Transformer size required to start a 300HP, 460V motor

[petersonra]

I never even heard of benshaw. We use ab, Siemens, and Schneider soft starts, and once in a blue moon motortronics. Customers do not want to pay most of the time for more than absolutely necessary. I have never had any trouble with the smaller contactors and we have a bunch of them in the field. I never had any trouble with the soft starts that only have two legs of scrs either. They seem to work just fine. Sometimes cheaper is more than adequate and more money does not get you much more than a thinner wallet.

Of late there seems to be a trend toward putting VFDs on just about everything. They are cheap enough these days that especially in smaller sizes there is not a ton of difference between the cost of a VFD and the cost of a contactor and overload when you consider you can avoid some mechanical engineering time and maybe a gear box.

 

[excited_electrone]

The IEEE Red Book has a chart that shows the voltage drop by motor size based on the transformer feeding it for across the line starting. It is very useful for questions like this one. I don't have my book handy or I would post the chart.

Do post it whenever you get a chance! Thanks!

 

[d0nut]

Thanks for the reminder. I had forgotten about posting the chart.

 

[bwat]

Thanks for the reminder. I had forgotten about posting the chart.

This is great. Thanks for posting.

It backs up my earlier post of saying 500kVA might not cut it for 300HP (if you're trying to use the limit of 15% VD, such as NEC 695, on starting). You'd need at least 750kVA for that. And if you're truly starting 2 ATL, at different times, I wouldn't go less than 1000kVA.

 

[petersonra]

might be cheaper to get a smaller transformer and put in vfds. especially if your poco does not allow atl starts of motors of this size.

 

[Julius Right]

The are a few details necessary to final solution-if you need 500 or 750 kVA transformer. You need to know the utility 5 kV supply system apparent short-circuit power, the overhead line length and the total load on the transformer-or switchgear. A distance from transformer to switchgear and from switchgear to motor etc. As I calculated for 200 MVA system, 2 miles overhead line and 30-50 ft low voltage cable and 75% transformer load you need 750 kVA 6% in order to limit the start voltage drop to 15%.

 

[JEFF MILLAR]

Rent a generator, for temporary power