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Joined: Dec 2001
Posts: 300
M
Member
My dad has a farm with about 2 acres of greenhouses. If his mist systems go down for more than about an hour in the summer, he can loose everything. So 20 years ago he bought a PTO driven generator as a backup.

The voltmeter on the generator is shot and he wants to order a new one. He asked me to stop by and look at the old one because he thought it was odd. He's a bright guy. Actually got his engineering degree before returning to the family farm.

What he has is a regular old two screw terminal voltmeter. which is pigtailed right off two poles of the main circuit breaker. Nothing solid state, nothing digital. As simple as it gets except that each terminal is grounded through a capacitor. I'm assuming capacitors. They are 1/2" diameter, 1" long with a single wire leaving the center of one end and the metal "shell" clamped to the chassis of the generator. Two capacitors, each with a wire attached to the screw terminal of the voltmeter with the capacitor "shells" clamped to the chassis of the generator.

I told him to scrap the capacitor and volt meter and just buy a simple panel meter but I'm curious how this was supposed to work.

Joined: Oct 2006
Posts: 745
E
Member
Those capacitors are likely there to keep the meter from "bouncing" since generator outputs can fluctuate so much. The capacitors just act as shock absorbers to keep the meter's needle steady so that an "approximate" voltage reading can be obtained.


---Ed---

"But the guy at Home Depot said it would work."
Joined: Jul 2002
Posts: 8,412
Likes: 3
Member
Originally Posted by EV607797
Those capacitors are likely there to keep the meter from "bouncing" since generator outputs can fluctuate so much. The capacitors just act as shock absorbers to keep the meter's needle steady so that an "approximate" voltage reading can be obtained.

Yeah Ed,
These capacitors are also known as "smoothing capacitors".

I've seen these used here a few times in dairy sheds, the capacitors need to be sized correctly, or it will throw the meter readings out of wack.

Joined: Dec 2001
Posts: 300
M
Member
That was my best guess but smoothing at 60Hz seems unnecessary? I can't see a 60Hz vibration, my eyes aren't fast enough and the needle can't move enough to make a difference in 1/120 second can it?

And why use two caps to ground? I'd just put a single cap between the terminals to do this.

Still seems odd to me.

Joined: Oct 2002
Posts: 482
Z
Member
Actually, yes, the caps are VERY necessary. Analog (magnetic) meters are pretty sensitive by nature, and smoothing the voltage is imperitive if the device is to last.

You will not ever see a decent power supply without caps either.

The reason that there are 2 of the caps (one neutral-ground and one hot-ground) is because this is an AC circuit, and as we all know, voltage swings from side to side at 60Hz (US). Both sides are subject to the same "bounce".

Joined: Aug 2001
Posts: 7,520
P
Member
Hang on a minute guys...... How are you figuring smoothing capacitors on an A.C. circuit? If the meter were fed through a rectifier, then capacitors across the resultant half- or full-wave rectified D.C. would work to let the meter read the effective peak voltage, but if the caps are connected directly from each A.C. line to chassis, they can't act as smoothing capacitors because the polarity is changing 120 times per second. Also, if the meter were being fed rectified power in that case I would expect a reservoir/smoothing cap to be connected directly across the meter terminals.

My guess would be that the capacitors are more likely there to act as a form of suppressor against high frequency transients causing by switching loads, etc.





Joined: Mar 2005
Posts: 1,213
S
Member
Yeah, they're not smoothing capacitors, you can't smooth AC! If they were part of the rectification filter, they'd be on the DC side of the bridge.

I think they may be there to prevent unsafe voltages from building up in the generator. The caps will allow a small amount of currenet to leak through, and will maintain proper voltage of the coils. Otherwise, the open-circuit voltage of the generator could reach unsafe levels.


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