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Single phase VFD
- Thread starter gonzo
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No, I'm not failing to take it into account, it's just not relevant.
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Voltage is "The potential difference" between the two legs! If one leg is at +100v and another leg is at +60v at a point in time then the POTENTIAL DIFFERENCE is 40v at that point in time. Two of the generated 3 phase legs from a VFD will never be 180 degrees apart. They will only be 120 degrees apart so the biggest potential difference between the two legs will never be the sum of the two legs, It is the DIFFERENCE between the two legs.
Get rid of the yellow leg since we are only talking about 2 of the 3 phase legs. Draw a vertical line between the red and blue legs at any point on the x axis of the graph. The DIFFERENCE between the red and blue legs will be the output voltage.
If we are in a universe where the laws of physics are not relevant then you are CORRECT!
Get rid of the yellow leg since we are only talking about 2 of the 3 phase legs. Draw a vertical line between the red and blue legs at any point on the x axis of the graph. The DIFFERENCE between the red and blue legs will be the output voltage.
If we are in a universe where the laws of physics are not relevant then you are CORRECT!
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Mike,
Take a look at this and you'll have a better idea of how VFD's work. If you look at the image, all the input is turned into DC first. Then back to AC at whatever voltage the VFD specs. The conversion is much more sophisticated than a simple single phase/three phase.
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Then back to AC at whatever voltage the VFD specs - WRONG.
Unless the input AC current is run through a step up transformer before the rectifier or there is some sort of DC boost circuit between the rectifier and the output power transistors the VFD control can not arbitrarily make the output voltage ANY voltage it wants. It is limited to the DC voltage it has to work with.
If what you said were true I would be able to buy a cheap 240v VFD and run a 480v 3 phase motor with it.
I think there might be a built in 2x step up transformer between the AC input and the Rectifier on your 1ph 120v to 3ph 208v VFD. If said VFD is putting out more than 208v across any two output legs then the step up transformer is greater the 2x.
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Sorry, it is always AC voltage*1.414 for single phase AC = 311VDC output for 220V AC input.
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Yep, on this detail I am wrong and you are right. I will admit when I am wrong.
(arg... I knew that one too
)
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I gave a link to a single phase 120 in / 240 volt 3 phase VFD several posts back. I guess you ignored that. Hard to explain when you’re ignoring me
But you really had it right, “unless it runs it through a boost circuit”. They do. So a 240v in VFD can output 240v three phase.
Of course you’ve also agreed that we get 311VDC on rectifying 220. So do we really need that boost circuit?
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I know how a VFD works! You are ignoring the 120 degree offset between any 2 output legs of a vfd which you can not do!
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Mike
One last try. I’m not ignoring the phase difference of the output legs. I’m trying to explain why the output legs are not constrained by the input legs. The input all gets first converted to DC, which has no phase reference. Then that DC is used to construct the output.
A transformer is not the only way to convert voltage.
The only applicable law of physics is conservation of energy. Otherwise the electronics can create whatever output voltage they want.