Ice Breaker: Testing Capacitors

October 4, 2010
/ Print / Reprints /
ShareMore
/ Text Size+

Many single-phase motors used on refrigeration systems will incorporate some type of capacitor. They will either use a start capacitor to aid in its starting or a run capacitor to improve its running efficiency. Some motors will even use both. Troubleshooting the operation of these motors requires a technician to know how to test both run and start capacitors.

The first step in testing capacitors is a visual inspection. A run capacitor that is malformed or bulged in one area, especially at the top, normally indicates some type of defect. Start capacitors typically have a membrane on top which will blow open when they overheat.

Capacitors can fail open, shorted, and on rare occasions weak (its microfarad value being less than its rated value). The best method of testing capacitors is with a capacitance meter. This meter will identify all three problems. Many multifunctional meters sold today will include this feature. When purchasing a new meter, make sure it has this useful function.

ANALOG OHMMETER

If a technician does not have a capacitance meter, an analog ohmmeter can be used. However, this meter will only show if the capacitor is open or shorted.

Below is a typical procedure used to test capacitors using an analog meter:

• Remove the capacitor from the circuit.

• Discharge capacitor with a bleed resistor or voltmeter.

• Set an ohmmeter to its lowest scale.

• Touch the probe ends together and zero the ohmmeter.

• Place the ohmmeter leads on the capacitor terminals.

• Watch for one of the following indications for the capacitor:

Good: Needle will swing toward zero and then slowly return to infinity;

Shorted: Needle will swing toward zero and remain there;

Open: Needle will stay at infinity.

• Make a second test by reversing the leads of the ohmmeter.

• If your meter indicates the capacitor is either shorted or open, retest the capacitor on a higher ohm’s scale to confirm your findings.

Run capacitors can also be tested for being grounded. Place one of the ohmmeter leads on a terminal of the capacitor and one lead on the body. If the ohmmeter reads a resistance, the capacitor is grounded and needs to be replaced. Be sure to test both terminals of the capacitor.

Figure 1. Sample circuit that can be configured to measure applied voltage and current draw of a capacitor.

ANOTHER METHOD

There is a method of measuring the capacitance value of a capacitor without the use of a capacitance meter. Once a technician has verified the capacitor is not opened or shorted, he can determine its capacitance by measuring its applied voltage and current draw. Once these values are known, the following formula can be used to determine its capacitance:

Capacitance (µf) = (Measured Amperage x 2,650) ÷ Measured Voltage

Figure 1 is a sample circuit which can be configured to measure the applied voltage and current draw of a capacitor. Before applying any voltage to a capacitor, always test it for any shorts.

If the capacitor is shorted, do not proceed with the test. Do not apply a voltage higher than the rating of the capacitor - if unknown, use 110 volts. Do not apply voltage to a start capacitor for more than three seconds. Also place the start capacitor in a containment box before energizing. In the event of a failure, a start capacitor can overheat and expel flaming material. When testing run capacitors, only apply voltage long enough to obtain required readings.

Publication date: 10/04/2010

Did you enjoy this article? Click here to subscribe to The NEWS Magazine

Recent Articles by Joe Marchese

You must login or register in order to post a comment.

Multimedia

Videos

Image Galleries

2014 MCAA Annual Convention

Scenes from the 2014 MCAA Annual Convention in Scottsdale, Ariz.

Podcasts

NEWSmakers: Julian Scadden

Training is an ongoing process. Julian will discuss how you can generate maximum return on time and energy invested training by following a three part process. Listen to this podcast to get expert tips on training, tracking and follow up. 

More Podcasts

ACHRNEWS

NEWS 04-21-14 cover

2014 April 21

Check out the weekly edition of The NEWS today!

Table Of Contents Subscribe

SERVICE CALLS POLL

Which statement on service calls best applies to your business?
View Results Poll Archive

HVACR INDUSTRY STORE

plumbing-hvac.gif
2014 National Plumbing & HVAC Estimator

Every plumbing and HVAC estimator can use the cost estimates in this practical manual!

More Products

Clear Seas Research

 

Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

DON'T MISS A THING

Magazine image
 
Register today for complete access to ACHRNews.com. Get full access to the latest features, Extra Edition, and more.

STAY CONNECTED

facebook icontwitter iconyoutube iconLinkedIn i con