generator size for 60 hp motor

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arits74

Senior Member
Location
dixie arkansas
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working owner electrician
small near by town wants to put a generator on their city water well motor is a 60 hp 3 phase 230 volt hollow shaft motor.motor is a code c motor .generac says they need locked rotor amps of motor to size a generator.locked rotor amps for this motor is 582 by my math. generac says a 150 kw generator is only good for 324 locked rotor amps.motor nameplate says 146 running amps but is only pulling 100 amps.thoughts?the city says another electrician told them they only needed a 40 kw generator so now i have to work this out.
 
By my math, 40 kW can supply 100 amps @ 230V, 3-phase. Perhaps the other electrician took a look at his clamp-on ammeter, saw 100 amps, and did a quick calculation based on that?

I can't think of any situation where the Code allows you just take a quick reading and use that in your load calculations. The closest would be 220.87 (which I'm doubtful even applies to your situation), and even then you have to use 125% of the peak demand obtained from a minimum 30 day continuous load test.

That's not even mentioning the fact that just because a generator can handle the running current, doesn't mean it can handle the starting current. You seem to know the right way to go about sizing the installation; now you have to figure out how to explain it to the City's representative so that he or she will understand. Unfortunately, that's usually the hard part, especially if there's another guy whispering in their ear that you only want a bigger generator so you can gouge the city for a bigger profit. My advice is to try to get a one-on-one, face-to-face meeting, and then take things slow to make sure they understand your explanation. If they won't believe you, you can always politely invite them to give you a call when the 40 kW generator stalls and dies when they try to use it to start their well pump.

One final thing to consider is the generator's warranty. I haven't looked too carefully at the warranty conditions, but I'm pretty sure you can void the warranty by connecting too large of a load to the generator (that includes a motor with too high of a locked-rotor current). In other words, if Generac says 150 kW is too small but you think it will still work okay, you may be voiding the warranty if you use it. Just one more thing to consider.
 
arits74 said:
small near by town wants to put a generator on their city water well motor is a 60 hp 3 phase 230 volt hollow shaft motor.motor is a code c motor .generac says they need locked rotor amps of motor to size a generator.locked rotor amps for this motor is 582 by my math. generac says a 150 kw generator is only good for 324 locked rotor amps.motor nameplate says 146 running amps but is only pulling 100 amps.thoughts?the city says another electrician told them they only needed a 40 kw generator so now i have to work this out.
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I did not work out any numbers for your 60 HP motor, but in my experience using a generator to back up a single large motor load is generally not very cost effective due to the inrush issues of a single large motor load. Six 10 HP motors would be a whole different matter.
There is another option to consider for this. Since this is a vertical hollow shaft pump, you can covert this to a a right angle gear drive to couple a gas or diesel engine to, and then your electric motor sits on top of the drive. All the parts and pieces are commercially available to do this as it is fairly common thing to do. The total overall cost will almost certainly be much less than a genset for this application.
 
Texie

The first thing I thought of was going to a gas or diesel pump but I had no idea it could share the same pump. Nice way to go except for the OP. :D
 
Duh...I think I just got you meaning....it will cut the EC out of the work?
Kidding aside, this is actually a pretty common thing to do with vertical shaft pumps in many applications as they readily lend themselves to this. I've done a number of them over the years. But, yes there's not much in it for the EC in conversion like this.:)
 
texie said:
I did not work out any numbers for your 60 HP motor, but in my experience using a generator to back up a single large motor load is generally not very cost effective due to the inrush issues of a single large motor load. Six 10 HP motors would be a whole different matter.
There is another option to consider for this. Since this is a vertical hollow shaft pump, you can covert this to a a right angle gear drive to couple a gas or diesel engine to, and then your electric motor sits on top of the drive. All the parts and pieces are commercially available to do this as it is fairly common thing to do. The total overall cost will almost certainly be much less than a genset for this application.
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they have the exact set up you speak of and use a tractor with a driveshaft hooked to the gearhead,but they also want a generator also.
 
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arits74 said:
small near by town wants to put a generator on their city water well motor is a 60 hp 3 phase 230 volt hollow shaft motor.motor is a code c motor .generac says they need locked rotor amps of motor to size a generator.locked rotor amps for this motor is 582 by my math. generac says a 150 kw generator is only good for 324 locked rotor amps.motor nameplate says 146 running amps but is only pulling 100 amps.thoughts?the city says another electrician told them they only needed a 40 kw generator so now i have to work this out.
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What is the motor starting method?
What is the intended genset fuel?
How many expected starts per hour?
My calculator indicates a 182 KW genset is required to reliably and consistently start a 60HP 3 phase DOL start motor.




The other electrician referenced is just mistaken, no make that wrong.
Undersized gensets starting motors will definitely shorten the prime mover and generator head life or more often with larger gensets and motors, cause generator head winding shorting and complementary fireworks.
This constitutes equipment abuse and is not covered by most warranties, I say most because somewhere some entity will honor the warranty, but I personally have never seen or know of a genset warranty repair/replacement granted on an overloaded or abused unit.
 
arits74 said:
they have the exact set up you speak of and use a tractor with a driveshaft hooked to the gearhead,but they also want a generator also.
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Ok, I think get a picture of a rural setting and the present setup is cumbersome. That said, they already have the makings of what they need if they want it to be a little more user friendly. If the run time from the backup (the gear drive), and this does not represent a lot of run time hours they could consider a propane fueled permanently installed engine. If the run time on the backup is more significant you could consider a properly sized diesel engine. If unattended startup upon power failure is also an issue, there are also solutions for this with the proper controls and a magnetic clutch. In my view this will be far more reliable and cost effective than a genset will be in an application like this.
Keep in mind that a genset large enough to start this will be running at significantly less than rated capacity and lead to very inefficient operation, and in the case of a diesel engine genset will lead to serious engine issues due to extended partial loading. In short, a genset for an application like this is a very poor choice.
 
I would think about using a soft start or a VFD to ramp the current up to the running current, but even then I would not use the 100 amp figure as when the motor and pump get older they can tend to pull more current, so I would use the motors FLA current with a little head room, 80 kva should do it.
 
hurk27 said:
I would think about using a soft start or a VFD to ramp the current up to the running current, but even then I would not use the 100 amp figure as when the motor and pump get older they can tend to pull more current, so I would use the motors FLA current with a little head room, 80 kva should do it.
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While I might agree with your thoughts, in the big picture, for this application, and the equipment that the OP says is already there, a generator is a dumb idea for this application. My whole point in my comments is that in a situation such as this , with a single large motor, a generator is a poor choice when the load could be simply driven directly by an IC engine, especially given that he already has a right angle gear drive. I realize we are electricians, but sometimes a mechanical solution is best. By the time you install all the gear, soft start, ATS, etc, for one motor you are looking at a lot of money and terrible efficiency in operation and also from a capital cost standpoint.
 
texie said:
While I might agree with your thoughts, in the big picture, for this application, and the equipment that the OP says is already there, a generator is a dumb idea for this application. My whole point in my comments is that in a situation such as this , with a single large motor, a generator is a poor choice when the load could be simply driven directly by an IC engine, especially given that he already has a right angle gear drive. I realize we are electricians, but sometimes a mechanical solution is best. By the time you install all the gear, soft start, ATS, etc, for one motor you are looking at a lot of money and terrible efficiency in operation and also from a capital cost standpoint.
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An IC engine has an efficiency of less than 30% whereas an electric drive may have efficiency greater than 80%. So a standby power supply from the utility instead of IC engine or generator, if feasible, is still better economical decision.
 
texie said:
While I might agree with your thoughts, in the big picture, for this application, and the equipment that the OP says is already there, a generator is a dumb idea for this application. My whole point in my comments is that in a situation such as this , with a single large motor, a generator is a poor choice when the load could be simply driven directly by an IC engine, especially given that he already has a right angle gear drive. I realize we are electricians, but sometimes a mechanical solution is best. By the time you install all the gear, soft start, ATS, etc, for one motor you are looking at a lot of money and terrible efficiency in operation and also from a capital cost standpoint.
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In all probability this is an unmanned, secluded, rural installation.
My experience with diesel IC engines used to operate as a backup to an electric motor is the mechanical issues with getting the engine to start, engage and properly drive the pump without human intervention and then when the utility electricity is restored, disengage, shutdown the IC engine and resume electric motor power again without a human present.
 
Sahib said:
So a standby power supply from the utility instead of IC engine or generator, if feasible, is still better economical decision.
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This is a ridiculous suggestion given what we know about the installation.


Just where is this standby power source going to come from that is not effected by an utility outage.

Are you going to have the utility run a new medium voltage circuit for miles just to run a single 60 HP motor?
 
iwire said:
Just where is this standby power source going to come from that is not effected by an utility outage.
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The pump is already run by an IC engine. To provide another IC engine or a new generator as standby is wasteful, if utility power supply is available or if there is feasibility of obtaining the same. If there is utility power outage, the IC engine would take care of the pump.
iwire said:
Are you going to have the utility run a new medium voltage circuit for miles just to run a single 60 HP motor?
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I already said,''if feasible''. Ask the right question.
 
iwire said:
This is a ridiculous suggestion given what we know about the installation.


Just where is this standby power source going to come from that is not effected by an utility outage.

Are you going to have the utility run a new medium voltage circuit for miles just to run a single 60 HP motor?
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Might I humbly suggest that you don't get into a spat with him?
It could lead to an infraction from a mod...........:p
 
Sahib said:
The pump is already run by an IC engine.
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No, the pump is run by electricity with IC back up via a tractor PTO.

To provide another IC engine or a new generator as standby is wasteful, if utility power supply is available or if there is feasibility of obtaining the same. If there is utility power outage, the IC engine would take care of the pump.
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You may want to go back and read the thread again.
 
i got a little more info today from a previous city employee.he says they have an old military surplus type generator that they used to use on this well motor,and its a 60 kw unit.the city is applying for a government grant to cover most of the cost of this project.thanks for the replys
 
Sahib said:
The pump is already run by an IC engine. To provide another IC engine or a new generator as standby is wasteful, if utility power supply is available or if there is feasibility of obtaining the same. If there is utility power outage, the IC engine would take care of the pump.

I already said,''if feasible''. Ask the right question.
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I do not see that in the OP's original post.
Given it is a water supply it may fall under 701 instead of a 702 install. Other options may not be available.
 
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