How to test a capacitor

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Tozguy

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This fan for our downdraft cooktop ventilator does not spin as quickly as it should and emits a weird noise.
About the only thing (in my extremely limited knowledge of electricity) that could be faulty is the run capacitor.
It is a 12,5 u. How can I test it to be sure that a new one will fix the problem?
Thanks in advance for your help.
IMG_0796.JPG
 
Before proceeding, disconnect the cap and short it out with a screwdriver to avoid potential shock.

A few things to check for:
1. Discoloration
2. Swollen case
3. Smell
4. Becomes hot during operation
5. Should not have low resistance. You can use an ohmmeter, but it may take a while to charge, and during that time, may appear "shorted" even if it's perfectly functional. Just connect the ohmmeter and wait a minute or two. The eventual resistance should read high, like greater than maybe 20,000 ohms.

Also, don't overlook the wires connected to it and the motor. Check for discoloration of the wires, which typically indicates a poor connection.

BTW, how do you know that's the run capacitor and not the starting capacitor? You can try disconnecting it and see what happens when you turn it on. If it doesn't start at all, that's the start cap and isn't the problem.
 
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To measure the value in microfarads you really need a tester of some kind- like this:
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I found one for around 6 $, they work well. Otherwise a simple test with an ohmmeter will simply tell you if the cap is shorted or open but not the value:

That test goes as follows: Connect ohmmeter to capacitor terminals. Reading should initially be low then move to infinity. Reverse test leads, same thing should happen. Each time you reverse leads you should see a swing from low ohms to very high. Easier to interpret results with an old-fashioned "needle" style meter. If cap is shorted reading will stay low. If cap is open reading will always be high.
-Mark
 
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Before going any further with the capacitor, I would check the bearings. These fans usually have oil impregnated bushings and the oil can dry up. A drop of oil on the bushings should eliminate the problem, I have rescued a number of fan motors in this way.
 
RJ is correct, it may not be the cap at all. Check for gunk and bearing problems in the motor first
 
If it isn't the motor...

If you have some resistors lying around you can do a quick test with a small DC power supply, a resistor and a volt meter. Disconnect the power and the leads to the capacitor. Be sure the capacitor is fully discharged. Charge it up to about 12V - 24V or so, connect the resistor/volt meter and you should see the charge bleed off quickly, then slowly to zero. I realize that many won't have resistors lying around but they are available online for cheap.

For a ~12uF cap, 200,000 Ohm resistor, the time constant is ~2.4 seconds. It may already have a resistor on it so either use that one if you can tell the value of it. In my example, the capacitor should be about 1/3 the starting voltage in 2.4 Seconds and about 0V in 12 seconds.

Also test it with no resistor. Touch the meter to the terminals briefly after charging and over a minute or so, maybe every 10 seconds. If it is discharging rapidly without a resistor then it probably has developed some internal resistance and should be replaced. The reason you don't want to keep the meter on constantly is that the meter has some load resistance itself. Probably not enough to matter in the case of 12uF but best to eliminate that possibility.

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Looks like we got it fellas.....but

I started with the test that tq mentioned in post 2. The multi tester set at the 200k ohm notch. The tester did not get off the initial reading of 1. as if the capacitor was open and stayed open.

While fiddling with the capacitor the fan fell off the shaft...what the ..... it turns out that the fan is held on the shaft by a collet that got loose somehow without my noticing. So while the fan was off the shaft I plugged in the motor just to see if it would start more easily without the fan. The capacitor just buzzed like crazy probably at 60 cycles and then another 'what the ....' came to mind.
IMG_0798.JPG

With the fan back on the shaft and TIGHT, I plugged the motor in again and BINGO, it runs, it runs quiet and at full speed, as if there is nothing wrong with the cap!
HOW COME?

My guess that it is a run capacitor was based on the wiring diagram.
IMG_0795.JPG

So now it looks like we won't have to spend $1800.00 on a new downdraft ventilator and I owe you all a beer for your help.

Fortunately we got it fixed without having to do the tongue test.
 
The fan blade probably acts like a flywheel somewhat so without it the motor acts confused and has trouble getting started- Just a guess
The cap test: if you used a digital meter the low ohm reading was probably so brief you didn't see it. Needle-type meters work better for this
-M
 
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