Sizing KVA of Transformer

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Sparky2791

Sparky2791

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Electrical Design
I know someone on this forum can break this down for me.



Making this simple , here is what I am trying to get a better handle on.



I am told I have (100) 5 HP motors All 208/3PH. Each motor is 3.8KW., .8 PF



Selecting a transformer size to properly feed these motors



3.8 x 100 = 380KW



Now do I divide this by .8 for transformer size?



380 / .8 = 475 so a 500KVA transformer.





Thanks for the reply.
 
You need to create a loading profile.

Will all of your motors be fully loaded at the same time?
Are the motors controlled by a VFD?
How many of the motors start at one time? Do some motors start while other motors are running? How long does it take for the motors to finish accelerating?
 
jim dungar said:
You need to create a loading profile.

Will all of your motors be fully loaded at the same time?
Are the motors controlled by a VFD?
How many of the motors start at one time? Do some motors start while other motors are running? How long does it take for the motors to finish accelerating?
Click to expand...

Yes of course.
Motor Duty factor is already included in the total KW
All motors on VFD's or RVS's
Motors will start intermittently
 
LibertyEngineering said:
Yes of course.
Motor Duty factor is already included in the total KW
All motors on VFD's or RVS's
Motors will start intermittently
Click to expand...

I mean a true loading profile.
100 motors running variable torque load at 50% capacity on VFDs, with 10 motor starting and stopping occasionally, is a different load profile than 100 motors running constant torque loads at 100% capacity with 50 of them starting and restarting every 10 minutes using an RVS with a 30sec acceleration time.
 
LibertyEngineering said:
Yes of course.
Motor Duty factor is already included in the total KW
All motors on VFD's or RVS's
Motors will start intermittently
Click to expand...
The idea suggested by the poster above is the loading profile that most likely describe how your loads behave.
  • Some motors are interlocked with another motor or
  • motors are required to be run simultaneously, or
  • the load will only run on emergencies, or
  • there are loads that act as backup equipment (redundant unit) should the running unit drop off, etc.
You will end up with a very big transformer if you just add the loads and plug in a fudge factor for future loads, IMHO.
 
Lots of issues here. The VFD will nullify the power factor if it’s an AFE or not if it’s diode front end. Need the VFD pF. Also the VFDs will consume power so you need their name plates. As already mentioned if you start and accelerate real slow with a max of 100% FLA on variable torque loads only (fans, centrifugal pumps) then you can size at just 380 / 0.95 or 400 kva. In fact that may be very realistic depending on actual load. If it’s all conveyors all starting and accelerating at say 150% of FLA at the same time you’d need at least 400 x 150% or 600 kVA. You might be able to take into account some acceptable voltage drop though and trim it a little.

Also another major concern here is short circuit. Chances are highly likely that if you put more than 10 VFDs on one transformer you will need to either use lube reactors on every one or semiconductor fuses. This will seriously blow up your budget. You will probably find it’s far more economical to put say 8 VFDs per transformer and run a lot of smaller transformers.

Finally why did you pick the lowest voltage? Those motors are most likely all going to be 9 or 12 lead. So 460 is “free”. You can run smaller VFD, cheaper transformer, cheaper smaller fuses etc.
 
topgone said:
The idea suggested by the poster above is the loading profile that most likely describe how your loads behave.
  • Some motors are interlocked with another motor or
  • motors are required to be run simultaneously, or
  • the load will only run on emergencies, or
  • there are loads that act as backup equipment (redundant unit) should the running unit drop off, etc.
You will end up with a very big transformer if you just add the loads and plug in a fudge factor for future loads, IMHO
Click to expand...
Well honestly this is not an untypical situation where as the designer we get very little information to complete a project. The above is an example I was applying to a waste water treatment plant but I tried to keep it simple in my question. I understand the concept of what jim dungar is asking but that is information we never will receive before designing the service size for the building because honestly I do not know if there is anyway to know for sure how the motors will operate together. What they did provide was a spreadsheet list of motors with their HP and the KW rating (using 746W per HP) plus a column indicating the duty cycle either 100% or 50%. The majority of the motors we listed as 100% duty cycle. I understand we could end up with a transformer much larger than needed but since the actual usage of the motors is not known beyond the duty cycle of each motor we must design around worst case scenario. What I was merely looking to ascertain is if my thinking is correct in converting the total kW of motor load into a kVA size of a transformer. This is the utility transfomer feeding the service to the building.
 
paulengr said:
Finally why did you pick the lowest voltage? Those motors are most likely all going to be 9 or 12 lead. So 460 is “free”. You can run smaller VFD, cheaper transformer, cheaper smaller fuses etc.
Click to expand...
Actual they are 480V motors. The transfomer I am referring to is the transfomer for the entire service to the building. Obviously there would be additional load added for lights and other power.
 
LibertyEngineering said:
What I was merely looking to ascertain is if my thinking is correct in converting the total kW of motor load into a kVA size of a transformer. This is the utility transfomer feeding the service to the building.
Click to expand...

I have never tried to calculate the motor KW, I have simply 1HP output requires 1kVA input.

I am pretty sure someone does have idea of which motors will be running, as flow impacts piping and storage. I would not be surprised that 25% of your listed motors are off-line because they are backup units.
 
paulengr said:
Lots of issues here. The VFD will nullify the power factor if it’s an AFE or not if it’s diode front end. Need the VFD pF. Also the VFDs will consume power so you need their name plates. As already mentioned if you start and accelerate real slow with a max of 100% FLA on variable torque loads only (fans, centrifugal pumps) then you can size at just 380 / 0.95 or 400 kva. In fact that may be very realistic depending on actual load. If it’s all conveyors all starting and accelerating at say 150% of FLA at the same time you’d need at least 400 x 150% or 600 kVA. You might be able to take into account some acceptable voltage drop though and trim it a little.

Also another major concern here is short circuit. Chances are highly likely that if you put more than 10 VFDs on one transformer you will need to either use lube reactors on every one or semiconductor fuses. This will seriously blow up your budget. You will probably find it’s far more economical to put say 8 VFDs per transformer and run a lot of smaller transformers.

Finally why did you pick the lowest voltage? Those motors are most likely all going to be 9 or 12 lead. So 460 is “free”. You can run smaller VFD, cheaper transformer, cheaper smaller fuses etc.
Click to expand...
On the regenerative VFDs, it is best to ask the manufacturer if the design of their VFDs contribute to the fault currents upstream! Majority of the VFDs I know are VSIs (non-regenerative with diode front-ends) as the input diodes blocks the current that might flow back! Still, it would be wise to ask the manufacturers for clarifications!
 
LibertyEngineering said:
Well honestly this is not an untypical situation where as the designer we get very little information to complete a project. The above is an example I was applying to a waste water treatment plant but I tried to keep it simple in my question. I understand the concept of what jim dungar is asking but that is information we never will receive before designing the service size for the building because honestly I do not know if there is anyway to know for sure how the motors will operate together. What they did provide was a spreadsheet list of motors with their HP and the KW rating (using 746W per HP) plus a column indicating the duty cycle either 100% or 50%. The majority of the motors we listed as 100% duty cycle. I understand we could end up with a transformer much larger than needed but since the actual usage of the motors is not known beyond the duty cycle of each motor we must design around worst case scenario. What I was merely looking to ascertain is if my thinking is correct in converting the total kW of motor load into a kVA size of a transformer. This is the utility transfomer feeding the service to the building.
Click to expand...
Then, my best advise for you is to sum up the kVA and you're good to go!
Never use the conversion formula (HP x 0,746 = kW and divide by PF)! You should use the rated Amps x Volts/1000 = kVA instead!
 
jim dungar said:
You need to create a loading profile.

Will all of your motors be fully loaded at the same time?
Are the motors controlled by a VFD?
How many of the motors start at one time? Do some motors start while other motors are running? How long does it take for the motors to finish accelerating?
Click to expand...
Does the NEC allow someone to decide sizing based on a loading profile? I know OP said transformer, and NEC doesn't address transformer sizing technically, but 450.3 would limit transformer undersizing.

OP said this is the utility transformer, not sure why he is sizing the POCO transformer, unless he works for the POCO.
 
electrofelon said:
Does the NEC allow someone to decide sizing based on a loading profile? I know OP said transformer, and NEC doesn't address transformer sizing technically, but 450.3 would limit transformer undersizing.

OP said this is the utility transformer, not sure why he is sizing the POCO transformer, unless he works for the POCO.
Click to expand...
That is a good point NEC Article 220 for service calculations would apply. We are sizing the utility transfomer because customer is taking service from POCO at 12470V so they will own these utility transformers. There are many buildings over a large campus. This is the first building.
 
LibertyEngineering said:
That is a good point NEC Article 220 for service calculations would apply. We are sizing the utility transfomer because customer is taking service from POCO at 12470V so they will own these utility transformers. There are many buildings over a large campus. This is the first building.
Click to expand...
Maybe we shouldn't be calling them "utility transformers" then 😉
 
electrofelon said:
Does the NEC allow someone to decide sizing based on a loading profile? I know OP said transformer, and NEC doesn't address transformer sizing technically, but 450.3 would limit transformer undersizing.

OP said this is the utility transformer, not sure why he is sizing the POCO transformer, unless he works for the POCO.
Click to expand...
The NEC has rules for sizing service entrance conductors, feeder conductors, branch circuit conductors and their protective devices. The NEC does not provide guidance on sizing transformers themselves. Given proper terminations it is allowable to have a properly primary side protected 10kVA transformer feed a secondary 1000A switchboard.

You are correct the OP is dealing with feeder and branch circuits not service entrance circuits.
 
jim dungar said:
You are correct the OP is dealing with feeder and branch circuits not service entrance circuits
Click to expand...
How is it not considered the service entrance when it is the service coming into the building from the Pad Mounted Transfomer. I guess I should look up definition of Service in Article 100
 
LibertyEngineering said:
How is it not considered the service entrance when it is the service coming into the building from the Pad Mounted Transfomer. I guess I should look up definition of Service in Article 100
Click to expand...
The NEC defines the service entrance as the demarcation point for the power company.

If there is no power company (utility) there is no true service entrance which would follow NEC article 230.

Try not to let your industry slang get in the way when applying NEC articles, try to use the NEC definitions instead.
 
I agree the terms "utility and service conductors" should not be used in this context.
transformers have to be sized per a article 220 load calc.
the motors fall under 220.50
 
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