gen sizing
gen sizing
I'll give it a shot...I have the Onan sizing program.
I plugged in the three motor loads, in three steps (1 15hp each step)for a non simultaneous start. You really need the NEMA code letter A-thru V (very important). I assumed Nema code letter G, starting across-the-line.
The 25 kva single phase load was entered as a receptacle type load with .9 power factor and entered into step 1, motors followed in steps 2, 3, 4. If a deviation is req'd from these assumptions, it changes everything.
You never mentioned fuel type so:
Nat. Gas = 85GGHG with PMG excitation 125 deg temp rise
Diesel = 80DGDA with PMG excitation 125 deg temp rise
Propane = 70GGHF with PMG excitation 125 deg temp rise
You get a slight reduction from the 100 kw unit you had previously. Very important to note... you stated the motors will not start simultaneously. Once you need to start two or more simultaneously you need a much larger unit, approximately 125 kw, such as would occur with manual jogging.
You could look at the 25 kva load more closely with regard to diversity, if you are interested in downsizing further. For arguments sake I removed the 25 kva load entirely and ran numbers again (therefore just considering the motor portion of the load):
Nat. Gas = 60GGHE with shunt excitation 125 deg temp rise
Diesel = 50DGCA with shunt excitation 125 deg temp rise
Propane = 60GGHE with shunt excitation 125 deg temp rise
So purely for arguments sake you need a genset with capacity rating somewhere between these two size groups, based on the diversity of the 25 kva single phase load.
If your looking for a reduction from the full load size group, Solid State soft start type starters on the motors would help, although I'm not sure how practical that would be, with consideration to starts per hour, the amount of jogging or bumping could exceed the duty rating of an otherwise addequately sized starter.... buts lets entertain the notion that you can use them, therefore motors loaded in 3 steps, with the 25 kva single phase load in step 1. I assumed the starters would be set to a maximum inrush of 300% of RLA, with length of ramp up set accordingly:
Nat. Gas = 70GGHF with PMG excitation 125 deg temp rise
Diesel = 80DGDA with PMG excitation 125 deg temp rise
Propane = 75GGHF with PMG excitation 125 deg temp rise
Remember the last group is utilizing soft Starts, not across the line, WITH the 25 kva single phase load included.
Well that was fun, I hope it helps.
MM
an added note: The natural gas downsizing from ATL to SS starters looks like it was achievable with an extended stack and full single phase output incorporated into the genset. Those type of things cost money, this is where a generator sales engineer could be helpful.