ECN Forum Forums General Topics of the Trade General Discussion Area Sizing 3 Phase Generator

That is part of the "Walk In" capability.

Just equipment? No other building systems, or lighting? The building is useless without them. You did mentioned... "Plus they would like to have a 60A /3 Phase Rooftop unit to run of the generator also" I would be wary of how that load interacts with the UPS. Maybe 2 gennies?

Then there's the Service Contract.... Maintenance on the Batteries, Generator, Testing, etc. If you're not willing to take that on, you might want to have a "quiet conversation" with a Company that would that specialize just in maintenance and system installs. Sub them the install with a little mark up for your trouble, bring them the inside circuits, and hand them a lifetime of maintenance..... One hand could wash the other there. Just an idea... You come out ahead, they take care of the problems.



[This message has been edited by e57 (edited 07-02-2004).]

I ran into this problem during a remodel of an existing building. Part of the remodel was to install an owner furnished UPS. The existing generator had been on site for over 20 years. Just getting tje generator up to code and operational was an PITA. The previous owner had not maintained it very well as he went broke.
The new owner already had the UPS from another site. This was a major headache we could not get the two to provide stable power at any time. Once the generator took over the load voltage started fluxuating wildly. The ups or gen seemed to compete to send voltageany were but at the correct level (480 3phase 150KVA). The manufactuers reps just pointed at each other and both told us to scrap the other item.
We finally convinced the owner to scrap them both. The cost over run was over 35K and the owner took a long time to pay. The owner had to get a new bank loan and the bank wanted to know why and argue about it. Owner knew he would have to pay since we did not supply any of it and it was hooked up exactly as specified.
I never want to hear that kind of machine screaming and see a gen shaking and pulling out tie down bolts again Also the UPS started stinking , thought it was going to burn.
The longest time they ran together was about 45 sec. the first time. Did not allow then to run that long ever again.
By the way both units ran very well on thier own if the other was not in the circuit.

We all have different levels of experience with equipment operation, but not all at the same time, so often hard-learned advice like nesparky wrote should be considered. Thin specs and low bids can precipitate this kind of insanity. Went through these kind of problems about 10-15 years ago at several facilities.

With an accumulation of switchmode supplies and UPS support, this site may be a “data center” with an inexperienced electrical designer writing bad specs and nearly useless drawings. Sometimes his client has high expectations that he has not detailed to the design/engineering folks figuring he’ll get a low-bid system that will somehow do the job to his unstated expectations. Some “fast-track”/”just-in time” commercial property management outfits are notorious for attempting to try to pull this off, and often it will involve gotta-have-it-last-week rehabilitation or tenant-improvement contracts, trying to snow another contractor with their obnoxious, urgent-but-still-tight-budget railroading. Some never learn, or they learn very well to feign the poor innocent dummy over and over.

Although to some degree everyone is on their own learning curve with standby systems and how they interact with loads, there are some basic ways that all but very precisely specified combinations of equipment operate. On the load end, motors and switchmode supplies being commodity items work best with a “stiff” electrical source that is a pure sinusoidal voltage, but their load on a given AC-power system can cause short {sub-cycle/harmonic} or longer-term [sluggish/dimming] currents that affect voltage at their AC-power terminals.

Generally on the source side, utility transfomers like to see ‘linear’ and fixed or slowly varying loads—ideally with a sinusoidal current that is best in phase wit the voltage waveform. Gensets are not as “stiff,” being pickier about load character, and UPSs are even less tolerant of changing load character; being even a less-stiff source. Gensets and UPSs can fight like the Hatfields and the McCoys if there haven’t been careful negotiations prior to the “meeting.”




[This message has been edited by Bjarney (edited 07-03-2004).]

My job involves working with experimental electric motors and variable speed drives. We regularly push systems into strange states to see what happens. If you are ever in the greater Boston area and want to see the simplest situation of _two_ devices duking it out, drop me a line.

A VSD has the same sort of output as a UPS inverter, taking an internal DC source and running it through transistors to produce AC. In the case of a UPS, you are trying to synthesize fixed voltage, fixed frequency AC, and in the case of a VSD you are synthesizing variable voltage, variable frequency AC to change the speed of the motor.

One operating state that we regularly see has the motor with only an inertial load (very low drag, but high mass) spinning at very near to synchronous speed. We can see tremendous sub-harmonic pulsing as the shaft speed oscillates up and down. We are seeing an oscillation where energy is going back and forth between the DC rail capacitors and the rotating mass on the motor. Current to the motor fluctuates between near zero and very high. The noise from the motor is something else, as we hear the VSD switching frequency, overlaid 60 Hz, and the sub-harmonic variation.

-Jon

Thanks for all the replys. Tho we have installed several engineered UPS / Generator systems for our clients in the past I needed to make sure I had my concerns covered in this design/build venture. I have had meetings with our generator & UPS distributers and everything is on track to working in harmony to get the design rolling. I was really looking for a quick reference to sizing for budgetary reasons and you all have provided plenty of info. I'll keep in touch!