Sizing Transformer for 2000HP, 4160V motor 2

[buzz53]

What size of transformer is neccessary for a 2000Hp, 4160V motor utilizing reduce voltage start?

 

[waross]

2000 Hp x 750 /1000 = 1500 KVA / .8 (power factor) = 1875 KVA
2000 Hp x 750 /1000 = 1500 KVA / 1.0 (power factor) = 1500 KVA
Use the impedance of the transformer to determine the voltage drop under starting current. You may want to go to a larger transformer or one with a lower impedance to improve starting voltage. You can improve the power factor with a capacitor bank to reduce the current drawn from the transformer.
yours

 

[aolalde]

At least 10% Service Factor and assumming it is an Induction motor will lead to 2000 kVA Transformer min.

 

[jraef]

2000kVA is a minimum for running it, but a quick and dirty rule of thumb is 1.5xHP to avoid a voltage drop of over 5% at start-up if you don't know anything else about the motor, starting method, load conditions, transformer and supply system. That would make it 3500kVA.

To be done correctly though, you really should do a study that includes a transient motor starting analysis. I use SKM software Link to SKM TMS. I have also used a good one from one of our posters, Marke, which is much more reasonably priced (US$22 vs US$3,000!). LM Photonics Electrical Calculations Software link. It compares well to the SKM unless you want to do more than just that one motor and know a lot more about the system.

There is also a freebie available from GE for use with autotransformer starters GE Engineer's Tools link, and you can adapt it by plugging numbers. It is an Excel spreadsheet that requires you to enable macros, so do that at your own risk, but I have it and I didn't have any problems. I have compared its results against my SKM software and thought the GE freebie was a little loose however.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[JBD]

Output HP *.746w/HP = Output kW.
Output kW/effciency = Input kW
Input kW/power factor = Input kVA

The above formulas give the continuous (running) kVA of the motor. Of more concern is the inrush (starting) current of the motor and the subsequent voltage regulation of the transformer.

1.5 kVA/HP seems to be a time proven estimate.

 

[waross]

Hi friends.
My suggestion was for a Bare minimum, and I did suggest that the size may have to be increased after considering the impedance of the transformer.
aolalde
Don't forget, it is a reduced voltage start. I have no problem with 2000 KVA.
Re 5% voltage drop on starting. Agreed but I have never seen the 5% rule applied to an industrial installation.
With combined loads, I agree that a larger size may be required for motor starting, but I understood that this was One motor, one transformer.
Have I been away too long?
3500 KVA seems like expensive over-kill.
On the other hand it has been a long time since I was involved with code compliance for starting motors large enough to "Dim the lights."