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Old 14-09-2016, 10:48   #1
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Capacitor uf Rating

I have a capacitor regulated generator, it's output when at 60hz is right at 110 VAC. (Nexgen 3.5 KW)
Ideally I'd like to see low 120's VAC. I believe I can change the uf rating of the capacitor and increase output voltage, I believe going up in uf will increase voltage?
Spec calls for a 40 uf cap, I will first replace it with another new 40 uf cap to see if that fixes the problem, but if it doesn't, what rated uf cap should I look for to increase the voltage appox 15 VAC?
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Old 14-09-2016, 11:32   #2
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Re: Capacitor uf Rating

a64, your problem may not be the capacitor but one of the diodes in the rectifier. If the capacitor change doesn't increase the voltage then check out the diodes. The capacitor, as I understand it, is just part of a rectifier to feed a dc voltage for excitation.

Did the generator ever put out 120 + vac?

Perhaps a call to Nexgen would get you more information and assistance.
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Old 14-09-2016, 15:41   #3
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Re: Capacitor uf Rating

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a64, your problem may not be the capacitor but one of the diodes in the rectifier. If the capacitor change doesn't increase the voltage then check out the diodes. The capacitor, as I understand it, is just part of a rectifier to feed a dc voltage for excitation.

Did the generator ever put out 120 + vac?

Perhaps a call to Nexgen would get you more information and assistance.

I don't know what it made voltage wise, I never checked or don't remember if I did, probably thought 110 was fine if I did, I hope it's not the diodes, cause I believe that requires disassembly, and I thought if a diode was bad, no current at all.
I did call Nexgen, his answer was to turn her up to 63.5 hz unloaded, right now I have it running at 60 hz but 110 V. I'd rather leave hz where it's supposed to be and increase voltage if possible.
I was thinking I could go to a 45 uf cap to increase voltage.


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Old 14-09-2016, 15:49   #4
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Re: Capacitor uf Rating

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Originally Posted by DeepFrz View Post
a64, your problem may not be the capacitor but one of the diodes in the rectifier. If the capacitor change doesn't increase the voltage then check out the diodes. The capacitor, as I understand it, is just part of a rectifier to feed a dc voltage for excitation.

Did the generator ever put out 120 + vac?

Perhaps a call to Nexgen would get you more information and assistance.
If he lost a diode, i doubt he'd have 110 and still be operating.....depending on the type rectifier.

Do you have a schematic of the generator by chance?

if I remember correctly on a rectifier like a full wave, the cap is there just to smooth out the ripple.

As shown here:

https://www.google.com/search?q=full...LZnfaQKfscM%3A
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Old 14-09-2016, 15:51   #5
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Re: Capacitor uf Rating

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Originally Posted by a64pilot View Post
I don't know what it made voltage wise, I never checked or don't remember if I did, probably thought 110 was fine if I did, I hope it's not the diodes, cause I believe that requires disassembly, and I thought if a diode was bad, no current at all.
I did call Nexgen, his answer was to turn her up to 63.5 hz unloaded, right now I have it running at 60 hz but 110 V. I'd rather leave hz where it's supposed to be and increase voltage if possible.
I was thinking I could go to a 45 uf cap to increase voltage.


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That does not sound right to me. I would think you would want to stick with 60hz on the nose. Most appliances will handle a big voltage variance. Why do you need 120?

http://www.50hz60hz.com
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Old 14-09-2016, 16:05   #6
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Re: Capacitor uf Rating

Actually, it sounds like you don't really have a problem.

Replacing a filter cap though isn't likely to increase voltage in my experience.
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Old 14-09-2016, 17:06   #7
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Re: Capacitor uf Rating

About the diodes. I was thinking that maybe they were higher resistance and thereby reducing excitation current. I'm not sure that diodes do that but thought it was worth mentioning. I can't see how increasing the size of the cap. could raise the output voltage. Might be worthwhile cranking up to 63 hz at no load and checking the frequency and voltage when you increase the load. I'm pretty sure that 63 hz, 5% variation, wont cause any problems.
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Old 14-09-2016, 17:12   #8
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Re: Capacitor uf Rating

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Originally Posted by DeepFrz View Post
About the diodes. I was thinking that maybe they were higher resistance and thereby reducing excitation current. I'm not sure that diodes do that but thought it was worth mentioning. I can't see how increasing the size of the cap. could raise the output voltage. Might be worthwhile cranking up to 63 hz at no load and checking the frequency and voltage when you increase the load. I'm pretty sure that 63 hz, 5% variation, wont cause any problems.
The diodes are there to either allow the positive half cycle or the negative to pass thru depending on biasing. I think it's .3 to .7 volts to forward bias depend on make

Electronics is old school and not easy to just learn on the fly........be happy with the 110 if it's within the tolerance of the generators predicted output
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Old 14-09-2016, 17:19   #9
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Re: Capacitor uf Rating

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That does not sound right to me. I would think you would want to stick with 60hz on the nose. Most appliances will handle a big voltage variance. Why do you need 120?

http://www.50hz60hz.com

I want 120 cause that is standard voltage and things like my AC compressors will run cooler and draw less amps at the higher voltage, and when I really load it up, hz drops to 58 or so, but voltage sags to 105 and you can hear the AC blowers slow down.
Normal shore power voltage is around the low 120's and therefore I assume "things" are designed with that voltage in mind.
Plus, if I can correct the voltage with a $12 capacitor that takes 30 sec to change out, why not?


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Old 14-09-2016, 20:48   #10
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Re: Capacitor uf Rating

[QUOTE=a64pilot;2213374]
Plus, if I can correct the voltage with a $12 capacitor that takes 30 sec to change out, why not?

Where did you come up with the idea that changing the filter would change the system voltage and have anything to do with the freq?
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Old 15-09-2016, 11:02   #11
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Re: Capacitor uf Rating

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Where did you come up with the idea that changing the filter would change the system voltage and have anything to do with the freq?
It's not a filter capacitor. The capacitor is used with auxiliary windings for stator excitation. This is in lieu of a more sophisticated voltage regulation system.
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Old 15-09-2016, 11:14   #12
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Re: Capacitor uf Rating

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It's not a filter capacitor. The capacitor is used with auxiliary windings for stator excitation. This is in lieu of a more sophisticated voltage regulation system.
Yes, and I think that changing the uf rating will change the voltage, correct?
If so then does going up to say a 45 where stock is 40, will that increase voltage or decrease, and if so by how much?
Or is that something I just need to buy a Cap and see what happens?
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Old 15-09-2016, 11:32   #13
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Re: Capacitor uf Rating

If you are dropping from 60hz to 58hz under heavy load, why not dial it up to 62hz and see if it drops to 60. And maybe the voltage stays at 110 or more?
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Old 15-09-2016, 12:05   #14
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Re: Capacitor uf Rating

Without the schematic I just guessing. But I suspect the adjustment the Nexgen rep mentions is to a variable resistor that is in the same circuit to which the capicator belongs.

Altering resistance on a voltage divider network which then affects the charge rate of the cap and consequently the switching of the circuit.

Another guess is that lowering the cap value will increase voltage, not raising.

But I'd tweek the Hz first.


Edit,... BTW, it supplies 30amps at 120volts, correct? Are you sure you are not reaching the borderline of the gens capacity. Falling voltage, falling hz, ........revs dropping? any black smoke?
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Old 15-09-2016, 12:12   #15
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Re: Capacitor uf Rating

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Originally Posted by a64pilot View Post
Yes, and I think that changing the uf rating will change the voltage, correct?
If so then does going up to say a 45 where stock is 40, will that increase voltage or decrease, and if so by how much?
Or is that something I just need to buy a Cap and see what happens?
The manual on my generator (not NextGen) calls for a 31.5uf. From the factory it had 35uf in it. When the originals failed, I replaced with 31.5uf. I too found the voltage too low and replaced again with 35uf. I'm now happy.

YMMV
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