Using an APC UPS connected to a diesel generator, how 'clean' does the output need to be?

Someone posted a thread some time ago about the possibility of daisy chaining UPSes and the consensus of opinion was that most UPS' produce an approximation of the AC sine wave. Hence the next UPS downstream would kick in prematurely because of the 'poor' quality of the supply (with the exception of some Liebert UPS' which can produce sine wave output).
I have a client whose office is in a remote area and he has just been notified of some major power outages in the next few months. He already has a substantial APC UPS (because the power in his area is so flakey), but would like to factor in a generator (ideally diesel as he is a biodiesel advocate).
Are there any specific details when buying a generator that would indicate the quality of the AC output? Alternatively is there any way you could condition the output from a generator so as not to fail the UPS' input quality criteria?
Many seem to quote forms of automatic voltage regulation, but it's a big difference powering plant equipment to office equipment.
Any insight gratefully received, Jason.
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Our old Enfield twin opposed diesel of 50 odd years vintage seems to produce a very clean wave until its gets loaded with some switch mode power units then that waveform gets a tad distorted, but then again the incoming mains ain't as clean as it once was!.
FWIW the APC UPS's we have on that seem to tick fine.....
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Tony Sayer


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Jason Arthurs wrote:

I cant give you a clear reply, but can offer a few tidbits.
Computers should not be run on private gens. Overvoltage is the lesser of the issues: the main one is surges. Generators are inductive sources, so as one load switches off you get large spikes on the line. Computer PSUs can cope happily with mains borne surges, but not necessarily with the heavier generator surges. Either install some proper surge absorbing or expect a pc supply failure rate. No, I dont recommend the surge absorbers sold in shops.
With a ups, the question is what can the ups handle. If its an electronic charging unit, as is likely, the answer will be no more than your computers. In which case again youre liable to run into trouble.
Generator surge magnitudes can be reduced - with parallel surge absorbers, series R or L, then parallel surge absorbers & capacitor - by also running loads that can surge absorb, eg filament light bulbs and to some extent transformers.
Finally, most gens give a sensible output waveform, but I've heard the very small cheap gens can make messy waveforms.
Waveform conditioning... the main options are low pass LCs, and resonant constant voltage transformers. Really you shouldnt need this type of filtering, all your equipment will be bridge rectifier input, and these should run on more or less anything waveshape-wise.
Experience has taught me to expect widely varying voltages off generators, your UPSes might go into the wrong mode at times when V_in is running low.
IOW portable generators are quite crude.
NT
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I have..
MGE Pulsar EX30 (Online, 3KVA) APC Smart-UPS 1400 (1KVA, listed as "Generator compatible") APC Back-UPS RS 500 (500VA) Liebert Powersure 400 BPC PowerPal 1000
They all happily switch to mains power when my generator is running The Generator is a Briggs & Stratton BSP 7500LE (It has been replaced with the Pro Max 7500 - E) but this is a petrol generator.
Sparks...
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BT use APC UPS units to power bits of exchanges. These are backed up by the diesel generators.
The units supplied are rackmounted PCs running Unix and they carry on fine (just as well as the billing goes through them).
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Jason Arthurs wrote:

Not all APC units are equal however. The BackUPS range basically pass the mains feed through to the protected devices (with some surge protection) unless the power fails when they switch to the inverter supply.
The SmartUPS ones are what they call "line interactive" and carry out more intensive power conditioning being able to fill in brown outs etc.
Hence I would expect the more expensive SmartUPS units to better cope with a generator supply. However I do know somone who ran a computer on a small honda genny with a 1kVA backUPS APC after it and had no problems.
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Cheers,

John.

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