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Old 26-07-2013, 19:27   #1
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What Does the Capacitor in a Generator Do?

i have in my hand a capacitor.
my generator stopped producing power and i was told this is most likely why.
what does it actually do for the generator?
is this what excites it?
does it affect power output?
should i upsize it?
just confused, as i never messed with an AC generator before.
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Old 26-07-2013, 19:40   #2
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Re: what does the capacitor in a generator do?

there was another symptom before it failed.
i would try to power up the air-conditioner,
it would load to 18 amps, and then would cut out.
at 3500 watts (i think) it should give me 40 amps for the 5 seconds it needs, or at least bog down trying... any ideas on that one?
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Old 26-07-2013, 20:02   #3
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Re: what does the capacitor in a generator do?

If it is a larger can-shaped capacitor of around 30-50uF, then it is used to regulate the excite circuit on the generator. The generator windings will be such that they are saturated around the nominal voltage and the excite circuit with the capacitor in it will keep it regulated around that nominal voltage as the loads vary.

Don't upsize the uF rating because it is matched to your exciter circuit. The voltage rating doesn't matter, and higher is generally better.

When these capacitors start to go bad, you will generally notice poor voltage regulation for a bit before they go completely. If you were only monitoring the amperage load and not the voltage output, you may have seen the the amperage spike up as the voltage sagged.

These run capacitors are extremely common and very inexpensive (I literally buy them on the street in Columbia, Panama and Guatemala for $10 USD), so it is cheap to just buy a new one and try it out to see if it fixes your problem. If so, keep 3-4 on hand as spares.

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Old 26-07-2013, 20:05   #4
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Re: what does the capacitor in a generator do?

yes, i was monitoring volts and amps.
i found mine for $10-15.
it struck me as odd that it would not even try to make the power, it would just unload.
i hope this fixes the issue, and thanks for the idea, i will buy at least two!
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Old 26-07-2013, 21:27   #5
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Re: what does the capacitor in a generator do?

If you have a capacitor measurement position on your AVO, measure the uF. If not take it to the place where you are buying get them to check it, if its similar to the rating stamped on the can, its OK, buy a new one anyway. Its just good practice to determine if it was faulty in the first place.

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Old 27-07-2013, 04:27   #6
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Re: what does the capacitor in a generator do?

the rating on the can was 15uf, that is what i ordered. if i would have not seen your reply colemj, i would have believed this, but then again, i don't think i would have...

Quote:
A capacitor is like a storage tank for electricity When
Power is needed such as surge watts to power up an ac unit
lol
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Old 27-07-2013, 05:16   #7
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Re: what does the capacitor in a generator do?

You can also check caps with a regular Volt/Ohm Meter. Just put it on the resistance scale and see if the cap charges and discharges. If it's a normal electrolytic capacitor, it should start charging.

Better explained here:

How to Test Aluminum Electrolytic Capacitors With a Multimeter | eHow
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Old 27-07-2013, 08:01   #8
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Re: What Does the Capacitor in a Generator Do?

The problem with using a multimeter to check run capacitors used in this application is that they often check out fine using the procedures mentioned, but fail to operate when used with high demands. I think of this as "weak", but there is probably a more technical term.

When the capacitor has reached this state, it will fail totally within a short time and the above tests will work correctly. But in the meantime, one could be chasing ghosts and doing surgery thinking the capacitor is OK, when it really isn't.

For $10, it is always better to first swap out the capacitor as a troubleshooting test.

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Old 27-07-2013, 08:06   #9
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Re: what does the capacitor in a generator do?

Quote:
Originally Posted by scoobert View Post
the rating on the can was 15uf, that is what i ordered. if i would have not seen your reply colemj, i would have believed this, but then again, i don't think i would have...
I don't understand what you are saying here. Capacitors are a form of energy storage. My reply meant to convey that - in your case storing energy in the excite circuit to be used when needed.

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Old 27-07-2013, 12:51   #10
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Re: What Does the Capacitor in a Generator Do?

Did changing the cap fix the problem?

Electrolytic capacitors are notoriously flaky, depending on their manufacture, so failures are common. You may just have had a weak component that died young. The replacement may last forever.

Upsizing the capacity (15uF) won't make it last longer.... upsizing the voltage rating is sometimes useful, but not always.

Best idea, is to just keep a few replacement capacitors on hand.
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Old 27-07-2013, 13:09   #11
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Re: what does the capacitor in a generator do?

Quote:
Originally Posted by colemj View Post
I don't understand what you are saying here. Capacitors are a form of energy storage. My reply meant to convey that - in your case storing energy in the excite circuit to be used when needed.

Mark
i was told that by someone else on youtube.
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Old 27-07-2013, 15:15   #12
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Re: What Does the Capacitor in a Generator Do?

Quote:
Originally Posted by colemj View Post
The problem with using a multimeter to check run capacitors used in this application is that they often check out fine using the procedures mentioned, but fail to operate when used with high demands. I think of this as "weak", but there is probably a more technical term.

When the capacitor has reached this state, it will fail totally within a short time and the above tests will work correctly. But in the meantime, one could be chasing ghosts and doing surgery thinking the capacitor is OK, when it really isn't.

For $10, it is always better to first swap out the capacitor as a troubleshooting test.

Mark
You can always "shotgun" all suspected caps. You can also look for a bulge ............ah, let me rephrase that. You can look to see if the top of the capacitor has pushed up somewhat. I have seen the phenomenon you speak of but it's rare.
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Old 27-07-2013, 15:22   #13
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Re: What Does the Capacitor in a Generator Do?

Quote:
Originally Posted by thomm225 View Post
You can always "shotgun" all suspected caps. You can also look for a bulge ............ah, let me rephrase that. You can look to see if the top of the capacitor has pushed up somewhat. I have seen the phenomena you speak of but it's rare.
Is this a gun thread now?

There is only one run capacitor in this system, so don't need to replace a bunch randomly - only one. I have two right now that test OK with a multimeter but won't run the full generator load. I haven't seen a bulging one yet. The best test is while it is running under a lower load, these caps get very warm before they fail completely. A good cap runs cool under all loads.

Scoobert, this hasn't been mentioned yet, but when working with these caps, short the terminals with a screwdriver before removing them - they can put you on your butt. Don't just pull the leads and grab the terminals - a partially working cap can still poke you.

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Old 27-07-2013, 15:43   #14
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Re: What Does the Capacitor in a Generator Do?

Quote:
Originally Posted by colemj View Post
Is this a gun thread now?

There is only one run capacitor in this system, so don't need to replace a bunch randomly - only one. I have two right now that test OK with a multimeter but won't run the full generator load. I haven't seen a bulging one yet. The best test is while it is running under a lower load, these caps get very warm before they fail completely. A good cap runs cool under all loads.

Scoobert, this hasn't been mentioned yet, but when working with these caps, short the terminals with a screwdriver before removing them - they can put you on your butt. Don't just pull the leads and grab the terminals - a partially working cap can still poke you.

Mark
Must be a big ass cap. I'm now worried about the Original Prankster....Scoobert.
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Old 27-07-2013, 15:58   #15
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Re: What Does the Capacitor in a Generator Do?

I always test the capacitor by sticking the wires to my tongue.
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