Dayton dual motor run capacitor, # 6FLP0, 440C, 35/3 MFD, 4 1/8" oval capacitor (C)

Electric Motor Starting & Run Capacitor Types
Installation Guide to Electric Motor Start-Boost or Run Capacitors

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Types of electric motor start & run capacitors:

This article explains and gives an identification guide to types of electric motor capacitors: motor starting capacitor, motor run capacitor, dual-run capacitors, and hard start capacitors used on electric motors such as air conditioner & heat pump compressors, fan motors, some well pumps & some heating equipment motors. We explain and illustrate how to recognize different types of motor capacitors and how to determine the use or identity of the different connector terminals on the capacitor.

This air conditioning, heat pump, or other electric motor repair article series 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.

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Types of Electric Motor Capacitors including Air Conditioning Compressor or Other Electric Motor Starting Capacitors

Start Capacitor Global (C) InspectApediaHow to tell what type of capacitor is installed on your motor - difference between a starting capacitor, a run capacitor and a dual run capacitor

Capacitors are electric devices that, by storing and then releasing an electric charge. In many electric motors there are two types of capacitors present, a starting capacitor and a run capacitor.

Dual run capacitors support the operation of two motors from a single advice, such as an air conditioner compressor motor and a fan motor. Hard start capacitors give an extra large starting boost to hard-starting electric motors, possibly extending their service life.

[Click to enlarge any image]

For example, air conditioner and heat pump compressor motors (and lots of other electrical motors) that run on two-phase (220V) or single phase (120V) electrical power include a capacitor in the start circuit to help get the motor spinning and a capacitor can be put into the "run" circuit of the motor as well to increase motor efficiency.

Start Capacitors: Get a Motor Spinning

A starting capacitor such as the Global universal start capacitor shown at above-left has the single job of giving a large voltage boost to give extra turning force or torque to a motor to start it spinning in the right direction by providing about double that nominal system voltage.

A start capacitor 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.

Starting capacitors will be of much higher microfarad uf or MFD rating than run capacitors. As you may notice from the forest of connection points on the top of the round capacitor shown above, there may be multiple wiring connections depending on the capacitor's use and features - connections we explain in this article series.

The start capacitor rating or power in microfarads (uf or MFD) required depends on the size, voltage, and other properties of the motor that it's starting. Typical ranges for electrical motors likely to be found in a home (air conditioning compressor motors, fans, pumps) are from around 150 to 480 mfd and at voltages of 120-240-440. Prices for a start capacitor range widely from under $10. U.S. to over $100.

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. You may hear a compressor or fan motor humming or observe that it's getting hot.

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.

Run Capacitors are Used to Improve Electric Motor Power & Efficiency

Motor run capacitor by Dayton - oval, 2 terminal (C) InspectApedia

Run capacitor: once the motor has started, 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.

The oval run capacitor sold5 by Dayton (made in China) and shown at left is the simplest two-terminal run capacitor you'll encounter and is sold in a range of mfd ratings, voltage ranges, and in oval and round shapes.

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]

Technical notes: 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 - ELECTRIC MOTOR RUN DIRECTION) to get the motor spinning at start-up. (Three phase electric motors do not use start/run capacitors.)

Electric motors that use start/run capacitors may be PSC (permanent split capacitor) and CSR / CSCR (capacitor start, capacitor run) designs. Unlike a PSC motor, a CSR/CSCR motor must also have a starting relay that will cut the start capacitor out of the electrical circuit once the motor has gotten up to run speed. The run capacitor then remains in the circuit to keep the motor spinning properly.

Dual run capacitors operate two different electric motors at once

Dayton dual motor run capacitor, # 6FLP0, 440C, 35/3 MFD, 4 1/8" oval capacitor (C) Dayton dual motor run capacitor, round (C) InspectApediaA dual run capacitor for electric motors combines two different microfarad-rated (MFD/uf) capacitors in a single housing or container such as the Dayton capacitor examples shown at left, and provides 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).

To spot a dual run capacitor that operates both the compressor and the fan motor, look for three terminals on the capacitor: one will be marked "common" or may be marked with a red dot to indicate that role.

A dual run capacitor will always have [at least] three wiring terminals - as you can see on both examples above, while a simple single-use "run" capacitor will have only two terminals providing a single mfd capacity.

In contrast with a dual run capacitor that serves two motors, single-run capacitor serves just one motor.


Which terminal is which on the dual run capacitor?

On a dual run capacitor the terminals will be marked "C", "HERM" and "FAN". Check the number of connecting clips at each of the three terminals. Usually you'll find that

How to read the rating data on a dual run electric motor capacitor

The rating on a dual run cap will include two MFD ratings and a voltage range, such as 45/5/440 expressed in numbers separated by a slash and interpreted as follows:

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.

Hard-start start capacitors or "booster" capacitors?

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

We give capacitor wiring details at HOW TO WIRE up a START CAPACITOR

These dual run caps are available at Grainger ( as well as electrical and HVACR suppliers. Dayton Electric Manufacturing company, 14441 W Il Route 60, Lake Forest, IL 60045 (847) 535-1000, produces electric motors, fans, pumps, electric capacitors, audio capacitors, tools, even hand trucks.

Examples & Sources of Start / Run Motor Capacitors

Article Series Contents


Continue reading at LOCATE the STARTING CAPACITOR or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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