Motor Run capacitor

[JIMMIE]

The HVAC guy told my wife the motor run capacitor on my AC unit needed changing because it measured 4 instead of 5. He didn't say 4 or 5 what, I'm assuming uF. Does this really need to be replaced?

Jimmie

[grumpy]

No that is small difference I don't think that will make any difference

[grumpy]

Jimmie you can reduce capacitor in value as long as motor start up but you can not increase, increase will cause start windings draw excessive currents and eventually possible burn out, I have found some motors may even run much cooler by reducing run cap. in value.

[hrhofmann]

Why was he looking at the AC? If it is not running, then he might be right. +/- 20% is about the maximum tolerance for thosse type of capacitors.

[The Daring Dufas]

A new capacitor that size is not that expensive and from the value of

5uf I have to assume it's the run capacitor for the blower or condenser fan motor. If the tech tries to charge you more than $20.00 for the cap which wholesales for $3 to $5, he may be trying to rip you off. The cap value dropping from 5uf to 4uf is quite a bit and could be an indication of a capacitor that's failing. I've had to replace a lot of defective capacitors for customers in the last decade because all the damn things are coming from China. If you decide to have it replaced, ask the tech to replace it with a higher voltage rated capacitor. If it rated at 370 volts AC ask the tech to install one rated at 440 volts AC.

TDD

[JIMMIE]

Ive never heard of anyone measuring the things before. While I am not officially an HVAC tech, I work on radar systems, I wind up doing a lot of the preventative maintenance on the ones at work and our tech orders dont call for checking caps for capacitance. What they do call for is measuring the start and run currents and recording them to show trends and verifying the right capacitor is being used. If the current is out of spec the first thing on the trouble chart is change the cap. Over 25 years Ive changed a lot of them but never because I measured the capacitance.. I know the environmental techs dont measure capacitance either, they do check leakage current along with all the other checks. I also know that even the best multimeters tend not to be that accurate at measuring capacitance and I doubt like heck he is carrying a capacitor analyzer around with him. Just didnt sound like a good reason to fork out $100.

Jimmie

[cjt]

That's really at the core of the issue -- HVAC techs can get away with charging $100 to spend a few seconds changing a $10 part, so they have a huge incentive to find "bad" ones.

[gregz]

I'll bet it was the same last season. What is the tolerance written on it. What temperature was is. They reduce when warm.

I say replace it. This might work if it's the same voltage rating. From granger.

Greg

[hrhofmann]

If the AC works ok, leave well enough alone. These periiodic checks are fine, but checking capacitance sounds like the tech was hunting for some way to maake some $$ because nothing else was wrong.

[krw]

20% tolerance (and more) is normal for such capacitors. There really isn't any need to make them better than that. If the motor runs, the capacitor is fine.

[clare]

On the other hand, if the capacitor IS borderline, and the AC fails to start on a very hot day next summer, good luck getting a tech to look at it and fix it in a timely manner. The time to fix it is before it quits, and when you do not need it.

[The Daring Dufas]

I check the darn things all the time because power surges and near lightning strikes pop a lot of capacitors in AC systems. For folks living in rural areas, I've been installing surge arresters on the furnace and outdoor condensing units because of not only capacitor damage but newer units have a lot of expensive circuit boards where the older systems relied on relay logic controls. The more conscientious HVAC techs use a capacitor tester on a regular basis because more damage is caused to very expensive motors and compressors when the Chinese caps go "POP!". Most of the HVAC techs who have a capacitor tester carry this one or a similar one:

My own test gear tends to be more elaborate thus more expensive since I also do circuit board repair work on a lot of different things. If you have electrical/electronic repair experience, you should be able to get your own 5uf capacitor at any motor rewind/repair shop, HVAC supply house or W.W. Grainger. I always replace them with the 440 volt rated capacitors.

TDD

[Jules Richardson]

So do motor capacitors typically fail in a way where it's only the actual capacitance that drops, rather than the ESR which climbs (the latter being what often causes problems* in electronic circuits)? The description of the meter in the link you posted doesn't seem to make any mention of ESR, so I assume it's not relevant in motor appications?

• the real issue being that the capacitance can still check out OK on a meter, but the high ESR will affect the circuit and cause it to fail.

cheers

Jules

[grumpy]

*************** Good Policy**********************

[Twayne]

No? A 20% change would make no difference? Whoosh!

[Twayne]

So instead of 4, it migiht only be 3.2? Now you're really scrapiing bottom.

[The Daring Dufas]

Motor run/start capacitors are an extremely inexpensive commodity and I tend to look at them like disposable batteries here lately because so many of the Chinese caps fail. It seems like every little hit from a voltage spike coming from the electrical service punches holes in the dielectric of the darn things. A simple test on every service call is the prudent thing to do because of the nature of the equipment. I have seen sources for very high quality American manufactured capacitors but they're very expensive. o_O

TDD

[Jules Richardson]

Sure, I don't doubt that they're under high stress and that testing of capacitance is wise - I just wondered if it was different to the electronics world, where high ESR normally warrants replacement long before a meter capable of measuring capacitance alone might show up any faults.

Maybe it's that high ESR just isn't an issue for motor start/run like it (usually) is for electronic circuits, so capacitance is the only thing there that techs really need to check.

cheers

Jules

[clare]

ESR, or eqeivalent series resistance, is a definite issue in electronics where the capacitor is part of a tuned circuit - particularly an R-C tank circuit.

Extra resistance throws all the timing out.

Not a big issue on a motor cap as far as I know

[gregz]

Last year I got some kind of surge/ spike that took out the suppressor I had on my compressor line. I installed a new one again. Ac is ok.

Greg