What would cause an AC unit to drag the generator down?

Hey guys. So we have 3 reverse cycle AC units. All 3 work great on shore power but for some reason, lately, the aft unit will put a strain on the generator to the point where it’ll trip the breaker after a short time. I have played around with taking the load off so all it had to do was run that ac and it still does it. No problems while using heat. Plenty of water flow.

Again, it’s only while on the generator. With that one off, we can run the other two, water heater, battery charger and the microwave with no problems. Something doesn’t add up.

Inexpensive solutions only please...thank you

Have you watched amperage draw for this unit both on shore power and generator? As well as confirmed generator output voltage and frequency (both at no load and under a moderate load)? If not, those are some good things to check that can narrow down what might be out of whack.

It sounds like your compressor is over loading the system maybe going bad.

The same happened to me with a NEW 12BTU Dometic unit. I had to install a "smart/Soft Start" unit to help the compressor initiate the start sequence.

https://www.dometic.com/en/pl/produ...s/accessories/dometic-smartstart-iii-_-215771

Steve you need to meter the volts and frequency when the suspect AC unit comes on. Normal genny volts are in the range of 115-125Vac, although some Westies like yours can be a bit sloppy with voltage control. Seen some lower, some higher.

Frequency should be about 61-63Hz unloaded. With a normal load should be about 60Hz. Starting a heavy load can drop to 57, but should recover to 60.

If either (or both) Vac and Hz are low, the AC unit will draw more amps, and that can make the difference between running and tripping.

Do you have a ring style amp meter? In addition to the other suggestions, I would measure amp loads on shore power for the unit and for the compressor when it kicks in. If all 3 units are the same BTU, check the starting and running amps on all 3. A unit with a marginal compressor may start fine on shore because of additional available amps, but have trouble starting on the generator with less capacity. It would be good to identify or eliminate the air conditioner as the cause of the problem.

Ted

Steve
New problem or always been that way?

You didn't mention the size of your generator. We have two, a 12.5kw and a 7.7kw. The smaller generator will not run our three AC units simultaneously without tripping the breaker.

Amp clamp. Do yourself a favor and buy the AC/DC amp scale meter. Until then we’re all guessing with you.

catalinajack said:

You didn't mention the size of your generator. We have two, a 12.5kw and a 7.7kw. The smaller generator will not run our three AC units simultaneously without tripping the breaker.

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Depends on the AC units too. My 3 units (2x 10k and a 16k) run fine on my 6.5kw generator. It's a pretty heavy load for it, but still enough headroom for the water heater.

Thanks everyone. We are in the car on our way to the boat right now. It’ll be later tonight once kids are asleep when I can take a look at it.

Here’s what I do know. The hertz on the generator do drop lower than 60 and will stay there while the aft compressor is running. So I wonder if the compressor has bad bearings or something causing an excess load. It’s not just a initial start up.

This is a new problem. We used to be able to run 2 of the 3 and the rest of the boat with no problem. We still can run the entire boat and forward and middle ac’s. Just not the aft.

I have a digital multimeter onboard. What exactly do I need to test with it?

Tonight I’ll mess with it and give more info. Maybe tomorrow I’ll fire the generator and take videos of what’s going on.

Thanks everyone!

This might help:
https://youtu.be/zr4Ow1JT9jU

you will need to borrow/buy a clamp on meter and make a homemade cable to give you access the wires (Hot, Neutral, Ground) inside your shore power cable. The clamp on meter should go on the Hot (Black wire) then turn one ACs one at time to measure their current draw (make sure you are on the active shore power if you have two of them).

Hey guys. Real quick we just got to the boat. We cranked all 3 ac’s in shore power and the rear ran for a while then tripped at the compressor. I fired up the generator, turned all 3 ac’s off and then recorded the gauges while my wife turned on the aft ac. It takes a huge huge when it kicks on but then has a massive draw that seems to get worse over minutes. I had her turn it off and let the generator recover. Then I flipped on the central AC and it had very little load. The. My wife turned on the v birth ac. With both the V birth and central AC, the load still way less than with just the aft.

So back to shore power I turned on the aft ac. The compressor turned on and was blowing cold. But it got hot as **** within minutes. I turned it off and informed my wife that we will be sleeping in the v birth tonight.

I called topsies and that’s about when she realized she forgot to bring wine. Might be a long night.

Here are some pics for now. I’ll have to edit my 5 minute video of my kids going nuts while I try to get the gauges while the ac kicks on.

Unloaded (mostly)

Aft ac on

Suspy ac

As always...inexpensive solutions only please.

So it even tripped the breaker on shore power?

Sounds like you are going to be buying a new A/C for the aft cabin...

Ski in NC said:

So it even tripped the breaker on shore power?

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Yes

Comodave said:

Sounds like you are going to be buying a new A/C for the aft cabin...

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I said....inexpensive solutions only. No stating the obvious.

That is the inexpensive solution, the expense solution is to have a mechanic come to the boat and do the replacement and charge you double for what it will cost you to do it yourself.

Can your meter test capacitance, or microfarads? A crappy capacitor can cause all sorts of grief with a refrigeration machine. A good meter has that setting and a capacitor can be tested for zero bucks. If it is not in spec you are looking at ten or twenty bucks.

My best guess is your compressor bit the dust. They don't last forever. Time to replace it.

I replaced my main 12K BTU compressor last November. It dragged our genny down too.

Alaskan Sea-Duction said:

My best guess is your compressor bit the dust. They don't last forever. Time to replace it.

I replaced my main 12K BTU compressor last November. It dragged our genny down too.

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How much was that?

I thought these ac’s were relatively new.

You really need to get a clamp meter and read how many amps the unit is drawing. It could be as simple as a breaker tripping before it should. Or a bad (marginal) capacitor. Or a device used to control start sequence. Don't even think about big expense items like a compressor until you do the basic troubleshooting.

Ski in NC said:

You really need to get a clamp meter and read how many amps the unit is drawing. It could be as simple as a breaker tripping before it should. Or a bad (marginal) capacitor. Or a device used to control start sequence. Don't even think about big expense items like a compressor until you do the basic troubleshooting.

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Yeah I know. Just wondering how much a compressor costs.

What’s a clamp meter? Like I said, I have a multimeter. I’ll YouTube how test amps and the capacitor.

Steve91T said:

Yeah I know. Just wondering how much a compressor costs.

What’s a clamp meter? Like I said, I have a multimeter. I’ll YouTube how test amps and the capacitor.

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The clamp on meter is what's in the video on post #11.

Ted

High Wire said:

This might help:
https://youtu.be/zr4Ow1JT9jU

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Excellent video! Really good explanation of how to use a multimeter. Thanks for sharing!

If the current meter goes from almost 0A to 18A and stays up there, then you have a bad capacitor on the AC unit that you will need to replace.
your current meter should have gone from 0A to 18A then drops to about 12A or 13A for normal unit.
good luck,

Ski in NC said:

Can your meter test capacitance, or microfarads? A crappy capacitor can cause all sorts of grief with a refrigeration machine. A good meter has that setting and a capacitor can be tested for zero bucks. If it is not in spec you are looking at ten or twenty bucks.

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I will place a small bet that Ski is correct,bad capacitor.

Ski in NC said:

Can your meter test capacitance, or microfarads? A crappy capacitor can cause all sorts of grief with a refrigeration machine. A good meter has that setting and a capacitor can be tested for zero bucks. If it is not in spec you are looking at ten or twenty bucks.

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I agree with ski. As as an inexpensive fix to try. There are usually start and run capacitors. Different mfd ratings and do a different job. Since your compressor seems to be reaching full speed, my guess is that it is the run capacitor.

Steve91T said:

How much was that?

I thought these ac’s were relatively new.

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I believe it was around 6 boat dollars, parts and labor.

My units are separated, meaning the compressor is separate from the air handler.