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In-line capacitors to prevent high draw?

  • Thread starter KMS - Brett Anderson
  • Start date
K

KMS - Brett Anderson

Jan 1, 1970
0
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
 
J

John Gilmer

Jan 1, 1970
0
KMS - Brett Anderson said:
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).

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.
 
J

John Gilmer

Jan 1, 1970
0
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.

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.
 
C

Charles Perry

Jan 1, 1970
0
Correcting the power factor?

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.
 
S

sammmm

Jan 1, 1970
0
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
 
S

SQLit

Jan 1, 1970
0
KMS - Brett Anderson said:
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.

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.
 
J

John Gilmer

Jan 1, 1970
0
sammmm said:
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

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!
 
B

Bob Peterson

Jan 1, 1970
0
KMS - Brett Anderson said:
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).

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.
 
K

KMS - Brett Anderson

Jan 1, 1970
0
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
 
K

KMS - Brett Anderson

Jan 1, 1970
0
Very helpful.

Thanks

Not

Brett Anderson

SQLit said:
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.
 
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