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Old 24-06-2018, 13:50   #1
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Generator Capacitor

So some may have read another thread about my generator voltage, but I hope for a more focused crowd response.

My generator is putting out too low a voltage and things are pointing to the capacitor. In a couple of days I'm going to be in a town that has a chance of having a replacement capacitor so I will be "getting into" it them.

My question is how many have replaced there generator capacitor with a different rated one to increase the voltage. If you have what did you change from and to (my is spec'd for 31uF 450V), what was the voltage before and after, how long has you generator ran since the change?

I ask as there are mixed search results that say it is ok to increase the uF size to increase voltage (but a lower value than spec'd will give low voltage).

I want my voltage "right" but not at the expense of burning out the generator.
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Old 24-06-2018, 14:31   #2
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Generator Capacitor

I know I answered before, but I went from a printed on the cap value of 35 to a printed on the cap value of 40, voltage went up I think about 10, maybe as much as 15. I was running low 100ís as in single digits, and am now running right at 120.
Since your last thread had me discovering that my Fluke 115 could measure UF value, which I didnít know, I measured the original cap that I replaced and had printed on it 35 was really right at 30. Itís now in the trash.
I assume as it got old, it lost its value and of course finally got so bad I needed to do something. Maybe your is the same? Maybe take you old one with you, they may can measure it? Maybe you just need a new one, not change value? I can tell you though that my air conditioner and other electric motors seem to ďlikeĒ the higher voltage and seem to run cooler and the compressors are quieter.
I have not measured the one in the generator, but itís supposed to be a 40. Maybe if I had just put in a new 35, I would be running at 110V?
I can confirm that on my generator that increasing UF value increased voltage. If I were you I think I would get a 31 and a 35. Iíd try the 31 first.
On my generator itís real easy to change a cap. Mine did not seem to hold a charge, but of course be careful.

My generator according to Nexgen calls for a 40, but mine had a 35, leaving me to assume the actual generator manufacturer bench tests the things and maybe will ďtrimĒ each one by changing UF value a little. I bet mine with a real 35 runs just a bit low, and with a 40, just a bit high.

Itís been a couple of years and I think about 300 hours. I believe if you stay around the 110 to 120 ish V range, your in spec.
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Old 24-06-2018, 14:34   #3
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Generator Capacitor

Many caps are meant for HVAC use and will come as 30+5 or 40+5 etc. that extra cap, the +5 is meant I believe for the fan on a HVAC unit and I believe you can just ignore it. If your going to a real HVAC store etc., they would know for sure.
It may be a little harder to get just a 31 or 35 single motor run cap.


On edit, I know he is looking for people other than me who has changed capacitance on their cap regulated generators, so any that have please speak up. Someone else had a couple of years ago when I posted about it.
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Old 24-06-2018, 17:44   #4
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Re: Generator Capacitor

I can measure the cap of the installed one. The purpose of the thread is to gather info because I want to fix the issue on the first try. If that means I buy a couple of captors just to throw them away that’s fine as it’s cheap compared to my time and costs of sitting around. I think my problem goes back 20 months, but I don’t recall ever seeing my generator make120V.
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Old 24-06-2018, 17:54   #5
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Re: Generator Capacitor

Be aware there is wide variation of tolerances in capacitor values. Many are + / - 20%.
Look for precision caps if you can; better than say %5.

Or take your meter with you when you buy.
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Old 24-06-2018, 17:55   #6
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Re: Generator Capacitor

Iíd just guess that target voltage is 115 with plus or minus 5 being good.
Mine will run a little higher not under a load and drop a little as you put a load on it, the Governor isnít perfect, it slows a little. I have forgotten how much, but its at least 1HZ. I think I run about 61 Hz unloaded and 59 loaded. I have adjusted that too, as it broke in, it slowed down some, or maybe the Governor springs just got a little weaker with age? Voltage does follow Hz, it slows down and drops voltage.
Have you measured your cap? if itís good then Iíd suggest you go up the next size, whatever that is and buy one size larger than that too.
I got lucky and a 40 worked, but I suspect that a good 35 was acceptable too, just mine went bad. I got mine on Amazon.
I have done a little reading and study in HVAC systems as Iím licensed and in that world OEM caps are often poor quality and donít last long, get a good GE cap if you can, or I have heard a Supco isnít bad either.
Iíd take my meter with me and measure the new one too.
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Old 24-06-2018, 22:01   #7
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Re: Generator Capacitor

My generator has several leads for the capacitor. Each lead is for a different voltage. Do you have the generator manual? It should tell you what capacitor and how to connect it to get different voltages.
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Old 27-06-2018, 16:38   #8
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Re: Generator Capacitor

I haven’t checked my capacitor yet because I’m still not somewhere I can get a replacement so touching it is asking for trouble. I must not be doing a good Amazon search as I expected a bunch of choices, but only seeing 2 for 31.5uF 450v ones.

But a quick math check shows that 31.5 @ +5% labeled and a 35 labeled @ -5% are almost the same.
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Old 27-06-2018, 17:16   #9
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Generator Capacitor

I see many 30 and 35, seems 31 or 31.5 to be an unusual spec.

I think unless your going to be somewhere for long enough to be Amazon delivery, a good HVAC supply house is your best bet. These things are motor run capacitors and are very common in HVAC systems, used usually for the compressor and often the condenser fan.
If you end up with one with three posts and called say a 30 +5. The three posts should be labeled herm, C, and fan.
The one labeled fan, ignore, itís for the fan motor on a HVAC system, the C and herm connect to the compressor and are the ones you want to connect to. A cap with a +5 or +3 or whatever is actually two caps in one tube, but as that is what is usually called for, it may not be easy to find a single cap, you may have to settle with a dual cap and ignore the fan connector.
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Old 27-06-2018, 18:22   #10
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Generator Capacitor

This may be if some interest to you. I was curious as to what the voltage at the outlet inside of a home in the US is supposed to be, cause we call it 110,115, and 120 interchangeably.
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Old 27-06-2018, 18:42   #11
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Re: Generator Capacitor

Yeah but my generator is going below 95 and that causes my battery charger to cycle
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Old 27-06-2018, 22:03   #12
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Re: Generator Capacitor

Try putting a resistive load such as a water heater or toaster on when the charger is running. Does the voltage go up or down?
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Old 28-06-2018, 00:35   #13
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Re: Generator Capacitor

Because capacitors are pure capacitive loads, capacitive currents raise the voltage at the capacitor terminals (the so-called "lift" phenomenon). So the rated voltage of the capacitor is higher than the voltage of the system.


In practice, when the current increases, the prime mover will increase the input energy, and the generator excitation current will continue to increase, keeping the voltage unchanged, when the prime mover exceeds the maximum power, or when the excitation current reaches the maximum, The current can no longer be increased, otherwise the voltage will decrease, the frequency will drop, and the serious will have to load or shut down, because the generator does not meet the load of electricity, is not allowed. That will lead to abnormal measuring instruments and electrical equipment, serious possible accidents, so the state has regulations, power plants must be in accordance with the national standard of power generation.
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Old 28-06-2018, 04:04   #14
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Re: Generator Capacitor

Quote:
Originally Posted by transmitterdan View Post
Try putting a resistive load such as a water heater or toaster on when the charger is running. Does the voltage go up or down?
Voltage goes up with just a resistance load. I just looked at the spreadsheet of my readings again and I think the voltage is running lower than when I took them a few weeks ago.
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Old 28-06-2018, 10:17   #15
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Re: Generator Capacitor

Quote:
Originally Posted by sailorboy1 View Post
Voltage goes up with just a resistance load. I just looked at the spreadsheet of my readings again and I think the voltage is running lower than when I took them a few weeks ago.
Then most likely the capacitor you have now is drifting down in capacitance. You should try to find the exact type as original and replace the one that is failing apparently rapidly.

There are 2 things that murder capacitors in generators. The first is heat, the second is voltage. The voltage on the capacitor is not the same as the output voltage of the generator. It is much higher than that. Be sure you get a capacitor with the same or higher voltage and temperature rating.

It is ok to mount the capacitor somewhere outside the generator as long as you make sure no one can touch the terminals and that the capacitor case is grounded. It may help longevity to put the capacitor in a cooler place.
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