Onan 17.5kw Genset Shaking when loaded above 40 Amps - Help please

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admillington

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With all of Bart's issues and Latestarter's comments about harmonic things, I checked over all the engines on board. One possible issue I noticed was that my 2012 Onan 17.5KW Genset shakes when loaded above 40 amps - Genset has 166 hours on the clock - serviced every year - always loaded gently to start off with and cooled down before being shut down.

I took a video today - is this a problem or not?

http://youtu.be/ge6lXgIsXzI

Many Thanks


Andrew
 
more than anything that sounds as if the engine itself is running roughly.

One other possibility is that at higher revs, the engine 'leans' more to one side under increased torque, and loads the rubber doughnuts on that side, and they fail to absorb the vibration fully.

EDIT

just seen your reply. If vibration is related to load,are the dampers for the genny side in good order ?
 
Not been following earlier posts, nor an expert on this genet, so commenting generally:
Presume 400 (ish) line volts, 3 phase, 50 (Hz) ish, and the 40 amps is line current, so running near full rated load? Are you running with a balanced load (ie, all phases pulling same current level)? Also, if bulk of load is mechanical, is it constant through its motor drive rotation cycle? (Best checked by viewing load current on a scope perhaps). Electrical load unbalance can cause similar vibration issues as mechanical balance issues.
 
sarabande said:
more than anything that sounds as if the engine itself is running roughly.

One other possibility is that at higher revs, the engine 'leans' more to one side under increased torque, and loads the rubber doughnuts on that side, and they fail to absorb the vibration fully.

EDIT

just seen your reply. If vibration is related to load,are the dampers for the genny side in good order ?
Click to expand...

The rubber mounts look in good order. There is no smoke from the exhaust at any load.
 
Jean said:
Not been following earlier posts, nor an expert on this genet, so commenting generally:
Presume 400 (ish) line volts, 3 phase, 50 (Hz) ish, and the 40 amps is line current, so running near full rated load? Are you running with a balanced load (ie, all phases pulling same current level)? Also, if bulk of load is mechanical, is it constant through its motor drive rotation cycle? (Best checked by viewing load current on a scope perhaps). Electrical load unbalance can cause similar vibration issues as mechanical balance issues.
Click to expand...

The genset is single phase 240v 17.5 kW so I work out at 40amps it is about 10 kW of load so no where near the full load.
 
admillington, I've had a couple of those gensets. From the video, the engine seems fine mechanically but there is a wobble. The mounts are very soft normally on these, so it is normal for them to wobble plenty, but from that video it looks like you might have slightly more wobble than normal so perhaps you've lost an engine mount at the back or something. Nothing serious, and Onan will fix no doubt.

That isn't the sort of wobble you'd expect to get if the crankshaft damper covered in BartW's thread had failed. Those damper things deal with higher frequency and small amplitude vibrations and harmonics, caused by the torque spikes of the firing strokes plus the "tug of war" between the linear inertia of the pistons and the torsional inertia of the crankshaft, which results in the rpm of the crankshaft constantly increasing and decreasing at quite high Hz. Your engine sounds fine, and those Kubota engines are generally very good. Ref post#4, there is no "lean" possible as Sarabande describes, because the alternator (ie the load) is attached to the bell housing so the engine mounts do not see the torque of the engine
 
jfm said:
perhaps you've lost an engine mount at the back or something.
...
the alternator (ie the load) is attached to the bell housing so the engine mounts do not see the torque of the engine
Click to expand...
Well, along the lines of your last sentence, I'd rather rule out the engine mounts.
Why do you think that the higher load per se could make the whole (engine+alternator) set wobble more, at constant rpm?
I could think of a higher wobble whenever the load is significantly increased, 'cause the engine might slow down for half a second, before restoring the 1500 rpm.
But that should be momentary, not constant. And should happen also, for instance, when the load jumps from zero to 30A.
Mind, not that I have any better idea, TBH...
Fwiw, I agree with all your other thoughts, including the fact that the engine doesn't sound rough at all, to my ears.
 
MapisM said:
Well, along the lines of your last sentence, I'd rather rule out the engine mounts.
Why do you think that the higher load per se could make the whole (engine+alternator) set wobble more, at constant rpm?
I could think of a higher wobble whenever the load is significantly increased, 'cause the engine might slow down for half a second, before restoring the 1500 rpm.
But that should be momentary, not constant. And should happen also, for instance, when the load jumps from zero to 30A.
Mind, not that I have any better idea, TBH...
Fwiw, I agree with all your other thoughts, including the fact that the engine doesn't sound rough at all, to my ears.
Click to expand...
I agree your line of thinking MM and actually I do not see a reason why a higher load per se could make the whole (engine+alternator) set wobble more, at constant rpm, so I deliberately ignored that reported symptom (!) and I didn't say I agreed it was occurring(!). For sure there can be a perception of more going on as the load increases (the whole engine gets louder as each firing stroke is a bigger event) but I agree that more wobble due only to more load (at same rpm) isn't rational!
 
We have a 3 cylinder 7.5 kva onan, with mechanical governing.

Interestingly it occasionally wobbles, but only when either the tank was low, or when the fuel filters were dirty.

we fitted an extra lift pump,(now have 2 the same type as your video, one on top of the tank & before a racor, the other in the standard place) and regularly change the small fuel filter. This improved matters.
 
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david_bagshaw said:
We have a 3 cylinder 7.5 kva onan, with mechanical governing.

Interestingly it occasionally wobbles, but only when either the tank was low, or when the fuel filters were dirty.

we fitted an extra lift pump,(now have 2 the same type as your video, one on top of the tank & before a racor, the other in the standard place) and regularly change the small fuel filter. This improved matters.
Click to expand...

Still have the issue despite trying everything and help from Berthons who were the Onan dealers for many years. What is quite clear is that the service from Cummins in the UK is appalling and I will ensure that the next boat I order is not fitted with an Onan. Clearly they get away with it as generators in the UK are used so little.


-Andrew
 
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Out of interest how much of the load is inductive, ie fridges a/c chargers etc.

Is it the same with say all rings and the oven on, water heater, connected heaters to give the same load. would be interesting to see if there is any movement then. I take it all the mounts have all the bolts in.
 
david_bagshaw said:
Out of interest how much of the load is inductive, ie fridges a/c chargers etc.

Is it the same with say all rings and the oven on, water heater, connected heaters to give the same load. would be interesting to see if there is any movement then. I take it all the mounts have all the bolts in.
Click to expand...

To get it up to the level of load, I needed to fire up all the air conditioners water heater, plus the kettle - so a mix of loads - The odd thing is that it stops shaking at 55 amps onwards.

I complained to Cummins today and had a call from the UK service manager and it seems there are been a reorganisation and hence the lack of service. They have assured me that they will go send an engineer to have a look and I will update the post when the solution is found.

-Andrew
 
It fixed - Lee from Onan Cummins visited the boat today and worked out that the problem was due to the mechanical governor on the engine not being set correctly - he adjusted it and all ok now......






admillington said:
To get it up to the level of load, I needed to fire up all the air conditioners water heater, plus the kettle - so a mix of loads - The odd thing is that it stops shaking at 55 amps onwards.

I complained to Cummins today and had a call from the UK service manager and it seems there are been a reorganisation and hence the lack of service. They have assured me that they will go send an engineer to have a look and I will update the post when the solution is found.

-Andrew
Click to expand...
 
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