Generator size for a 125HP induction type motor

[SANFRID]

I would like to run our 125hp induction motor using generator, and with a use of a soft starter, what size of generator should i use. I have a 270KVA standby generator, would this be sufficient since i have a soft starter to use. pls advice, thanks.

 

[itsmoked]

That should be enough. However, some generators freq-out when fed a soft starter's input. The pulsating load causes the generator's voltage regulator to spazz-out. You also have said nothing about the other loads on the generator. Any other loads will certainly impact the situation.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Spazz-out... is that an official ANSI / IEEE term there Keith?

A very gross rule-of-thumb is that if you are using a solid state starter (notwithstanding Keith's valid concerns), the generator kW should be at 1.5 - 2X the HP rating of the motor (2 - 2.5X the kW rating), unless it is the ONLY motor on there and you can accept a lower than normal voltage drop, i.e. the soft starter will not drop out on low voltage/low frequency. So you should be fine with 270kW for a 125HP motor all by itself.

To Keith's concern. most generator mfrs have now solved the problems with non-linear loads on the generators, they had to by virtue of the ubiquitous nature of VFDs and soft starters. So they added more/better filtering to the AVR sensing circuit. But since you didn't provide any details, for all we know you are planning on using a 15-20 year old or a really cheap genset and that might rear its ugly head for you.

Then in addition to Keith's concern on the effects of a solid state soft starter on the generator, there are some soft starters that themselves run afoul of being used behind a portable generator as well. Not so much any more because most are now digital, but you didn't specify any details and if your plan is to use an older analog soft starter, you could run into that too. Some older analog and early digital designs used a single Phase Locked Loop circuit to determine the zero-cross point of the incoming sine wave of one of the phases so that they could coordinate the SCR gate timing. But frequency drift made them "spazz-out" too and begin to misfire. Again, problem solved for the most part but if you use older stuff, you may run up against it.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376