Generator Sizing

[adamscb]

I'm sizing a generator we will be renting at my food processing plant. This generator will feed an MCC with a total of 440hp, so approximately 550A. My question is do I have to size generators according to the inrush upon startup as well? This could mean the difference between a 1MVA and 500kVA generator.

 

[mgookin]

Someone smarter than me can help you better but the first question that comes to mind is do they all start at the same time?
Is there something (like a logic program) to prevent them from all starting at the same time, for example, upon loss and restoration of power?

 

[adamscb]

These will be started separately. They're not on the same process 'system', but are still fed from the same MCC. So I don't see a reason why we'd start them at the same time.

 

[texie]

I'm sizing a generator we will be renting at my food processing plant. This generator will feed an MCC with a total of 440hp, so approximately 550A. My question is do I have to size generators according to the inrush upon startup as well? This could mean the difference between a 1MVA and 500kVA generator.

Genset sizing for a load such as this is not for the uninitiated and there are a lot engineering judgments that need to be made. All the major players have very good sizing software available for their machines. Notice I said "their" machines as all gensets of a given size do not have the exact same characteristics.

 

[JoeStillman]

It can be done longhand pretty easily. You need to calculate the motor starting kVA at each step. A good rule of thumb is 6 x FLA on startup.

1. Start the largest motor first (if you can; process will dictate.)
2. With that motor running at full capacity, start the next largest motor. Add its starting kVA to the first motor's running kVA.
3. With motors 1 and 2 running, start motor 3. Add its starting kVA to their running kVA.
4. Lather, rinse, repeat.

The step with the highest motor starting kVA will help determine if a generator rated for the running kVA will be able to start all the motors in order.

This looks like a job for Spreadsheet Man!

 

[JoeStillman]

 

[Ingenieur]

I'm sizing a generator we will be renting at my food processing plant. This generator will feed an MCC with a total of 440hp, so approximately 550A. My question is do I have to size generators according to the inrush upon startup as well? This could mean the difference between a 1MVA and 500kVA generator.

the alternator starting kva needs to be > than the motor starting kva
this can be found in the spec sheet

when doing this the gen usually ends up being 1.5 to 2 times larger than the steady state load

you can stage staring to minimize this

free sizing software http://www.kohlerpower.com/industri...onNumber=13261&nodeNumber=1&contentNumber=104)

 

[Ingenieur]

typical 500 kw data sheet http://www.kohlerpower.com/onlinecatalog/pdf/g5355.pdf
page 2 has

Peak motor starting kVA: (35% dip for voltages below)
480 V 5M4024 (10 lead) 1350
480 V 5M4027 (12 lead) 1550

480 V 5M4028 (10 lead) 1800


let's say you want to start only 200 HP and starting kva is 1200 kva, any one of these gensets/alternators will do it
if the HP is 400 and starting kva is 2500 they won't
you need to up size

here's the 750 kw http://www.kohlerpower.com/onlinecatalog/pdf/g5385.pdf


Peak motor starting kVA: (35% dip for voltages below)

416 V, 480 V 5M4032 (10 lead) 2200 (60 Hz), 1650 (50 Hz)

416 V, 480 V 5M4034 (10 lead) 2600 (60 Hz), 2000 (50 Hz)

the 5M4032 alternator is too small
the 5M4034 should do it
but you still may want to go one size larger for reliability and lower stress on the unit