select gen set for start pumps 1

[gaux]

hi,
I want to start up 3- 300 kw (272 start up current cos 0,7) motor, 1- 90 kw ( 68 A start up current, cos 0,72), 1- 450 kw ( 396 A start up current cos 0,7) and the voltage is 4160 Volts... we are thinking on starting up with direct start and a generator driven by a diesel motor.

thanks a lot

 

[davidbeach]

Gen set manufacturers give away software for just that purpose.

 

[gaux]

ok, but i´ll like to know your opinion. thanks in advance

 

[davidbeach]

I don't have an opinion, I'd just use the sizing program and then probably go a size larger. My guess is that you will be in the 2MW-2.5MW range. Will also depend on starting order and spacing.

 

[gaux]

where could i get a sizing program on internet? yes, my mechanical calculations,talks me on 2193 kva, and 1500 kw nominal, but if i get all except one, in example 450 kw i get with the starting current a 3,8 MVA... how could i size the gen set? i don´t know....

 

[davidbeach]

Go to the web site of your favorite gen set manufacturer, or call their local rep in your area.

 

[waross]

My rule of thumb is the sum of the KVAs of all the motors running plus 300% of the next motor to start. If the last motor to start is not the largest, check each step to ensure that at no point in the starting cycle do you exceed the sum of the KVAs of the running motors plus 300% of KVA of the motor starting. The more motors you have, the more forgiving this sizing is.



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

 

[gaux]

thanks a lot...

 

[gaux]

and if we use a soft starter for each motor?

 

[waross]

With soft starters you may get away with 300% current limit and use the same rule of thumb but with less voltage and frequency upset as each motor starts.
If this were my project, I would run the numbers and see how the demand matches available Gen-Set sizes. The set selection may be a no-brainer. However, with soft starters, if I was close on a smaller Gen-Set, I may remind myself that the starting current is lagging the running current quite a bit and the total current is less than the sum of the parts. I may convince myself that I can go as low as 125% of total load. But, 125% of total may be more than the original rule of thumb.
When I consider the problems I have had with a lot of undersized sets (sized by others) and the problems I have not had sizing sets with my rule of thumb and the problems that were anticipated (by suppliers) but didn't materialize with over sized sets, I tend to avoid cutting corners with Gen-Sets.
If this will be prime power, seriously consider adding another 10% on top of everything. The extra 24% may make the difference between 15,000 Hrs and 30,000 Hrs or more between major engine rebuilds. You will probably calculate the hours per year of expected running and compare that figure to 15,000 Hrs and 30,000 Hrs.


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