Advise needed. Diesel generator loading issues

[HomeCert ltd]

Hi everyone,

I would really appreciate some advise. I'm a domestic electrician but my neighbors asked me for some help on a issue he has and here I am.

So my neighbor has a tree surgeon company, they have a yard where the cut and dry logs as well. So they've just got a new drying system installed and have a new 20KVA generator to power the yard. A few weeks ago the generator started spewing oil, the manufactures said it's because the generator is be under loaded, so they said they should ideally run the generator at 12KVA to be most efficient.

I went to the yard today and the supply is 3-phase. I measure the current drawn on each phases as L1:4.4amps - L2:0.4amps - L3-1.0amps

Total loading 5.8amps. If I put this into KVA I get 4.0KVA on 230v and 5.8KVA on 400v.

I worked out 12KVA to be around 17.3amps

Any if the above i correct or am I completely wrong somewhere. Also any advise on the best way to increase the loading, the yard is ran 24/7 and is in constant us. I was thinking of a dummy load, but wanted to get other opinions or ideas! I've attached a pic or 2 of the generator and controls.

Any advise is welcome.

 

[Lucien Nunes]

Not sure how you got those numbers!
5.8*230 = 1.3kVA
I'd agree that a 20kVA genny is likely to gunk up if run for long periods with only 1.3kVA load*, as the engine will run cool, with bore glazing, ring sticking etc. as a result. There's a difference between peak efficiency and minimum continuous load though - I don't think you need to load to 12kVA to keep it happy! A dummy load will simply waste fuel however, and he is already wasting fuel by running a set of that size for 1300W of load.

It sounds like the genny is far too large, but one would have to know what the normal and peak loads are for the installation. Any info on this? If they have a heavy load when sawing etc. but a light 24-hour load for the drying fans, that would suggest getting a small genny for the 24-hour load only.

*Strictly, as far as the engine is concerned, it's the real power (kW) load that matters, but it's fine to work with kVA as the power factor probably won't be that far from unity.

 

[Deleted member 105166]

If the set is spewing oil, there is something fundamentally wrong.
The light load you have across the three phases will not make it do this.
As a rule, with any three phase generator, you should aim to balance the phases as evenly as you can and it's not a bad thing to ballast the load with halogen floodlights or heaters to ensure there is always some demand on each phase, although doing this to 55% of its rating seems excessive and will waste fuel.
Thisparticular set is rated 22kVA or 17.4kW, so across the three phases will be good for around 25A per phase at 230V

 

[HomeCert ltd]

Thanks for the replies. This is definitely not my usual field.

When I went there today they informed me the usage it was using at the time was the same it would us literally 24/7. As the boiler consumption is wood chipping and literally the only additional use is a arm which move chipping in to the furnace from time to time, light loads.

The generator only feeds the containers and all other machinery, everything else chainsaws etc... are petrol powered.

They've just spent £6k on the generator as their electrician who wired it up informed them they required a 20KVA generator.

I think there trying their best to use the generator they've purchased as otherwise it's money down the drain.

 

[Deleted member 105166]

As the loads are apparently so small, it would be interesting to see the calculations that resulted in speccing a 22kVA 3-phase set, when it appears quite likely that a 6kVA single phase set would be sufficient, better suited, smaller and cheaper to run.

 

[Lucien Nunes]

Indeed. That would chug along at 1300VA round the clock and support the odd kettle or microwave during the day.

 

[HomeCert ltd]

I just wanted to check my measurements, so I simply held a clamp meter on L1, L2 and L3 to measure the current being drawn. Out if curiousity how do you work out the KVA, is it simply the KW rating of what's being drawn.

 

[davesparks]

I just wanted to check my measurements, so I simply held a clamp meter on L1, L2 and L3 to measure the current being drawn. Out if curiousity how do you work out the KVA, is it simply the KW rating of what's being drawn.

Your profile says you’ve completed the 2330, but you don’t know power calculations? This was definitely a part of the 2330 course when I did it.

 

[davesparks]

I can’t see how the poor loading would cause oil to spew out of the generator. However being under loaded and out of balance is not going to be doing it any favours!

The only time I’ve seen a generator spew out oil was when the hire company sent one out to us without the dipstick in the engine.
Shortly after starting it it started to run a bit rough, I opened the access door to see oil spewing out of the dipstick tube!

 

[HomeCert ltd]

it's been around 10years and mainly I just do House bashing, and maintence etc.... and I haven't done 3-phase since my early years.

I'm guessing you would add up all machinery/loads etc.... in KWs and divide by .8 to get KVA. Does it work the same on 3 phase?

I'm not posting on here to start a ----ing war, was just after some general advice in a field I don't specify or work in.

 

[HomeCert ltd]

I've just seen this:
Underloading Diesel Generator - https://hyundaipowerequipment.co.uk/underloading-diesel-generator

Talks about underloading producing a lot of soot but not specificially oil

 

[Lucien Nunes]

You might not regularly work with generators but there's nothing mysterious about the power they produce - calculate the load power just as in a normal installation. Apparent power = total of line currents x phase-neutral voltage.

The only thing that is a bit more relevant is power factor, because real power (kW) is limited by engine mechanical output, while apparent power (kVA) is limited by alternator winding current. So generators tend to give both ratings, neither of which may be exceeded, and are often rated on the basis of a mixed load having a typical pf of 0.8. For extreme load pf, other limitations may apply, but that is not relevant here.

We don't know precisely what was going wrong with the engine but continuous low load causes the problems mentioned in your link. If it gets very bad, with all the piston rings stuck in their grooves etc, this can increase blow-by that overcomes the breather and forces oil out, and/or unburnt fuel gets into the crankcase and causes the level to rise until it pours out. Left unchecked, the engine will eventually just be circulating diesel in the lubrication system. I've seen engines in this condition, fairly comprehensively knackered.

 

[HomeCert ltd]

If they were to let the generator run on a higher load for a few hours every few days, this should help eliminate this issue? As the it would be running more excessively and get rid of and excess oil or spot it may be building up?

Maybe plugging in like a 12kw 3-phase heater for a few hours here and there will do the trick?

 

[Lucien Nunes]

a.k.a. Italian Tune-up. Might slow the build-up process somewhat but probably not enough if the unit is running long hours, and suddenly working a glazed engine hard can cause premature wear as the surfaces don't hold a proper lubricating film throughout. Then you get polishing; instead of varnish filling in the honed texture of the bore, the rings run hot and take metal off the top.

There's no quick fix, if you want good engine life. Diesels just don't like running light for hours on end.

 

[HandySparks]

As the loads are apparently so small, it would be interesting to see the calculations that resulted in speccing a 22kVA 3-phase set, when it appears quite likely that a 6kVA single phase set would be sufficient, better suited, smaller and cheaper to run.

Quite agree. Your neighbour is due an explanation from the original electrician as to why the generator is so grossly over specified. Was there an allowance for some future large item of equipment?