Selection of a generator

Daer Signode, Welcome to the forum. There are few more inputs which would decide a size ( Is it ( generator) going to be on "Continuously" or is it standby duty. If you are running the motor on generator 24 X 7 then you have to oversize it a bit (around 15-20%). I suggest you use a softstarter instead of a Y-D starter. That will help you in reducing the Kva rating of Generator as the starting would be smooth. ( Y-D starters are not popular nowadays since the cost of softstarters have reduced). I would suggest a 300 kva Gennie with softstarter for 24 X 7 operation. Good luck, Chaterpilar

All reputable gen set suppliers have a sizing program where they plug in the code of your motor (such as Code G) and all the info you have supplied plus what type of voltage and frequency dip you can live with and it picks a unit. Altitude, ambient temp, humidity (slightly), fuel type (natural gas - diesel - gasoline - etc) as well as alternator characteristics (continuous duty, other loads on line when you start the big motor, etc) all effect the engine selection, alternator selection, cooling selection. An engine may perform at high altitude (due to the turbocharger) but the generator may overheat as the fan is blowing 15% less dense air at high altitude. And if you specified 'standby' duty, you will be at 130 *C rise on the name plate and will not have any extra copper to absorb heat at a high altitude, whereas if you specify continuous 80*C rise you'll have some extra copper. Different engines inside the same supplier portfolio react differently to load, so it is not a simple formula. There are different turbocharger pressure ratios across different models and wildly different across different manufacturers. Their is no easy solution if it is miss sized, especially on the small ones where you are using all the cylinders.

Check my profile (click my name). I won't even give you a rule of thumb formula today when 20 years ago i would have. The engines are different today.

Is that your only load?

If it is, then Genset should be 100KW+ with Speed Governor resonse times~5 Seconds for 0>100% load step.And Excitation Control same order.

But again-if this is the only load-why a genset+Motor+Star Delta---at all?

Why not directly couple Prime mover to final load?

Or if you do not want to change conceptions drastically-- at least do not bring in the STAR-DELTA racket-go electronic soft start.

JRaef explained so well only last week!

The primary issue is ,if the motors are under load when starting .My sawmill runs off a 100kw generator ,initially I start the 125 hp motor that drives blade with no load .I use a home made step starter that consists of two starters with a time delay which is essentially a soft starter at 10% of the cost .I can still run another 25- 50 hp because once the 125hp is spinning there is plenty of starting power available.

I was told to stay around 125% or 100 kw runs 125 hp

These are only practical applications that you might apply although as the other posts suggested there are allot of variables .