Adding VFD Power and Motor Power to size Transformer?

[Peakhunter]

Does anyone know if it is correct to add both the VFD FLA with the Motor FLA to properly size an upstream transformer?

Most of the calculations seem to ignore the VFD completely yet it is downstream of the transformer.

 

[ptonsparky]

I would size the transformer as if it were an across the line start for the motor(s). That will be the most severe loading of the transformer. Do not add both.

 

[iceworm]

Assuming you mean the VFD is feeding the motor.

430.122 requires the conductors to be rated for the 1.25 x VFD input as opposed to 1.25 x T430.250 value. I don't ever recall seeing the VFD input to be less than the T430.250 value

And then there is 430.122.B for VFDs with bypass to use 1.25 x art 430.6 value. Anybody that wants to jump in here and explain that one clearly would be met with gratitude.

So, yeah, use 1.25 x VFD input, plus, as tom said, upsize the xfm to that required for maintaining Voltage under starting current.

 

[Jraef]

VFD rated input power is actually always lower than the motor FLA, because the VFD effectively corrects the motor power factor to .95.

You can actually size the transformer SMALLER for a VFD than if it were Across-the-Line, but of course if you do size it for Across-the-Line then you are building in some serious future growth potential (or allowing for bypass of the VFD). This is because the VFD will reduce or can even totally eliminate the motor starting current (it's the only technology that can do that). So if you sized the transformer for the VFD input kVA, you are fine, so long as you can take as much time as necessary to accelerate the motor. If you NEED fast acceleration, then size the transformer for 150% of the VFD rating. That will be the maximum possible current that the VFD can handle anyway.

 

[bwat]

To echo the statements made here already with a real world example I just had a few months ago, I had (2) 450HP / 480V motors being driven from VFDs with maybe 50-75VA of other auxiliary loads at a site. 450HP is 515A from table. We were concerned that a 1MVA transformer might be cutting it too close to supply all of this and also left no room for growth, so we up-sized it all the way to 1.5MVA. It turns out we would have probably been just fine at 1MVA based on actual startup and running current measurements during commissioning. I don't think we ever even break 1000A (~830kVA) at full load, and starting is nothing but a smooth ramp up in current.

Our motors were driving centrifugal pumps with a very friendly ramp up time in the VFD, which goes to Jraef's point about the VFD, load, and starting characteristic playing a role in virtually eliminating the starting current when using a VFD in certain applications.

 

[Jraef]

15 or so years ago I had a project for an oil pipeline in central California that involved 4 x 2500HP 4160V pumps and when originally built in the 1950s, they started them Across-The-Line. But now every time they started a pump, the lights dimmed in the nearby town. So they proposed Soft Starters, which is where I got involved. But even with those, the software we used to determine voltage drop still showed greater than 5% on startup of the first one, almost 20% starting #4 with the other 3 running. The only thing that would work was using VFDs and each VFD was able to accelerate its pump without ever exceeding the motor FLC. They didn't need to vary the speed so these were the world's most expensive soft starters, but it solved the problem.