3 Phase motors on Emergency Generators 10

[Kwpower]

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?

 

[davidbeach]

Possible: Yes. Desirable: Probably not.

 

[dpc]

Certainly possible.

David Castor

http://www.cvoes.com

 

[rasevskii]

One minute for an emergency generator to start up and "come on line" is pretty long. If you actually lost a phase, a lot of damage could occur unless your motor protections are state of the art...

rasevskii

 

[waross]

Many automatic transfer switches will detect a phase loss and disconnect from the grid completely. Then the generator starting sequence is initiated.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

"One minute for an emergency generator to start up and "come on line" is pretty long."

Not to be OT, I guess it all depends on the size of the emergency generator. The most we got from a 2,000 kVA emergency generator was 1 min 42 seconds on startup!
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!

 

[resqcapt19]

If it is a true "emergency" generator, NEC Article 700 application, the code requires the emergency power to be online within 10 seconds. If it is a "legally required standby system", Article 701 requires it to be online within 60 seconds.

 

[jraef]

Back to your original question: If a motor was already running when the phase was lost, it will likely continue to run, but will begin overheating. If the load on the motor is light it may run for a long time without tripping anything, so one minute could easily fall into that category. However, one minute on single phase at a load low enough to not cause an overload trip is not likely long enough to cause major damage to the motor.

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.


"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

 

[burnt2x]

OT:

"If it is a "legally required standby system", Article 701 requires it to be online within 60 seconds."

Very true. Big emergency generators do not qualify as a "legally required standby system" owing to its inherent longer starting time/ cutting-in.

 

[davidbeach]

I've seen 2MW gen sets make the 10 sec start required of "emergency" generators. Rather ugly, but it can be done.

 

[peebee]

Davidbeach, can you clarify "rather ugly"?

 

[davidbeach]

The only way to get there is start with the throttle wide open. Lots of cold black smoke during start; smoke that doesn't move away from the end of the stack very rapidly at the same time the radiator fan is starting to draw in tremendous quantities of fresh air.

 

[roydm]

Slightly off topic but wouldn't most three phase motor overload units trip on out of ballance with a phase missing?

 

[waross]

It depends.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[cjw81]

Interesting thread. Concerning smoke, in hospitals NYC we are required to either connect generator to 30 % of the nameplate rating or 50% of connected load. If either is not the case we must connect to a load-bank annually in increments of 30,50,75 and 100% in order to prevent unburned fuel from accumulating in the exhaust stack. Pyrometers also help you to keep temps at all times above 325F to avoid turning sulfur dioxide to sulfuric acid and rotting out said exhaust.
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.

 

[jraef]

Slightly off topic but wouldn't most three phase motor overload units trip on out of ballance with a phase missing?

As Bill said, it depends.

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

 

[waross]

I have seen thermal differential trips that would trip on less than full load current but as you say, may not trip on a combination of light loads and phase loss. Back to it depends.

Bill
--------------------
"Why not the best?"
Jimmy Carter