Smouser
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- Amesbury, Wiltshire, UK
I need to spend a day with you Dave for some VFD educationI suspect they do it exactly the way I have:
400v class often have two banks of capacitors wired in series because...
Big electrolytics that can take 600v at high ripple currents are *a lot* more expensive than ones that can take 400v.
By putting them in series the capacitors (or banks of two+) are each only seeing 330v or so, the voltage is split across them (like wiring two 6v bulbs in series to run off 12v) - there are resistors to help keep the voltage equal as well as bleed off the capacitors at power-off.
with 3-phase 400v input, the capacitors (or banks) in series are charged up as one with rectified 400v (about 600v peak) to give 600v bus voltage across them - this then goes off to be chopped up to make a simulated sine wave. This is how they deliver the 600v peak to power a 400v RMS (Root Mean Square) motor - this is easiest to think of as the "area between zero and the sine wave", averaged out the 600v peaks would be a continuous 400v (hope that makes sense).
With 240v input (single or wimpy American 3-phase) the capacitors can only charge to around 330v, so the inverter can't deliver the 600v peaks - so motors have to be wired for 240v (delta) instead of 400v (Star)
For 240 input 400v output, instead of charging the two capacitors (or banks) as one block, each is charged by the rectified 240v separately, one for positive 240v (+330v peak) the other for the negative half of the AC current, also giving 330v peak, but -330v.
The inverter's tapped across the positive and negative ends, giving more than the 600v or so required to drive a 400v motor.
To do this it's quite easy: take the incoming neutral to the point between the capacitors (or banks), the rectifier's + to +v, - to -v, giving 330v on each.
BUT: the capacitors are only being charged half the time on + or - AC waveform, so they have higher charging and discharge currents - either needing to be higher capacitance or reducing their lifespan, and in both cases the current through the rectifier increases. This is why they need to be de-rated, to allow the components leading up to the actuall chopping circuits to survive
My VFD was modded by finding the common point between capacitor banks and fitting a terminal to take mains neutral, feeding live to the rectifier - this is a "voltage doubler", it gives a less well-regulated "raw DC" on the capacitors, so I run a 5HP VFD into a 3HP motor - and it works.
I'm pretty sure (99.9% certain) this is how the commercial 240-415 VFDs do it - if they're thorough they'll fit larger capacitors and a higher-current rectifier, but not so much better that they can justify some of the prices, I doubt the Chinese makers of the reasonably-priced ones milk it as much as some in the US and UK...
Dave H. (the other one)