Transformer Sizing for Large Motor and Several Small Motors

[plangetee]

Hello all,

I'm looking to size a transformer to power the following loads. The power source is 460 VAC, 3ph, 60 Hz.

Loads:
Four (4) 1-phase 59.8 kVA Trans-Rectifiers
40.0 HP total of 3-phase motors
72.0 KW total of 3-phase heaters

The Maximum (460 V) Phase-Current I calculated at 470.18 Amps for the above loads. You may take this number as a "given". I used 480 V system voltage in this calculation.

Also to be powered from the SAME Transformer:
One (1) 200-HP Pump.
This load is "Across the Line" (Full-Voltage) and there is no option to make this motor a Wye-Delta start or any other "soft-start" method to reduce in-rush current upon start. The in-rush current is 1380 Amps.

The 200 HP Pump is the main concern in this problem. The 200 HP Pump will be turned on and off six times a day. The other loads on the transformer will NOT be turned off and on during the normal course of the day - these loads will always be on.

I do not have the option into splitting the loads into two different transformers.

I am trying to find a suitably-sized power transformer to power all these loads. The main concern is the 200 HP Pump here. After doing some calculations I thought a 750 kVA will be suitable for all these loads after doing a few calculations...however I dont have a great basis for these calculations.

I would like some professional opinions on what formulas to use to calculate the transformer necessary to power the loads with the constraints given. If you could cite the source from which you developed your calculations that would help greatly.

Thanks in advance!!

 

[jraef]

An across-the-line starting transformer rule-of-thumb I have used successfully for 25+ years is;
Minimum transformer kVA = 3X motor HP.

So in your case, the smallest xfmr I would consider would be 600kVA with no other loads. 750kVA would provide a little fudge factor to that, but what I would also do is subtract the kVA of the constant loads. Not accounting for PF on the motors, you have 225kVA already on when you want to start the 200HP motor, you might want to consider a 1000kVA transformer.

The proper way to do this though would be with a good TMS study using SKM or ETAP.


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