NEC Power Exam 114

[StinkyTofu]

Hi All,

I'm having a hard time understanding the problem and what is given/being asked. We are given an AC voltage of 679 V. Is this the RMS Line to Line voltage (or average or peak)? How do we know that this is equal to the load Peak Line to Line voltage? I've been tring to figure this out using the following resource (page 12)

https://cas.web.cern.ch/cas/Warrington/PDF/Visintini.pdf

Can anybody lend an insight or share good resources to share? Thanks in advance!

 

[Ryan P.E.]

The basics of a VFD drive are: AC supply -> Rectifier Section to DC -> Inverter Section to PWM AC

The voltage V=679V across the DC link is a DC voltage, not AC. If you imagine closing 2 transistors to complete a phase-to-phase circuit, the peak voltage phase-to-phase of the "PWM AC" waveform is 679V.

Additional real world example: You can probably imagine that 679V comes from 480V *sqrt(2). Just be careful since the DC link is not always going to be sqrt(2)*the rated LL voltage of the VFD. A VFD vendor gave a training presentation for Medium voltage drives we just bought and in the beginning when they talked about theory, they mentioned their 480V drives and said the DC link was 679V using the formula. When they talked about their 4160 medium voltage drive's DC link voltage, it was not 4160*sqrt(2), so don't always make the assumption. I am just bringing this up since the numbers 480 and 679 are obviously related so it can give you more insight into VFDs