In-line capacitors to prevent high draw?

I have an 11kw generator powering my house during very regular power outages.

It will run the house fine, with the exception of the central air conditioning compressor (wife will melt without AC). When the compressor tries to engage (rated 80 amp start up), the entire house blacks out, the generator loads up, and the compressor takes about 10 seconds to fire up. After it starts, all else returns to normal.

Given that we lose power here about 30 times a year, for anything from 10 minutes to 8 hours, I'm looking for a fix for this issue. Also take as a given that I own this generator, it's installed, it's staying. I'm not prepared to spend 4 times the dollars on a 25kw just to be able to start the AC compressor.

Can anyone tell me why I shouldn't put a bank of capacitors on the line to the compressor? Seems to me that this would isolate the start up surge of the compressor from the generator (which runs the whole house through an automatic switch).

In case it matters, the house is in Geauga County, Ohio. Russell Township to be exact.

Thanks

Brett Anderson

Reply to
KMS - Brett Anderson
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Consider using a voltage starting relay to switch the bank of capacitors in and out.

I suggest the following "experiment:" Get some caps and wire them through a simple toggle switch with a good sized rating. (You might use a two pole, 20 amp, switch and put half the caps on each pole.)

When you start the compressor,have the switch "ON". (I forgot to add that the connection should be at the contactor for the compressor.) Observe start time and measure peak and running currents with a clamp on meter. When the machine is running, switch out the caps.

Use you clamp on meter and your ears (the sound of the compressor and the generator) to determine the best value for both a "start' capacitor, and a "run" capacitor.

As I said, the start cap can be automatically switch out with a "potential" type starting relay. If you are using a LOT of caps, you might consider dividing the starting cap load among more than one potential relay.

I believe the BEST place to put this network is at the compressor. This may cause problems with getting problems fixed under warrentee.

Reply to
John Gilmer

Yes, it can. I think the responder was describing the start capacitor would be in parallel with the compressor although the original poster's language seems to imply he was thinking putting a cap "in'line" or in series with the the generator and compressor motor.

I agree with the responder. The start-up current for the motor lasts for a few seconds and then goes away. Often, motor manufacturers add a start capacitor and a run capacitor to be able to start motors on smaller ampacity services without external source of start-up torque. The start cap, as the responder describes, is only in the circuit for start-up. Many motors with start caps have a contactor "ring" that moves based on the centripetal force of the spinning rotor. Once a certain speed is reached, the start cap is taken out of the circuit.

Basically, the start cap is "storing" emf for the surge of energy needed to start the motor. The efficiency of the system would be comprised if the start cap (often rather large) stayed in the circuit continuously.

Decent schematics are in the link below:

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Reply to
Dave P.

Well, a "regular" start cap also provides a phase shift for the start winding.

The caps I am talking about will help cancal the inductive load on the generator while the compressor is just starting.

On more consideration, a voltage sensing relay and a cap bank could be placed at the generator.

About 30 years ago I tryed to run a small piece of medical equipment from a cheap inverter. The inverter just would not start the load but the motor in the equipment mad an hell of a racket in the attempt. I borrowed some caps from a friend in the HVAC business and fooled around until I found a size that would let the unit start. It started but ran with some noise and I found that disconnecting the cap after the motor started stopped the noise. So I got the potential relay.

The only problem is that the generator voltage may not "droop" enough to engage the potential relay. That's why it might help to have the relay after the contactor so that it starts out de-engerized. A slow operate (NC) relay might also work; anything to keep the cap network in the circuit long enought to start the compressor without dragging down everything.

Very interesting but beside the point.

Reply to
John Gilmer

On Wed, 19 May 2004 13:02:35 -0400 John Gilmer wrote: | | |> |> Basically, the start cap is "storing" emf for the surge of energy needed | to |> start the motor. The efficiency of the system would be comprised if the |> start cap (often rather large) stayed in the circuit continuously. | | Well, a "regular" start cap also provides a phase shift for the start | winding.

Which is a separate winding at a different angle so that there is some degree of rotation in the magnetic field long enough for the motor to get going in the proper direction.

| The caps I am talking about will help cancal the inductive load on the | generator while the compressor is just starting.

Correcting the power factor?

Reply to
phil-news-nospam

Exactly. This is how static var systems work on large (very large) machines such as electric shovels and rock crushers. You switch capacitors (shunt) in and out to supply the vars required during starting, rotor lockup, torque surges, etc. When a motor starts, its power factor will often drop to 20 to

30 percent. Most of the starting current is inductive. A shunt capacitor can supply this reactive current, reducing the magnitude of the current drawn from the utility. This reduces the amount of voltage drop.

Charles Perry P.E.

Reply to
Charles Perry

don't. you'll smoke more than the generator. invest in a larger generator. it'll be cheaper in the long run. good luck, sammm

Reply to
sammmm

Your going to shorten the life of the compressor each time it starts with the generator. What is it going to cost you when the compressor fails because of your induced low voltage condition? What is it going to cost if the generator crapps and takes the compressor with it?

It is your equipment, I would not do it to mine.

Reply to
SQLit

We who live where the light go off several times a year have a lot of discussion about genertors.

So far, we haven't gotten any kind of generator. Since "camping out" isn't so bad, the only important use for a generator is to run the deep well pump and keep the ice box cold. In such a case, the machine would run no more than a few hours a day in 1/2 hour sessions. With the addition of a "Pig tail" and a plug/outlet the pump can be served with a four wire extension cord. The ice box can be served with a three wire cord. A generator that would "work" would cost around $500 and maybe less.

Beyond that level, the expense and trouble increase. When you decide to run the generator "full time" you get more wear and tear and will end up replacing it sooner.

Wiring a new panel with a built in transfer switch would run about $600 to $700. Trouble is that once things are open we will get a serious case of "while we are doing this is would not be much trouble to do that!" It's not hard to talk yourself into a BIG generator with a 500# propane tank (propane keeps better than diesel) and an automatic transfer swtich. This could run up to $10,000.

Right now we are just waiting for WalMart to get in some cheap generators!

Were I the original poster, I would get a window air conditioner and maintain a "cold room" when the power goes out. It's tempting (and quite possible if you cut out the resistance heating) to run a heat pump with a modest sized (5 kW or more) generator but then we would lost the fun of the fireplace and getting out all the camping gear!

Reply to
John Gilmer

The startup surge is very low power factor and the addition of a capacitor bank could well help you out. I am not sure if there are code issues for using capacitors in a residence, but take care to use proper precautions with your capacitor bank or you may have other problems.

Reply to
Bob Peterson

| don't. you'll smoke more than the generator. | invest in a larger generator. | it'll be cheaper in the long run. | good luck, sammm

Or alternatively, a 2nd separate generator exclusively for the motor.

Reply to
phil-news-nospam

We use the generator to run the well pump, and the sump pumps, given that we're in a flood plain, with a basement, and the power only usually goes out during heavy storms.....

The AC is a nice touch, that I could live without, but I have a WIFE. You all know how that goes...... During the "Big" blackout last year, we were the only house within sight with power, but we had no AC. She bitched the entire time......

The generator is already bought and paid for (after 14 blackouts in 2 months in 2003), so that's a given. It's natural gas, which is not the best, but better than nothing. I'd prefer propane, but don't have the tank, while I did have the natural gas line.

Thanks

Brett Anderson

Reply to
KMS - Brett Anderson

Very helpful.

Thanks

Not

Brett Anderson

Reply to
KMS - Brett Anderson

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