VFD on Line vs. Load Side of Transformer

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sch

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Texas
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Electrical Engineer
Hi, I have a 480/277V service and need to run a several hundred horsepower motor at 2300V. My currently intent is to have a 480/277V distribution board, then a 480:2400V transformer, then a VFD, then the motor. This would be switchboard->step-up transformer->VFD->motor. I understand that this drive is very expensive, so I was wondering about placing the VFD between the board and the transformer. This would be switchboard->VFD->step-up transformer->motor. This option would have a sine wave filter. I have heard mixed thoughts on this approach. It would be cheaper but difficult on the transformer due to harmonics or losses.

Are there any opinions on this set up? I just want to make sure I am considering all angles and accounting for potential issues.

Thank you all in advance.
 
When frequency varies in a distribution transformer, increasing frequency leads to smaller transformer size but higher core and winding losses, while decreasing frequency causes the transformer to operate as if overvolted, increasing magnetic flux, core losses, and risking saturation, making the transformer run hotter and potentially failing. Transformers are designed for a specific frequency, and operating outside of this range can lead to inefficiency, overheating, and damage.
 
A motor is essentially a transformer with a rotating secondary. When you change the frequency driving the motor you have to simultaneously change the applied voltage. So even though the transformer is being used at lower frequency, it won't be overvolted because the voltage is simultaneously reduced. So at first glance this will work just fine.

Here is a link to a previous discussion of this topic. Ignore the back and forth and just look at @Jraef 's post, which goes into some of the details that you need to be aware of:

480 VFD feeding a 480\4160 stepup transformer

I share Besoeker's concern. _If_ the motor is designed to operate at 4160V and 60Hz, and is fed with a 480V to 4160V 60 Hz transformer...and then the VFD is being operated at 48Hz with an appropriately reduced output voltage, then the transformer is probably fine. Perhaps a suitable filter on...
forums.mikeholt.com
 
My thoughts were to place a line reactor pre transformer on the primary side 480 vac. This would reduce the reactive power on the primary, reducing the load on the transformer (then drive) itself. Thoughts?
 
Why does it HAVE to be at 2300V? A several hundred HP motor at 480V 3 phase is not that bad of an idea. I mean a 300 HP motor is only 361 amps and requires a 600 amp breaker.
 
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