading at TEST a MOTOR START or RUN CAPACITOR or select a topic from the More Reading links or topic ARTICLE INDEX shown below.


Suggested citation for this web page

HOW a STARTING CAPACITOR WORKS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

More Reading

Green link shows where you are in this article series.


OR use the Search Box found below at Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Publisher's Google+ Page by Daniel Friedman

Click to Show or Hide Citations & References

Support & See Fewer Advertisements

From Google's Contributor website: Contribute a few dollars each month. See fewer ads. The money you contribute helps fund the sites you visit.

Google-Contributor supports websites while reducing advertisements. You can support InspectApedia with a contribution of any amount you wish. Or you can contribute nothing and we'll still keep our website free to all readers - supported by advertising. Either approach is OK.