Capacitor question 1

[innov]

How does one know if a capacitor is bad on a 1/2 hp 'permanent split capacitor' motor?
I had a motor that stopped running; so I checked out the capacitor (7.5 uf) with an old style c-checker (tested fine), but doesn't drive the motor. I then installed a slightly larger capacitor (11 uf), which seemed to do fine....amp draw is less than max....wondering what's going on here?

Will the larger capacitor harm the motor?
Do capacitors go weak?
Is there some other way to test them out?

 

[waross]

Will the larger capacitor harm the motor
Probably not if the current is not excessive. There is such a wide tolerance on capacitors that the motor has to work with a wide range of values. The current is a good indicator.
Yes capacitors go bad.
You can calculate the impedance of the capacitor at 60Hz. and energize it with 120 volts and measure the current. Electrolytic capacitors occasionally explode. Not often but the risk with a suspect capacitor is much higher. Don't test the capacitor with your face over it as is often the practice on the bench. Have something solid between you and the capacitor when energising it on the bench.

 

[jraef]

Slight possibility of a failing contact on the speed switch as well, i.e. high resistance that is being overcome for the time being by the higher cap voltage. The usual failure mode of the speed switch is a failure to close at rest, so the start cap is out of the circuit and the motor can't start. But if the contacts are burned or full of crud they can close, but resist the cap energy, giving the same effect.

Amen to that waross!
I had a cap explode on me when I was a rookie, wasn't even aware that could happen. We had a Plexiglas panel over the test area making it necessary to reach around with the leads. It was set up by one of the veterans who had since retired, I thought it was a pain in the rear and planned on removing it, assuming it was installed for some kind of archaic devices that we no longer worked with. Boy was I wrong! Probably saved my eyesight, the Plexiglas was covered with hot splatter. I hung it up on the wall behind the test area as a reminder to future generations, with a sign that said "Think it won't happen to you? Think again!"

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[motorsdirect]

Hi Jraef
Just to point out that this is a permanent cap type motor that is being talked about..so a centrifugal type switch would not be a factor on this type of motor.
regards
Jeff

http://www.motors-direct.co.uk

 

[itsmoked]

To answer specifically one of your questions innov; Yes, capacitors of that type can "go weak". This is because they are chemical devices that can start to change - messing with the capacitance. Once they start "going weak" they are not far from total failure and should certainly be replaced!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[innov]

ok then, I suppose this all should answer my question; thank you

does anyone know of any 3 phase motors that I could drop right in there as a replacement? it's a 48YZ frame size 230 volts 1/2 hp.I tried Grainger and other local shops and the smallest they had in 3ph was a 56YZ frame, so I'd have to bore out the blower wheel collar and, among other things, rebalance the wheel, which is out of the question.

 

[waross]

Try the Baldor motor web-site. My Baldor CD lists one 48YZ at 1140 RPM. CFM 3036A
There are quite a few 48 frame size motors.
Yours

 

[machmech]

Hi Folks

I Googled capacitor test and came up with a generator manufacturing outfit (U.S.) that actually advises testing a cap by charging w/large battery and touching wires together for spark to confirm good...I hope they check in here and revise their site.

Keith, Are you stating that even though a cap reads good on meter its potential is less then good, or weak. Or will a good reading on meter always confirm good cap?

Chuck

 

[jraef]

Just to point out that this is a permanent cap type motor that is being talked about

D'oh!

 

[itsmoked]

Hi machmech; Specifically once a capacitor starts reading "less than it used to" (like 5% or more) it should be considered end-of-life. If you don't know what value it was, a common state of affairs, then I would suggest calling it end-of-life once it has dropped 10% below its listed value. Besides the less the capacitance the less the starting torque.
Others here may have a tighter or looser tolerance. Depending on the actual application I might just say the heck with it and run it until failure... But a lot of folks are, (understandably), more conservative or their application less forrgiving and wouldn't want to do this.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[machmech]

Hi Keith

Thanks for this information, It just so happens we had a two speed fan go on the fritz it would run at very low speed until hot, I performed a very quick check...(was getting dangerously close to Friday night go home) and found the switch and cap checked out good.

One winding had a high 98 ohms reading and no shorts to ground, I will dissect it when the time comes to replace motor. My unprofessional guess would be the winding.

Chuck

 

[waross]

Hi jraef and itsmoked;
Not to be argumentative, but do you fellows trust a multi-meter type capacitor tester for reliable checking of motor starting and split capacitor electrolytics?
I prefer applying 120 volts and checking the current, but it rarely comes to that. Usually the capacitor is obviously dead, and often the first test is to replace the cpacitor. If the motor runs, scrap the old cap.
respectfully

 

[itsmoked]

Yeah those Friday problems can make some people break out in hives. I fortunately, like this stuff more than 'just' about anything else and start getting the shakes if I haven't considered something from a technical point of view within 2 or 3 days. I take tech journals on trips to Hawaii.

walross (hehe you caught that other response?)

We wouldn't want any argumentative behavior around here no...

Kind of a toss up. The meters are supposed to read the actual capacitance. If it is changed when reading it with a meter then the cap is drying out and will be failing soon.(In fact it is already failing in providing its original capacitance)

Your 120V test may show you that it is failing (right this moment.. bang!) and can also show that, as they say, "it's still a capacitor" but unless you have some form of calibration it is only qualitative. (which may be enough)

If you don't have a spare cap to swap in then the 'measure its capacitance with a meter and compare it to what it sez on the side' may be your only option. BTW I find the DMM and bench meter capacitance meters to be surprisingly accurate.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[imok2]

Try this web page

http://www.xpedio.carrier.com/idc/groups/public/documents/techlit/570-968.pdf?SMSESSION=NO