Thread: Do I need a generator? and are they all the same?

It's seems inefficient to use a genset to power a compressor.
Why not purchase a compressor with a combustion engine direct.
You should be able to do it for far less that buying a 3P genset.
If you like your compressor you could purchase a Kohler or Honda engine and replace the motor on the compressor?
Just checked on eBay and there are good quality used 70 cfm Airman diesel air screw compressors going for $8k.

In general terms there's a difference between analog generators and inverter ones. Inverter generators have the capacity to destroy some devices with complex electronics. Friends of mine for instance had some high end imported freezers destroyed by an inverter type. (they're off grid and imported those super duper freezers for extra energy conservation - doh!)

Don't really think in your case that's going to matter with running compressors or dust extractors off it, nor, in that price range, should it be a problem. I have a 4kw analog unit sitting in the barn for our frequent power outages and no problems with it running high end electronics at all.

thanks Bob, the thing is, I bought the compressor new in late 2014.... reluctant to sell it now (big depreciation)

I may put the extractor on the generator. either one of the compressor or the extractor has to go on the genset.

Originally Posted by Albert

thanks Bob, the thing is, I bought the compressor new in late 2014.... reluctant to sell it now (big depreciation)

I may put the extractor on the generator. either one of the compressor or the extractor has to go on the genset.

Sure I can understand that but I was just offering the cheapest solution.

That's why I also suggested a compressor motor replacement.
That way you can store the existing motor and the whole unit can then be sold with either motor or engine or both which can be hand

Extractor on the genset is not a good idea. It means the genset has to run just about all the time - that's lots of fuel, ear and tear, noise and exhaust to deal with.

I see where you are coming from...

the purpose of the Genset is to serve as the prime power, not a backup, I only use my machines 2 hours a day max and possibly 4 hours on a weekend.

I will definitely look at the feasibility about replacing the motor on the compressor - need to talk to my electrician.

thanks bud, I dont think the compressor/extractor will have a problem with the genset... its just the cost of it.

When I had my 3 phase installed the line company did ask me if I wanted to upgrade to 100amp, and I happily rejected because it costed $5000 to do this. now I wish I can go back in time...

Is the current (55.2A) above measured, calculated or the sum addition from the motor nameplates?

If it is not a measured value, it may pay to have an electrician actually measure with a tong meter.

You may well be drawing less than the 55A.

Compressors and dust extractors will pretty much draw close to their rated current during normal operation.

Compressors may even draw more than their rated current for the last part of the charging cycle.
Dust extractors will draw less current the more they are throttled but on normal operation they run at their full rated power.

The current drawn by the sander will be load dependent, free running it will be significantly less, but on full load it will probably draw more than its rated current.

Originally Posted by Albert

I have come to a stage where I may need a 20/22kva generator.

background:
I have 63amp 3 phase supply
existing worse case loading:
18.6kw (sander) + 7.5kw (Compressor) + 4kw (Extractor) = roughly 55.2amp

the problem I have is that the extractor is underpowered, the sander requries extraction of 6300 m3/hr, to get an extractor capable handling this kind of extraction demand I need a small commercial system which possibly sits between 7.5kw - 11kw

I am thinking to put the compressor on the generator as the compressor needs to sit outside the shed anyway, the generator is on star-delta.

thoughts and comments?

If I proceed to buy a generator, are they all the same? (apart from cheaper ones has smaller tank and other bell and whistles) I've got quoted from $10k NZD to $20k NZD for a 20kva unit

I'm glad I don't live next door to you. A 20KVA generator would be how noisy?

A word of warning, my experience with 3 phase generators is that they are very finicky in regards to balancing the current draw across each phase and diesel generators like to run at 70% or greater load, otherwise the cylinders tend to glaze.

after confirming that you need a supply greater than 63A, perhaps revisit the cost of upgrading your supply wires.

Originally Posted by ian

I'm glad I don't live next door to you. A 20KVA generator would be how noisy?

A word of warning, my experience with 3 phase generators is that they are very finicky in regards to balancing the current draw across each phase and diesel generators like to run at 70% or greater load, otherwise the cylinders tend to glaze.

after confirming that you need a supply greater than 63A, perhaps revisit the cost of upgrading your supply wires.

The line company has finally responded and they are looking at this right now to see if the upgrade to 100amp or bigger can be done.

if it can be done with less $ than a genset then I will do it.

I work in the power utility industry in Aus and as such have a fairly decent appreciation of the run costs of generators over long periods of time (such as when things go boom).

The purchase cost of a good quality 100A capable generator is only the first part of the issue. I'm not familiar with your setup, so assuming you are running a 22kVA genny for serious length of time (as in all the working hours of the week), you'll be paying a lot in fuel. Not to mention your hours clock will be spinning over at a rate of knots, so maintenance is also going to catch up with you, if not in cost then at least in down time

Cheaper inverter units are square wave or modified sine - they're the ones that explode electronics. Motors will run warmer and less efficient (due to the interesting harmonics), but will mostly not care unless run at heavy loads all of the time. Dust extraction doesn't count for this because the motor is running at design speed, so cooling is not affected, I'm talking about machines where they run at lowered RPMs and higher current draws - think an overburdened table saw or spindle moulder running for hours with that "I'm working too hard" sound going on.

All that said, by the time you're up to 20kVA, many generators will be 240/415 AC all that way from the windings, so inverters are not required - it gets cheaper as they get bigger to just do it right from the beginning. This means that your waveform may have some harmonics, but it will actually be voltage regulation that will be your killer in terms of cost. More expensive units should have a faster responding voltage regulation system, leading to fewer dips, sags and underfrequency trips. Expensive units can also be quieter (you can get down to 63dBA @ 1m units if you have to worry about people hearing the chugging).

When specified for backup purposes, generators for high importance loads (think datacentres) are normally spec'd at 150%. You seem to have done this for the generator -> compressor matchup. The slow increase in load of the compressor as it runs works right into the hands of the generator as throttle should be able to keep up with the increased demand from the compressor. However, as an intermittent load, unless your shop is sucking some serious compressed air, the generator is going to be spending a lot of time at idle, which as mentioned before, is bad for the prime mover component.

I don't know the regulatory scheme for NZ with regards to who can do supply upgrades, sounds like it's like what it used to be in much of AU where the supply authority was the only one allowed to do the work. If so, you're stuck with the price, whatever it comes out to.

Most metering is 100A rated (current transformers are used for higher amperages), so you shouldn't have issues there, unless your supply authority is particularly tight*** and has procured 63A meters as well as 100A.
Your service fuses will need replacement to 100A and so will your main isolator.
You incoming supply cables will more likely than not require replacement as the AS/NZS ratings for the most common service cable is only 63A/ph (although because of arcane derating nonsense the same size is normally good for 100A single phase, go figure). If it's just cabling, that's normally relatively cheap, hopefully around AU$20 - 30/m + labour.

Where you'll get screwed is if you have to contribute to network upgrades - that could be big bucks if you start paying for transformers and street wiring.

All of this normally works out that once you've paid the capital cost for the generator and you start having to sort out the delivery of a couple of hundred litres of diesel a week, network supplied electricity starts becoming pretty cheap provided it's only your service that you're upgrading. Street upgrades will take a while longer to realise the benefits - many years. But you'll have the advantage of a supply that someone else maintains and who will be, to a degree, responsibly for the reliability of the connection. A generator will be all your own problem and if it blows, you're stranded with no help.

I probably should have checked with you before, but I'm assuming based on the size of all of this that you're probably doing this for a living (or at least part of it). If so, go on the network and pay for the upgrade. If it's for very occasional usage, the generator may win out, but the ongoing maintenance will be your problem. Too much usage and you're paying for fuel and oil. Too little usage and you'll have to turn it on and run it loaded for no other reason than to make sure it doesn't rust up and seize.

Just an update, met the technician from the line company to go through my requirement onsite a few days ago to clarify the requirement (their initial assumption was incorrect)

Got the quote and it will cost me about $500NZD ($450 AUD?) to upgrade from 63amp 3 phase to 100amp 3 phase

a fraction of the Genset cost!!

Win!

It's good that you have solved the problem particularly as I am not certain a 20/22KVA generator will start a 7.5KW compressor. The starting current of an induction motor is probably four times the running load. If that was the case a VFD, which can deliver a soft start, would have been a solution (might have even allowed a smaller gen set.)

However , theimage21 addressed most of the associated problems and it is all academic now anyway.

Regards
Paul