ele motor capacitors

Dose anyone know a low-tech way for me to test them?

A good quality DVM (Fluke, e.g.) testing for leakage.

There is an ESR meter kit available from someone in Australia.

You need a heavy rheostat and a multimeter. If possible, you also want an isolation transformer, control transformer, or filament transformer (any one will do, but the lower the secondary voltage the safer the following procedure will be).

Put the rheostat in series with the capacitor and connect them across the secondary of the transformer. Measure the voltage across the series pair (should be full secondary voltage). Now measure the voltage across the rheostat. Adjust the rheostat until the voltage is exactly half the voltage across the pair. Now disconnect power, remove the capacitor under test, and measure the resistance of the rheostat (ohm scale). This value will be the reactance (Xc) of the capacitor at 60 Hz.

Then apply the following formula:

Xc = 1/(2*pi*f*C)

transformed to solve for C, and with f set to 60 Hz like so:

C = 1/(377*Xc)

With Xc expressed in ohms, C will be expressed in Farads. Multiply the answer by 1,000,000 to get microfarads.

Of course if you have a digital meter that reads capacitance, you can skip all this and just use the meter to read capacitance directly. Even Radio Shack sells a meter that will do this.

Now if you just want to check and see if the capacitor has any life, charge it up with the ohmmeter then reverse the leads. If the needle "kicks" (analog meter), the capacitor still has some life. With experience, you can judge just about how much capacitance it has by how the needle kicks.

Also try reading it on a high ohms scale. It should charge up to nearly infinite ohms unless it has gone leaky (electrolytic start capacitors may only charge up to a couple hundred kilohms, but run capacitors should go all the way to infinite). If it jumps to zero ohms and stays there, it is shorted.

If you have an oscilloscope, you can test the breakdown voltage of the capacitor. Use a 100 megohm resistor in series with the capacitor, a transformer with a secondary voltage higher than the capacitor's rated breakdown voltage, and hook the oscilloscope across the capacitor. The point where the sinewave flattops is the breakdown voltage of the capacitor. (The high resistance protects the capacitor from damage.)

NOTE: this last procedure requires use of dangerous high voltages and a *floating* secondary (don't even *think* about doing this test unless you have the appropriate isolation transformer). Observe all appropriate safety precautions, and make sure your oscilloscope is rated for the voltage you're going to use for the test.

Gary

The way I was taught was to use an analog multimeter set on the ohms scale (don't remember which one). When applied to the terminals of a good capacitor, the meter needle should move quickly to the right, and slowly drop back to the left. If it doesn't, it's faulty. I've used this method for years, although (like I said) I don't remember which resistance scale I used.

Dave Young

Gary Coffman wrote: [how to measure capacitance]

Nice post - very informative. I've got it filed under Electrical/How To. Bob

Perhaps goes without saying-- but short the capacitor (preferably through a wirewound resistor, but a screwdriver is cheaper than a meter) for a bit first. Otherwise you could wreck your meter. Capacitors have something called "soakage" and will sort of re-charge themselves a bit if just shorted for a very brief instant.

Best regards, Spehro Pefhany

LOL. This is how I first learned to re-build d'Arsonval movements!

Hooked a charged capacitor up to my Lafayette Radio mulitimeter, when it was switched to ohms. Bent the pointer really well.

Jim

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Jim,

Do you mean straightening the needle or rewinding the deflector coil? :-)) Carl

Jim

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And, if you are real game ... move the meter through 90 degrees as in lay flat, then stand up, and rebalance the movement via sliding the little coils of wire (weights) around each of the cross arms. This is very close to #2 below, tinker with bearings, because there is a great chance of damaging the bearings when attempting balance.

Carl

Jim,

When I was doing ham radio in the early '60s, I ran afoul of a few meters and the d'Arsonval movements. After a while I learned not to tinker with them. They were very unforgiving.

The idea that you repaired the open in the coil leaves me in awe !

Nice job.

Carl

yes, that's a topper for sure. my best efforts include repair of a vacuum tube rectifier, a dryer timing motor, and a Victor O2 guage that my neighbor opened fully before cracking the bottle. this guy said he taught shop at the local high school. --Loren