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Capacitor?

masteele7

Sep 14, 2022
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Hi. I have a 15kw generator and a 6 ton heat pump. When the thermostat calls for heat, and the contactor tries to engage the pump sucks all the electricity, trying to start, and the 24v control board goes into a 5 minute time delay. I was wondering if I could put a capacitor across the 24v side of the 110v/24v transformer that would give the control board enough power till the pump starts, to stop this from happening, and if so, what size capacitor could I use? Thanks
 

Minder

Apr 24, 2015
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Feed the transformer from a separate 120vac supply.
Either something is wrong with the device, or the wire sizing feeding the equipment is undersized.
 

masteele7

Sep 14, 2022
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Feed the transformer from a separate 120vac supply.
Either something is wrong with the device, or the wire sizing feeding the equipment is undersized.
There is no separate 120v supply. The pump works fine on street power. The only time I have a problem is when I'm running the house from the generator. I think that initial spike in power is sending the control board into delay mode when the compressor starts, like it does when you turn the power off then back on. That's why I thought a capacitor on the 24v side might supply enough juice till the compressor is running.
 

AnalogKid

Jun 10, 2015
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You need to know three things:

1. The current drawn by the control board.

2. The lowest power source voltage that can still run the control board. IOW, the control board input voltage range.

3. The amount of time the capacitor has to supply charge before the normal power source is re-established.

With these, you can calculate the hold-up capacitor size. Note that the cap probably will be pretty large, as in over 100,000 uF.

An alternative is a small 24 V sealed lead-acid battery (SLA). Or a lithium cell plus a boost converter. There are several ways to achieve your goal, depending on your skill set.

ak
 

Bluejets

Oct 5, 2014
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If there is not enough capacity in the generator, you risk running the pump continually on the start winding and cooking the motor.
That is if it ever gets going.

Motors can take up to 6 times full load current at startup.
If that capacity is not there, the voltage drops significantly and never recovers.
Your generator is simply not big enough.
No capacitor known will correct that.
 

masteele7

Sep 14, 2022
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If there is not enough capacity in the generator, you risk running the pump continually on the start winding and cooking the motor.
That is if it ever gets going.

Motors can take up to 6 times full load current at startup.
If that capacity is not there, the voltage drops significantly and never recovers.
Your generator is simply not big enough.
No capacitor known will correct that.
Thanks! That is one thing I was concerned about. I didn't know that you could run a pump continually on the start winding. Is there a way to tell if that's happening? I'm able to run the pump if I manually push the contacts together. I did that just to see if it would kill the generator, and the generator, and pump kept running. My thinking was that once the pump got up to speed I'd be all set.
 

masteele7

Sep 14, 2022
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You need to know three things:

1. The current drawn by the control board.

2. The lowest power source voltage that can still run the control board. IOW, the control board input voltage range.

3. The amount of time the capacitor has to supply charge before the normal power source is re-established.

With these, you can calculate the hold-up capacitor size. Note that the cap probably will be pretty large, as in over 100,000 uF.

An alternative is a small 24 V sealed lead-acid battery (SLA). Or a lithium cell plus a boost converter. There are several ways to achieve your goal, depending on your skill set.

ak
I'll have to check into that. Thanks. Right now I'm concerned about what Bluejets mentioned about the pump burning up if it runs on the start winding continually.
 

roughshawd

Jul 13, 2020
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Here we are again! With our little thermistor in one hand and power company in the other!
Assume your thermostat says to your braniac powerful completely ficticious by magically working 'heat pump'
"Its getting cold, but the power if off and I can't get you to start..." because its running off of a UPS source when the power browns out.
You heat pump shouldn't say..."Well start the generator you dumb magical but incredibly trustworthy thermostat, I already know that its getting cold..." It should say "Generator Start sequence activating, thermostat can wait..." So what you need is a generator starter, with relay and everything, and not a simple clicker because it gets cold.
 

Bluejets

Oct 5, 2014
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Is there a way to tell if that's happening?
Yes, you pretty much answered your own question in your first post.

When the thermostat calls for heat, and the contactor tries to engage the pump sucks all the electricity, trying to start, and the 24v control board goes into a 5 minute time delay.

Other than that a tong tester, commonly known otherwise as a current clamp meter.
And a voltmeter usually simultaneously required.

By your own words, "have to hold the contactor in" , pretty much covers what I said previously.
 

masteele7

Sep 14, 2022
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Yes, you pretty much answered your own question in your first post.



Other than that a tong tester, commonly known otherwise as a current clamp meter.
And a voltmeter usually simultaneously required.

By your own words, "have to hold the contactor in" , pretty much covers what I said previously.
I have a clamp meter. I left it clamped on to the main power for about a day, and the highest amp draw was 70a. That was with the AC turning on and off all day. The heat pump has a 50a breaker, and the genset puts out 63a. It seems to me that it should run the heat pump as long as I'm not using anything else to draw current at the same time.
 

Bluejets

Oct 5, 2014
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Did you read the part that said motors can draw up to six times full load current...???
Fine testing on mains supply where one has capacity to start and run this thing from now to eternity.
Because of this unlimited capacity, the motor starts and reduces from the maximum requirement in the blink of an eye.
You'll simply NOT see it. A scope or chart recorder would be required to see it.

You'll never see the max current draw on the genset by sticking your clamp meter on the mains.

Your problem is on the genset.
Get a bigger one.
 
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