Generator Sizing

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Tainted

Tainted

Senior Member
Location
New York
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Engineer (PE)
Hi everyone,

A customer wants (4) 2HP motors to be on a generator. Nothing else will be on that generator. My understanding is that you size the generator based on the locked rotor amps + continuous loads. If all 4 motors are going to be on at the same time, do I just add up all the locked rotor amps and size the generator based off of that? or do I just do it based on 1 largest locked rotor amp? The motors are dual rated 208V and 230V.
 
retirede said:
It will make a difference whether or not they will start simultaneously.
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So let's assume the 1HP motors have a LRA = 80A, would this mean the generator will have to be sized for 320 starting amps? Because they will start at the same time
 
Wow.
Stagger start would be better.
It’s the difference between a 37.5kVA and a 150 kVA
 
Hv&Lv said:
Wow.
Stagger start would be better.
It’s the difference between a 37.5kVA and a 150 kVA
Click to expand...
what do you mean stagger start? I was thinking of running 2 portable generators to feed the (4) 2HP motors, meaning I will have (2) 2HP motors per generator using 50A outlets. But the LRA is my question.
 
Tainted said:
what do you mean stagger start? I was thinking of running 2 portable generators to feed the (4) 2HP motors, meaning I will have (2) 2HP motors per generator using 50A outlets. But the LRA is my question.
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Start them at different times is what I mean by stagger start them. I’m just talking about 30 seconds or so difference.
You start them all at once your generator will have to be 4-5 times bigger than if you stagger start them.
 
Hv&Lv said:
Start them at different times is what I mean by stagger start them. I’m just talking about 30 seconds or so difference.
You start them all at once your generator will have to be 4-5 times bigger than if you stagger start them.
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Yea I understand that would be ideal, but there's these motors are not meant to be turned on at different times unfortunately. Does that mean I have to add all the locked rotor amps together and find a generator that can handle it?
 
Tainted said:
Yea I understand that would be ideal, but there's these motors are not meant to be turned on at different times unfortunately. Does that mean I have to add all the locked rotor amps together and find a generator that can handle it?
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Yep
 
electrofelon said:
Im not so sure about that. I think the "size the generator to the LRA" idea is an oversimplification and generally not really true. For example, I am looking at a spec sheet for a cummins generator, and it shows motor starting KVA at almost 4 times over generator nameplate rating.
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is that the same case for portable generators? What if I have (2) 2HP 15FLA motors on a portable generator, the LRA each motor has is 80A, what portable generator do I need? I was looking at Generac cutsheets but I’m ok with Cummins too
 
electrofelon said:
Im not so sure about that. I think the "size the generator to the LRA" idea is an oversimplification and generally not really true. For example, I am looking at a spec sheet for a cummins generator, and it shows motor starting KVA at almost 4 times over generator nameplate rating.
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Correct. Which is what I said in post #6
 
What type load? fixed displacement compressors? Fans? machinery drives? What type motors.? etc.
Why simultaneous starts, 1 second separation would allow single LRA rating, once one is running, the others start easier.
Lots of information missing. If absolutely simultaneous start needed (e.g lifting 4 corners of a load) then look as intertied VFD for the motors.

You need an EE - wait, you say your are one?
 
junkhound said:
What type load? fixed displacement compressors? Fans? machinery drives? What type motors.? etc.
Why simultaneous starts, 1 second separation would allow single LRA rating, once one is running, the others start easier.
Lots of information missing. If absolutely simultaneous start needed (e.g lifting 4 corners of a load) then look as intertied VFD for the motors.

You need an EE - wait, you say your are one?
Click to expand...
It’s 2 motors built into a unit, it has a controller but it doesn’t look like a VFD to me
 
Tainted said:
is that the same case for portable generators? What if I have (2) 2HP 15FLA motors on a portable generator, the LRA each motor has is 80A, what portable generator do I need? I was looking at Generac cutsheets but I’m ok with Cummins too
Click to expand...
You could consult with the manufacturer. They probably have data on motor starting. I think they even have software where they can analyze motor starts and should offer that service of sizing it. That said if you were talking a small portable generator they might not have detailed information on it or be able to offer much help. I know my 5500 watt generator will start my air compressor, not sure what the horsepower is I will see if I can find it.
 
electrofelon said:
You could consult with the manufacturer. They probably have data on motor starting. I think they even have software where they can analyze motor starts and should offer that service of sizing it. That said if you were talking a small portable generator they might not have detailed information on it or be able to offer much help. I know my 5500 watt generator will start my air compressor, not sure what the horsepower is I will see if I can find it.
Click to expand...
I just did that this morning, looked at their control wiring and it confirmed they do not start at the same time lol
 
Tainted said:
is that the same case for portable generators? What if I have (2) 2HP 15FLA motors on a portable generator, the LRA each motor has is 80A, what portable generator do I need? I was looking at Generac cutsheets but I’m ok with Cummins too
Click to expand...
To make it simpler, when choosing generators, try looking for the gensets starting kVA rating as well as the generator percent impedance. That is because even if you have enough engine power to provide for starting your motors, enough voltage dip could occur and your motor starters will drop out!
 
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