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

  • TEST a MOTOR START or RUN CAPACITOR - CONTENTS: description of electric motor capacitor test procedures to determine if a capacitor is damaged or working normally & test procedures to measure the capacitor's capacitance or microfarads, MFD, or uF to determine if it is working within its rated capacitance range.
  • POST a QUESTION or READ FAQs about installing a hard-start capacitor to get an air conditioner motor, fan motor, or other electric motor running.
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How to test an electric motor capacitor: this article gives a description of electric motor capacitor test procedures to determine if a capacitor is damaged or working normally & test procedures to measure the capacitor's capacitance or microfarads, MFD, or uF to determine if it is working within its rated capacitance range.

This article series gives electric motor start-run capacitor and hard-start capacitor installation instructions 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.

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Start or Run Capacitor Diagnostic Checks: How to Use a VOM or Multimeter to Test a Motor Starting Capacitor

Wiring diagram air conditioner (C) D FriedmanIf an electric motor that uses a starting or run capacitor won't run try replacing the capacitor. If the electric motor runs, check the current draw (AMPS) on both of the capacitor leads. You should see current draw on both leads. If not the capacitor is probably failed. Here are details:

There are two quick indicators of a bad electric motor start capacitor:

  1. The motor will draw higher than normal current when it is trying to start, having trouble starting, refusing to start at all, perhaps humming and overheating, and perhaps tripping an internal thermal overload switch or tripping the circuit breaker or blowing a fuse.

    [Click to enlarge any image or table]

    These symptoms may come and go depending on the operating state of the A/C or heat pump system: when the system is operating under load (high refrigerant pressures are present) the compressor motor will have a harder time starting than when the system has been turned off for some period and refrigerant pressures are equalized.

    Intermittent compressor hard-starting: A bad starter capacitor can cause the compressor to fail to start sometimes while other times it seems to start and run OK.

    Compressor hard starting, fan runs OK: If the compressor is having trouble starting or won't start at all but the condenser fan runs just fine, we figure it's a bad start capacitor or possibly a bad run capacitor.

    Compressor and condenser fan both won't start: we suspect a bad capacitor that is designated as a combination start/run capacitor unit wired to both devices.

    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.
  2. A quick test of the starter capacitor itself can indicate that it is faulty as we detail here.

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.

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.

Difference Between a Starting Capacitor, a Run Capacitor, and a Dual Run Capacitor

A starting capacitor has the single job of giving a very large voltage boost to a motor to start it spinning. It does not keep at the job once the motor is operating. The rating on a starting capacitor will include a high MFD number and the operating voltage range. Temperature ranges and other data may also be provided on all caps.

A run capacitor has the job of keeping an electric motor spinning. The rating on a run cap will include the MFD rating and voltage range.

Dual run capacitors combine two different capacitor ratings and provide run support for two different motors. In a common air conditioning application these would be the compressor (marked Herm), and the compressor/condenser unit fan (marked F or Fan).

The rating on a dual run cap will include two MFD ratings and a voltage range, such as 45/5/440 which translates as

How to Measure the Capacitance of a Run Capacitor or a Dual Run Capacitor

Dayton dual motor run capacitor, round (C) InspectApediaA standard digital VOM or multimeter that includes a MFD (microfarad) option is set (on its dial or selector) to MFD and with the capacitor disconnected from any other wiring the VOM probes are touched to two terminals on the capacitor.

If the uf/mfd reading on the meter is close to the rating stamped on the capacitor label then the device is in normal condition. For example on a 45MFD (or uf) capacitor your meter should read close to 45.

Watch out: do not attempt to touch or measure equipment with power on and wires in place. You could be injured or killed or could cause a fire or explosion. Watch out that capacitors can store energy and deliver a shock even after power has been turned off.

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


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:


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

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

A/C motor resistances (C) D FriedmanMost electrical problems in air conditioning systems are in the compressors and their relays or motor overload switches.

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

[click to enlarge the image at left]

A fractional horsepower electrical motor should show different electrical resistance between 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 labeled so that the three leads on a start/run capacitor can be wired correctly:

See How to INSTALL & WIRE UP 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).

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