Need to size generators

[inthedark]

SETUP:
32 pumps
480V, 16A, 3 ph, 10hp (from vendor, not sure if it makes sense......p = vi sqrt3 = (480V)(16A)(1.73) = 13286.4 VA
13286.4 VA / 746 W/hp = 17.8 hp, right?)

Assuming V & A correct:
(13286.4 VA x 1.25 safety) / pump x 32 pumps = 16608 VA x 32 = 531,456 VA = 532 kVA

Looking at generator rating, 3 ph:
82 kVA, 480 V, 98.6 A, 0.8 pf

Is it as simple as 98.6 A (generator rating) / 16 A (pump rating) = 6.1625 pumps per generator, so, 6 generators needed

OR

532 kVA / 82 kVA = 6.49 pumps per generator, so, 6 generators needed, again

As you can see, I am not an electrical engineer nor an electrician. Can anybody help with the proper approach and solution?

Thank you all in advance.

 

[davidbeach]

Might work if you never have to start the pumps. Best bet would be to contact the generator supply for their sizing program and use that to determine how many would run on one generator. Weird size though, where did 82kVA come from? Even better, why not just get the right size of generator for the job?

 

[rbulsara]

Hire good electrical consultants. How do you feel or think when an electrical engieer starts designing our civil engineering or environmental projects??

 

[dpc]

A good approximation for converting motor hp to electrical requirements: 1 hp = 1 kVA.

But you still have to deal with motor starting.

If you send your load data to gen set suppliers, they will probably be happy to give you an estimate of the required gen set size.

 

[aolalde]

The full load requirement for 32 motor-pump array will be:

32*1.732*0.480*16 = 425.65 kVA and your PF around 0.8

If you decide to have one large unit or several small units the starting sequence should be considered for inrush purposes.

For instance if only one Generator is selected and every motor is started in sequence (next motor started after the previous has accelerated to full speed), the starting requirements are:

31*1.732*0.480*16 + 6.5kVA*10HP = 477.36 kVA and your PF around 0.8

For four Generators, assuming motors start in sequence. (Only reason for four is 32/8 = 4), the starting load will be:
7*1.732*0.480*16 + 6.5*10 = 158.11 kVA and PF around 0.75, each of four generators.

However, if the eight motors start at the same time, the inrush load on each one of four generators will be:

8*6.5*10 = 520 kVA and PF around 0.4

Some generators will allow overload for a few seconds, then the size could be reduced but only after having the manufacturer specifications and the allowed voltage drop you could decide on that subject.

 

[rbulsara]

inthedark:

Only comment on your first calc. The motor is 10HP so it can't be 17.8HP as you arrived at. That should raise a flag to you. The key there is power factor. W=VA*pf. Assume a p.f. of .75 to .8 and redo your calc. It will be close to but never exact ct as actual HP may be slightly different than nameplate rating.

If interested refer to faq237-766.

 

[waross]

I don't understand where you got the 82KVA figure, but based on that;
98.6 amps/16 amps = 6.1625
4 pumps = 64 amps plus 34.6 amps starting allowance.
3 pumps at 100% current and one starting at a little over 300% current.
With motors only and no secondary loads to be affected, I would push it to 5 pumps with a starting allowance of a little over 200%.
Three phase generators in this size range are generally rated at 80% power factor.
80% of 82KVA=65.6 KW.
That is an allowance of 13.12 KW for each of 5 pumps.
10 hp = 7.5 kw. As long as the motor efficiency is greater than 57% you have adequate power.
I think a question that is just as important if not moreso than "How many motors?" is "Why are the motors drawing such high currents?"
Are the motors very inefficient?
Is the power factor of the motors very bad?
(Probably both)
Can the power factor of the motors be improved?
What is the actual horsepower load on the motors? If the pump curves show that the motors are running at 50% loading, you may be able to drive almost twice as many with the same generator.
I think that you may want to consider having your whole situation, (motor sizes, motor currents, generator sizes etc.) reviewed by a competent consultant.
respectfully

 

[mjd2000]

" How do you feel or think when an electrical engieer starts designing our civil engineering or environmental projects?? "

Lets stick to what we are good at.

Well said rbulsara.