Testing electric motor start capacitors

[Robert in Md.]

I have blown my rotophase and need to test the 4 capacitors. I googled it and they said to use a multimeter set on ohms. I am having no success with this method.
I kind of remember watching a repair shop put current across the terminals and then discharging the capacitor with a screwdriver. If there was a big arc the capacitor was OK. I;m not having any luck with this method.
Anyone know how to test these capacitors safely?

Robert in Md.

 

[John T]

Okay, heres the deal. To "really" test them accurately, you need a "real" capacitor tester that can do a leak test and give you the MFD value etc.

HOWEVER a body can do like myself and use my old reliable Simpson 260 Analog Ohm Meter which will show if its reallyyyyyyyyy bad but not necessarily if its good. I use what I call the "kickback test"

First (AFTER I dischaerge the capacitor) I set the meter on the R x 1 scale and put it across the leads and it better read an OPEN (near infinity) CIRCUIT and if not (i.e. it has continuity) its bad shorted out.

Second (AFTER discharging) I put the meter on the R X 10,000 scale and apply the leads AND THE METER SHOULD INITIALLY SWING OVER TO LOW RESISTANCE AS ITS CONDUCTING CURRENT AND CHARGING THE CAPACITOR, BUT THEN THE NEEDLE (as it becomes charged up) WILL (kickback) SWING BACK OVER TO AN OPEN CIRCUIT.

IFFFFFFF it passes BOTH tests you know its not real bad or dead shorted at least and it does take on current and charge up BUT THATS NO GUARANTEE AT HIGHER VOLTAGE AND ONCE WARMED UP IT CANT STILL FAIL. This test can show if its bad but NOT if its necessarily good

So thats my method, it will be interesting to reads of any others ABSENT a real capacitor tester.

John T

 

[36 Coupe]

I use subsitution to check motor caps.They can lose capacity.

 

[Dean]

That's how I do it, and I know of no other easy way without a capatance tester.

That said, has anyone tried to find a reasonably priced (or ANY) analog VOM lately?

Dean

 

[guido]

Hello Robert in MD,
The A/C and shorting is a good test
The volt meter is another test. Not having the capacitance test equipment, the best test is direct replacement. If you can't find an exact replacement, you can go safely put in a capacitor with up to 20% higher capacitance-
value
Guido.

 

[John T]

Mornin Counsellor, Years ago a body would trip over old analog meters in pawn shops but ya know I havent seen any in years. I"ve had the Simpson 260 and RCA WV 38A almost since high school in the early sixties and I"m just more comfy with them versus the modern digital that jumps all over at times. HOWEVER that kickback test is what I did mostly on smaller capacitors I dont think it acts quite the same on those big honkin can electrolytics but its been too long since I messed with this stuff

John T

 

[ink spot]

took mine out and went to the local electric supplier he had testor and checked it but didnt have replaement sent me to local motor shop he also checked it but not exact replacement sent me to water pump supplier he also tested it and had part all said was bad so i think there are a lot of places will test it for you just be sure you discharge before messing with it

 

[Mel in SW Iowa]

I also keep a good Simpson 260 for capacitor testing, but for accuracy, I have had good results with a Velleman DVM 6013 capacitance tester in the
10 - 50pF range. Its range is from .1pF - 20 mF.
I was surprised to find the accuracy, stability,
and repeatability that I have experienced so far
in a portable instrument that is so economically
priced. ($50-60 range)

 

[Dean]

For years I had a decent Radio Shack VOM but it ceased functioning a few years ago. and I've not found any obvious causes.

Since then I've been keeping my eyes open for a good Simpson but have not yet found one.

Dean

 

[36 Coupe]

The younger generation cant read the scales on an analog meter.I think Pete Rometti would have kicked my butt out the back door of his tv shop if I used an ohm meter to check a capacitor.

 

[DENNIS MIN]

John T,

That's the way Ive been taught sans a true cap checker.

D.

 

[barnE]

Charge it up and wait about 5 minutes. See if it holds charge. Spark test. Or charge with 12 volts and check with magnetic(old type)headphone. should be able to tell. No guarantee on capacitance. Dave

 

[JMOR]

With only a resistor, voltmeter, ammeter, ohmmeter, you can do a lot more testing than you imagine. Ohmmeter for shorts, ope, leakage. Connect in series with resistor & to power line and measure current & voltage, and calculate reactance & from that capacitance. What more do you need?

 

[Aaron Ford]

We have a dedicated cap checker at work, but my new Fluke has one onboard. Model 87V.

Aaron