Calculating generator size for 40HP Motor 1

[phstoon]

I have a customer who wants a generator to start a 40HP 480V electric motor. I used this formula Motor HP xKva per HP x (1000/(Volts x1.73)) = LRA
So:
40x5.59x(1000/480x1.73))
40 x 5.59 x (1000/830.4)
223.6 x 1.204
=269 Amps or 178Kw
I then use a manufacturer's caluclating program and come up with 80 kw which seems correct.
Can anyone tell me why this big discrepancy? Am I doing the math incorrectly?

 

[waross]

The right math on the wrong numbers.
Quick calculation.
40 HP = 45 Amps
45 Amps x 3 = 135 Amps.
135 Amps x 480 Volts x 1.73 / 1000 = 112 KVA
112 KVA x 0.8 = 90 KW
or, depending on the application
45 Amps x 2.5 = 112.5 Amps.
112.5 Amps x 480 Volts x 1.73/1000 = 94 KVA
94 KVA x 0.8 = 75 KW
It depends on an interesting combination of the following factors:
Prime rated or standby rated.
Are there other loads on the generator?
If so, how sensitive are they to voltage dips?
Is this a tight budget or a generous budget.
What percentage of the total load is the motor?
Does it have to work well or do you have to PROVE AND DOCUMENT that it will work well.
Is the set a single phase set, a three phase set or a three phase set re-connected and re-rated for single phase service?
Note:
I installed a lot of sets about 15 years ago using the factor of 3, and for some, a factor of 2.5
All the sets are working well and the customers are happy.
BUT
Within the last year I installed some generator sizing software from a major manufacturer. I ran the loading of as many of the old installs as I could remember as well as an install that I had done to the customer's satisfaction about a year ago. (An old set put back in service. I did not have the option of selecting the size.)
The software informed me that almost all of the installs would not be satisfactory.
Based on my experience and some reasonable assumptions, (no hidden surprises), the 80 KW figure looks good. You must have better software than I had. grin


Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

I agree with Bill, the software provided by a company who paint their generators yellow and black is quite conservative. It's pretty much guaranteed to meet or exceed the load requirements, and perhaps also guaranteed to sell you a slightly larger set than strictly necessary.


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If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

See faq237-766.

Rafiq Bulsara

http://www.srengineersct.com

 

[phstoon]

It is 3 phase 480 volts on a 12-lead reconnectable generator end. There is no other load on the generator.
Question; Where do you get the 40hp=45 Amps and why do you multiply by 3?
How would a PMG or other excitation boost help the application?

 

[waross]

Assumptions:
Full load current: 45 Amps.
Generator: Standby rated.
X 3: Rule of thumb. A generator capacity of three times the motor capacity will start a motor.
If you can tolerate voltage and frequency drops below accepted limits you may also use X 2.5 instead of three, but this is getting into the ART of generator sizing and it may not be wise to use X 2.5 if you don't have some previous experience with generator sizing.
Or use the Big Yellow software and Cover Your ASSets.

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

 

[jraef]

Quick and dirty rule of thumb for estimating 460V FLC for motors >20HP is 1.2A/HP, so 40HP = 48A. I would use that if nameplate data is not available, but that was derived before the advent of modern high efficiency motors. Bill was likely reading a motor data sheet somewhere.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Actually, my SWAG was based on "A little more than 10% of HP at 480 Volts." I checked a motor data sheet selected at random but it seemed low. I checked a reference table but it seemed high. I went with the SWAG. I like your number better than the numbers I found as well. I may use it myself next time (if i remember) grin.

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

 

[rbulsara]

This is another way of saying, treat HP= kVA as a quick rule of thumb. As typical power factor of 0.8 of motors or generators is pretty close to 0.746, the multiplier for HP to kW conversion.

Once you have kVA, it becomes simpler to derive amps.

Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

Rafiq,

Yes, I use that rough estimate myself and it's normally close enough.


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If we learn from our mistakes I'm getting a great education!