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How to size transformer for VSD?

Published date: 09 October 2018

In most simple way, size transformer power  that is designed for ATV supply as 1.5-2.5 times ATV power. To be precise, transformer kVA power has to cover active power consumption, reactive power and harmonics, also ovelroad if required by aplication. 
In case of ATV, cos phi is equal to 1, so there is no reactive power. But the harmonics have great influence on transformer power. 
ATV process without a passive filter achieves around 40% of THDi level (example: datasheet of ATV660C63Q4X1 shows 38%). And as you know, power factor is expressed as PF=1/sqrt(1+THDi^2), which in case of 40% THDi result is PF=0.93 . Besides that, consider rated ATV power as a power that reflects mechanical power on motor shaft. E.g. 220kW motor will take 220kW + motor losses from ATV. And ATV will consume 220kW+motor losses+ATV losses and all multiplied by PF. 
In case of heavy duty where ATV can go overload, apply overload factor. 

For instance ATV660C63Q4X1 in heavy duty. Motor is 500kW. ATV660C63Q4 input power is:  500kW + motor losses + 14kW (losses of ATV660C63Q4X1 from datasheet). Lets estimate motor losses and get ATV input power around 550kW. Then by applying power factor 0.93, the apparent power gives 550/0.93= 592kVA. As alternative, the apparent power not need to be calculated if available on datasheet. For ATV660C63Q4, the data shows 588kVA in heavy duty.

Next, on transformer you probably do not want to go to the same overload (1.5 times for 60secnds in HD) as with the motor, although transformer alows certian overload as well.
Therefore calculate transformer apparent power as overload from your application cycle * 588kVA. If you use max cycle with 1.5 times overload, I recommend to size transformer for 1.5*588kVA = 882kVA

For ATV660C63Q4X1 in heavy duty, use se transformer with rated power at least 882kVA
When it comes to transformer voltage, thing about the voltage drop on cables, chokes, etc. For 400V motor I would propose 415-420V transformer secondary rated voltage.

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