What size generator do i need??

[syence]

I've been tasked to order a 500 kW genset 460V/3 Phase/ 60 Hz to test fans we installed for our project. Permanent power is currently not available, hence the need for the generator. Before placing an order for this monster, I need confirmation that it will be sufficient. The 5 Fans are scheduled to tested one at a time, but we might be asked to test them all concurrently.

Specs for the largest of fan motor: 150 HP , RPM 1800, 460V/3 Phase/60 Hz. I can not find any information on the motor data sheet that spells out the amperage requirement. And unfortunately this info is what's required by the company renting me the generator. My questions: 1) Can the amperage requirement be determine from the HP and Volts?? If so, how would I figure this out?? 2) How much total amperage can a generator of this size provide??

Any help would be greatly appreciated.

Thank you,

Civil Engineer in an Electrical world

 

[X49]

You should get the motor amps directly from the manufacturer, but typically a 150kW 460V 3 phase motor would be 170-180A. Table 430.250 in the NEC estimates the typical full load current of such a motor at 180A. The generator's likely rated at around 750A.

I expect that the supplier will suggest a 650kW unit or larger.

Getting clarification on how many fans are required to run concurrently during testing could allow you to rent a much smaller genset.

 

[KllrWolf]

Have you looked at the motor nameplates? They should have the operating current on them.

You can get an estimated amperage with the following equation. If you have the motor efficiency and/or power factor, it is much more accurate. If you do not have them, you can look at the motor manufacturers website to find motors like them to get estimated values.

I=(((HP * .7456999) / eff.) * 1000) / (460 * 1.73 * p.f.)


The generator vendor would be able to tell you more about the ratings of whichever generator you are looking at.

 

[waross]

A quick estimate for generator sizing is HP x 3 = KVA of generator.
150 HP x 3 = 450 KVA.
A 500 KW generator should be capable of 625 KVA so you are safe.
I have several other generator sizing rules of thumb and they all say that a 500 KW generator will be adequate.
A standby generator should be rated at 3 times the amperage of the motor. If the generator is prime rated, it will be capable of 10% more current than rated for one hour out of ten. This is a great assistance for motor starting.

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

 

[syence]

Thanks for the replies. Very helpful.

Follow up:

x9: are suggesting that I can get away with a smaller generator for this size motor? If so, given the motor specs, what's the smallest generator I can rent for this application?

waross: you suggest the at the generator should be sized for three times the HP. If that's the case, then a 500 kW genset would not have the capacity to start any of the other 4 motors that are slightly smaller (125 HP, 100 HO, 50 HP, 50 HP)??

Again, I really appreciate your input.

Thanks

 

[burnt2x]

Check your math. 500 kW = 670HP. Best to start your motors one after the other, 500kW is more than enough.

 

[ScottyUK]

Cat's SpecSizer program reckons this would need an 800kW / 1000kVA prime-rated set to keep terminal voltage within 15% of nominal, assuming the motors are sequentially started. I am pretty sure Cat are conservative with their sizing, and you'd probably get away with a smaller set.

Obviously if you decide to relax your voltage tolerance a little further than I have then things get easier, but watch for contactors dropping out if you take control power from the generator bus. By Cat's reckoning a 500kW set will see a voltage dip well below 80% and frequency will drop to about 85% of nominal.

 

[waross]

When sizing a generator, you need about 3 KVA per HP to start a motor and 1 KVA per HP to run a motor.
Add up all running motors at 1 KVA per HP. Then use 3 KVA per HP for the motor to be started. Once the motor is started, its demand drops to 1 KVA per HP.
This is the simplest sizing that I can suggest and is slightly conservative compared to the sizing for many installations I have done. I installed a couple of dozen standby sets almost 20 years ago. All worked well and many are still in service. Cat's SpecSizer says that ALL will fail.
500 KW will deliver 625 KVA.
That is over 4:1 for the 150 HP motor.
When the 150 Motor is running there are 475 KVA available to start the 125 HP motor.
That is 3.8:1
Add the 125 HP motor and you have 350 KVA available to start the 50 HP motor.
That is 7:1
Now you have 300 KVA available to start the second 50 HP motor.
That is 6:1
All of the ratios of motor KVA to available generator KVA are well above 3:1.
I don't see a problem with starting the motors in the order of largest to smallest on a 500 KW/625 KVA set.
"I am pretty sure Cat are conservative with their sizing, and you'd probably get away with a smaller set."
Yes Scotty. Very conservative. Another little detail that is often not considered.
With the set running in droop control, it will be 3% fast with no load. You can probably subtract that 3% from the frequency dip when starting the first motor.
If the set has a Permanent Magnet Generator supplying power to the Automatic Voltage Regulator, the voltage dip may well be less also.
Yours
Bill

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

 

[ScottyUK]

Bill,

Anyone would think that Cat are in the business of selling gensets...

 

[waross]

Yes I agree.
The irony is that when motor large starting is part of the load, it is often hard to get enough load to keep the diesel engine happy. I installed an Onan set that almost never went above 25% load and often ran at 10% loading. The supplier threatened to void the warranty on the basis of under-loading.
However when that one large motor started, it took everything the set had and we had voltage and frequency dips. Over conservative software doesn't do much to avoid wet stacking.
I believe that gen set sizing may not be simply a matter of adding up equipment ratings but may include an understanding of the characteristics of the loads and a consideration of how they will interact. Some understanding of the care and feeding of a diesel engine may be helpful also.
Despite the economies of a turbo charger there are some installations that will perform better with a normally aspirated engine. When that one large motor took the Onan set from 10% load to 100%+ load, the turbo lag was not our friend.

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