A/C Vibration When Generator Running

jff

jff

Member
Mar 29, 2008
510
Atlanta
Boat Info
2017 Crest Pontoon
Engines
Yamaha
I've got a problem that is easy to reproduce. When I run the generator, my Salon A/C unit makes a terribly anoying vibration noise continuously. I'm told it has to do with the generator creating a certain harmonic. The problem is, the generator service guy I had out says that the generator is fine (it's a Kohler 7.2) and running in the normal range and the A/C guy I had out says the A/C is reacting to a certain frequency from the generator and there's no way to prevent that other than tune the generator. I posted about this some time ago but thought I would try one more time in case anyone had any fresh ideas? I'm tempted to schedule both of them at the same time next week and let them argue it out....

Thanks,
Jason
 
Connect a digital voltmeter to one of the 120 VAC outlet and ensure that you have about 117 volts and 60 Hz power. Then go from there.

Best regards,
Frank
 
I was getting ready to write that someone else had the exact same issue several months ago, but that was obviously you. My '05 340 A/C used to be a lot noisier than it is now. I dialed the fan speed down a bit, and I think that as it broke in, it smoothed out more. We leave it on at the dock (79 - 80 in the Summer), so it's getting used pretty regularly. Sorry I can't really help more.
 
Yeah, probably me although I was thinking I had posted that in a different form on CSR. What came ultimately out of that was that I did adjust the fan speeds but it didn't make any difference - still very, very noisy when running on gen.

I'll get a voltometer hooked up and probably get back out both the gen and AC guys (if possible at the same time, that should be interesting). The A/C in this case is under the stairs and in a nearly impossible place to reach (only way to get to it is to empty and remove the fridge, or via a tiny access panel next to the stairs but that's only for filter changing) so I fear that in reality the A/C may just not want to mess with it. Anyway ... thanks for all the suggestions.

Thanks,
Jason
 
Based on the post several months ago, I thought this was being attributed to the quality of voltage being supplied by the genset. But now it sounds more like the running genset is causing the whole AC unit to vibrate. To clarify, does the AC unit make this noise if you are running it on shore power but also have the genset running (and not using its electrical output)?
 
The A/C unit only makes this noise when I switch over to gen power.

Regards,
Jason
 
Ok, that's what I thought originally.
 
I guess I shouldn't have posted the same problem again, sorry. I suppose my question is, based on that fact - someone has to be wrong. Either my generator guy who says that ir's running in acceptable ranges or my A/C guy who says that the A/C will always react to these frequencies by vibrating. Surely the two can/did work together well at some point, know what I mean?

Thanks,
Jason
 
This one is almost more difficult to explain then it is to fix.

The two units are creating a resonance frequency. Neither unit has an issue.

One famous occurrence of this phenomenon is the collapse of the Tacoma Narrows bridge due to resonance frequency caused by wind. It's commonly cited as one of the great engineering disasters of modern times in which the designers failed to consider resonance as a potential source of failure.

http://www.youtube.com/watch?v=P0Fi1VcbpAI

Basically, the two units, the generator and the A/C unit are vibrating in such a way and at such a time/frequency that the combined vibration is causing the noise.

The solution is to get something heavy, say a piece of lead or solid block of steel and bolt it onto one of the items, say somewhere on the A/C unit where you can fit it. Bolt on something heavy and secure it well.

This extra mass will change the vibration frequency of the unit, causing the two units to operate out of phase. An example of this would be pushing a child on a swing. Push them in phase with the swing movement and with relatively gentle pushes you have the child swinging very high. Push them out of phase and their movement comes to a near stop. Your goal is to slightly change the vibration frequency of one of the units by adding mass to one of the units.

This is not rocket science. Just do it. Bolt some added mass at a random location then try it and see if the problem persists. Repeat, changing object or location until the problem goes away.

Please let us know how it goes.
 
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Wondering if you could first try to insert thick, heavy duty rubberized washers in between the current genset platform and the generator? I would try unbolting the first two bolts (those most readily accessible) and inserting a vibration/resonance isolation washers in between them, assuming I could pry-up enough space to fit them in there. Then rebolt and see what happens. Getting to the four bolts on my boat would be 'interesting'...but certainly that would be even better...taking the whole genset out would be an incredible PIA on my boat, may be easier on the OP's boat though.
 
Okay, some good tips for when I call the two repair services. I'm going to try and get each of them out there to discuss (and I'll throw out these suggestions). I think you guys are spot on, both are working correctly - that's been my problem, nobody can 'fix' either one since they both work fine, they just don't work together well. I'll throw out the ideas and see where it takes me. Thanks again.

Sincerely,
Jason
 
OK I guess I am still confused. Presentation's take on the problem is the same as mine was with my post this morning, but you said that this did not happen when the two units were running at the same time with the AC on shore power. I took that to mean that it only happens when you power the AC unit with the voltage supplied by the genset. This would seem to eliminate the harmonic cause and leaves only the actual supply of power as the variable.
 
That's true, it's only when I actually switch over to gen to power A/C, not just when gen is running (but not powering the A/C). So that must mean it has something to do with the supply of power, right? And if so, what does that mean?

Regards,
Jason
 
It means that the output power is either off voltage or, more likely, off frequency. If it's not generating 60 Hz A.C. it's because the engine is running at the wrong speed. Most decent digital voltmeters will read frequency. Stick the probes into an outlet and see if you're getting 60 Hz.

Best regards,
Frank
 
Unless, of course, the harmonics are only caused with the genset under the load required to operate the A/C. Harmonics are very specific in frequency. Try starting the genny at the dock and operating the A/C from the genny. Note the sound of the genny itself (how hard it's working). Duplicate your problem.

Then switch the A/C to runn off of shore power, and load the genny using the other A/C bus - one on shore, one on genny. Turn the water heater on, stove on... until the genny is loaded as if the A/C were running from it. Can you duplicate the problem by loading the genny without the A/C running off of it? If so, harmonics. If not, maybe harmonics, maybe not. Just an experiment.
 
Pointless. Get a meter and check the generator's RPMs. Why play games guessing?
 
fc3 said:
Pointless. Get a meter and check the generator's RPMs. Why play games guessing?
Click to expand...

I would normally agree, but an expert has verified that the genny was functioning properly. So, we are now thinking outside the proverbial box.
 
Depends on how he defines "functioning properly." Is 2% error, 5%. 10% OK? Do the spec call for .5 Hz but the tech thinks 2 Hz is good enough, or did he just check voltage and call it a day? Without data, it's all BS.

Best regards,
Frank
 
He was actually out and did a quick voltage test for free - it sounds like what I need to do is have him come back and do a full calibration/inspection on it as a next step.

Thanks for all the ideas though, these are very helpful.

Regards,
Jason
 
Jason, make sure he checks either the engine RPM or the generator's frequency output. Have him show you the results on the tester. RPMs should be either 1800 or 3600, depending on if the alternator is 4 pole or 2 pole. The frequency should be, of course, 60 Hz. Voltage should be 120 VAC plus or minus 5%, which is utility grade spec. The old 110, 115, and 117 voltage standards are passe.

Best regards,
Frank C
 
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