Load calculations for transformer sizing help

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DaXoXz

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Hi, I'm new to this forum. I'm an electrical engineer student, i'm currently trying to size a transformers bank to feed the intrinsic 480v loads of a MPU Turbogenerator.

The following loads are connected into the MCC's 480v Bus bar.

Pre/post lubrication Motor/pump: 2 hp, 460vac, 60hz, 1750 rpm, 2.9 A, p.f 0.8
Enclosure vent fan #1 Motor: 15 hp, 460vac, 60hz, 6.7 A, p.f 0.85
Oil cooler vent fan #1 Motor: 5hp, 460vac, 6.7A. 79%
Start Motor : Duty: 15 minutes inverse service 1760 RPM, 231 A. 0-150 hp, 0-460 vac, 0-4000 RPM.
VFD, Start Motor speed control: 50/60 hz, 0-150 hp, 0-460 vac, 0-4000 rpm
PCR Air conditioner:
Indoor Motor 1/3 HP, 460VAC, single phase, 1.1 A FLA
Outdoor Motor 1/5 HP, 460VAC, single phase, 1.4 A FLA
Compressor 460VAC, 3- phase, 39 LRA and RLA 5.5

1. I would normally use the apparent power equation S = sqrt(3) * VL * I , but i also read that with motors you need to include motors efficiency and power factor with the following:

S = (hp * 0.7457) / (P.F * EFF)

What's the most accurate way to do it? It's for my thesis.

2. How does the VFD affects the load calculation?

3. To size the bank should i add up the kVA and divide them by 3? And how should i do the voltage drop at secondary for each single phase transformer?

I know maybe i'm asking for a lot, i was supossed to size a 3-ph transformer first but they told me it needed to be a 3T bank and i'm confused on how to proceed.
 
Last edited:
DaX -
I have never had to write a thesis on load sizing. However I'm guessing you have to justify any assumptions/methodology you use. If that is the case, consider:
The equipment appears to be all North American Standard
Use the National Electrical Code to size the loads. In particular, do not use the motor name plate data, rather use article 430 tables for motor currents.
Check the NEC section on Adjustable Speed Drives for sizing the starter/VFD load. I don't have the NEC in front of me, however, I'll guess the loading is VFD nameplate current plus 25%.. If the VFD nameplate data is not available, I would use the NEC table 430 motor current for a 150 HP motor (plus 25%).

Motor starting inrush should not be a problem. All the motors are running before the starter motor is energized.

Add up all the currents, perform your calcualtion for Apparent Power S = sqrt(3) * VL * I

The motor efficiencies or power factors are not a concern. The transformer only cares about the currents. That is what causes the heat.

For one transformer, pick the next available size up from S. For a 3t bank, use S/3 for each. And, of course you will likely want to round up to the next available size.

As for voltage drops, consider the 3t bank as black box for a 3ph transformer. The equipment/loads can't tell the difference.

Here is a check for your calculations, I expect you will be in the 225kva range for the total.

Minor aside:
This is a pretty standard application for a 5MW TG. It is what I would do if I had to stamp the drawings.
Caution - University professors are known to not have any stable connections to reality.

Let us know how it comes out
 
iceworm said:
DaX -
I have never had to write a thesis on load sizing. However I'm guessing you have to justify any assumptions/methodology you use. If that is the case, consider:
The equipment appears to be all North American Standard
Use the National Electrical Code to size the loads. In particular, do not use the motor name plate data, rather use article 430 tables for motor currents.
Check the NEC section on Adjustable Speed Drives for sizing the starter/VFD load. I don't have the NEC in front of me, however, I'll guess the loading is VFD nameplate current plus 25%.. If the VFD nameplate data is not available, I would use the NEC table 430 motor current for a 150 HP motor (plus 25%).

Motor starting inrush should not be a problem. All the motors are running before the starter motor is energized.

Add up all the currents, perform your calcualtion for Apparent Power S = sqrt(3) * VL * I

The motor efficiencies or power factors are not a concern. The transformer only cares about the currents. That is what causes the heat.

For one transformer, pick the next available size up from S. For a 3t bank, use S/3 for each. And, of course you will likely want to round up to the next available size.

As for voltage drops, consider the 3t bank as black box for a 3ph transformer. The equipment/loads can't tell the difference.

Here is a check for your calculations, I expect you will be in the 225kva range for the total.

Minor aside:
This is a pretty standard application for a 5MW TG. It is what I would do if I had to stamp the drawings.
Caution - University professors are known to not have any stable connections to reality.

Let us know how it comes out
Click to expand...

Hi Iceworm, your reply was very helpful, thank you (fr)! I'm going to discuss it with my industrial tuthor but i think that's absolutely the way to go, my mind was all over the place after reading different national and international standards and guides.

I'm actually sizing the backup system of the black start generator for 5.2MW TGs and will design it's installation. I will post my calculations after i discuss this. When i have it all then im sending it to my University professor which i hope he aproves lol. Thank you again
 
iceworm said:
DaX -
I have never had to write a thesis on load sizing. However I'm guessing you have to justify any assumptions/methodology you use. If that is the case, consider:
The equipment appears to be all North American Standard
Use the National Electrical Code to size the loads. In particular, do not use the motor name plate data, rather use article 430 tables for motor currents.
Check the NEC section on Adjustable Speed Drives for sizing the starter/VFD load. I don't have the NEC in front of me, however, I'll guess the loading is VFD nameplate current plus 25%.. If the VFD nameplate data is not available, I would use the NEC table 430 motor current for a 150 HP motor (plus 25%).

Motor starting inrush should not be a problem. All the motors are running before the starter motor is energized.

Add up all the currents, perform your calcualtion for Apparent Power S = sqrt(3) * VL * I

The motor efficiencies or power factors are not a concern. The transformer only cares about the currents. That is what causes the heat.

For one transformer, pick the next available size up from S. For a 3t bank, use S/3 for each. And, of course you will likely want to round up to the next available size.

As for voltage drops, consider the 3t bank as black box for a 3ph transformer. The equipment/loads can't tell the difference.

Here is a check for your calculations, I expect you will be in the 225kva range for the total.

Minor aside:
This is a pretty standard application for a 5MW TG. It is what I would do if I had to stamp the drawings.
Caution - University professors are known to not have any stable connections to reality.

Let us know how it comes out
Click to expand...


I wanted to ask something about NEC 430.6 (A) exception No.3:

Exception No. 3: For a listed motor-operated appliance that is marked
with both motor horsepower and full-load current, the motor full-load
current marked on the nameplate of the appliance shall be used instead
of the horsepower rating on the appliance nameplate to determine the
ampacity or rating of the disconnecting means, the branch-circuit
conductors, the controller, the branch-circuit short-circuit and groundfault
protection, and any separate overload protection.


Does this exception count for motor driven oil pumps? motor-driven fan? If anyone knows feel free to reply, thank you!
 
DaXoXz said:
I wanted to ask something about NEC 430.6 (A) exception No.3:

Exception No. 3: For a listed motor-operated appliance that is marked
with both motor horsepower and full-load current, the motor full-load
current marked on the nameplate of the appliance shall be used instead
of the horsepower rating on the appliance nameplate to determine the
ampacity or rating of the disconnecting means, the branch-circuit
conductors, the controller, the branch-circuit short-circuit and groundfault
protection, and any separate overload protection.


Does this exception count for motor driven oil pumps? motor-driven fan? If anyone knows feel free to reply, thank you!
Click to expand...

NEC 430.6(A) Exception 3 merely says if there is a marking on the motor nameplate stating the HP and the full-load current, the motor full-load current figure should be used in determining the ampacity of the conductors, the rating of the disconnect, etc. FLA calculations based on the motor HP should not be used.
 
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