3 Phase motors on Emergency Generators
3 Phase motors on Emergency Generators
(OP)
Is it possible for a 3phase motor to single phase during the time a bldg loses power on 1 leg untill the Emergency generator starts about 1 min from loss of power to generator start up and come on line?





RE: 3 Phase motors on Emergency Generators
RE: 3 Phase motors on Emergency Generators
David Castor
www.cvoes.com
RE: 3 Phase motors on Emergency Generators
rasevskii
RE: 3 Phase motors on Emergency Generators
Bill
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"Why not the best?"
Jimmy Carter
RE: 3 Phase motors on Emergency Generators
Back to the OP, anything could happen, yes. What rasevskii said, your motor protection should prevent it from running when the power from your emergency generator cuts in!
RE: 3 Phase motors on Emergency Generators
RE: 3 Phase motors on Emergency Generators
If the motor was off when the phase was lost and then was called upon to start while still getting single phase power, it would not start and damage to it would be more rapid.
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RE: 3 Phase motors on Emergency Generators
Very true. Big emergency generators do not qualify as a "legally required standby system" owing to its inherent longer starting time/ cutting-in.
RE: 3 Phase motors on Emergency Generators
RE: 3 Phase motors on Emergency Generators
RE: 3 Phase motors on Emergency Generators
RE: 3 Phase motors on Emergency Generators
RE: 3 Phase motors on Emergency Generators
Single phasing until the generator comes online:
It depends a lot on the control scheme.
1>A phase is lost. The ATS starts the generator and when the voltage is stable, transfers the loads to the generator. The motor will probably carry through. But this scheme risks severe damage to larger motors on a random out of sync closing.
2> A phase is lost. The ATS disconnects the loads from the mains. The motor is now coasting. The ATS starts the generator and when the voltage is stable, transfers the loads to the generator. This scheme is what I see the most often. There is much less chance of the motor coasting long enough to have enough residual EMF to cause damage when it is re-energized.
Out of balance trips.
Again, it depends. The trips that I am familiar with have an inverse time delay on unbalance trips. It depends, may trip on unbalance, may not.
Been there, done that David. The sets that I worked on the most used non-electronic governor controls. Normal starting was full throttle.
On the turbo-charged sets that meant way too much fuel and the cloud of black smoke was common on start-up. block loading could also cause a smoke plume. Naturally aspirated sets were a lot cleaner starting and took block loading better.
Bill
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"Why not the best?"
Jimmy Carter
RE: 3 Phase motors on Emergency Generators
My 2 cents on the single phasing. Any 3ph motor can single phase and continue to run. If you have a fire pump connected to the emgen everything is designed to keep the fire pump running under almost any condition. NYC the code is to fuse the fire pump at 600% of the rating. It is tied to the incoming service before anything in the building. The fire pump ATS is set (by dip switches) to be the first to transfer to em power. The main breaker for the pump is mounted ON the generator buss panel.
RE: 3 Phase motors on Emergency Generators
Most Solid State OL relays now include some form of imbalance / phase loss protection. I highly recommend using SSOLs for that reason alone.
Older NEMA type bi-metal or melting alloy OL relays would NOT trip on phase loss or imbalance unless the current in the other two legs rose to exceed the trip curve from the heater selection / setting, which in many cases was never. So in older NEMA systems it was a good practice to have Phase Loss and/or Phase Imbalance protection relays.
Many IEC bi-metal OL relays have what is called a "differential trip" mechanism (some people use different terms, it's all the same). This is a spring loaded balancing bar that equalizes the trip spring pressure across all three sensing elements. So if you lose one phase, the trip bar is biased towards tripping faster if an overload occurs. But many people interpreted that to mean it will always trip on a phase loss so they called it "built-in phase loss protection" in their marketing materials and that is just not the case. If the motor loading is light enough, it may never trip.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: 3 Phase motors on Emergency Generators
Bill
--------------------
"Why not the best?"
Jimmy Carter