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Discussion Starter · #1 ·
So I under stand what a VFD is capable of, and I am aware of how it does most of what it does, but something that has bothered my for some time now is that I can't seem to understand HOW some of them can create a 3 phase load off of a single or "2 phase" line, can anyone give me an explanation just to ease my mind??
 

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Simple way to put it...i guess... Single phase input charges capacitors. You discharge caps
So I under stand what a VFD is capable of, and I am aware of how it does most of what it does, but something that has bothered my for some time now is that I can't seem to understand HOW some of them can create a 3 phase load off of a single or "2 phase" line, can anyone give me an explanation just to ease my mind??
If you have any electrical understanding then this simple explanation will help.
You feed a vfd single phase, turn it into dc and PWM a new 3 phase via transistors. I left out some stuff. but like i said, its a simple explanation.

Sent from my VS995 using Tapatalk
 

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Discussion Starter · #5 ·
Simple way to put it...i guess... Single phase input charges capacitors. You discharge caps If you have any electrical understanding then this simple explanation will help.
You feed a vfd single phase, turn it into dc and PWM a new 3 phase via transistors. I left out some stuff. but like i said, its a simple explanation.

Sent from my VS995 using Tapatalk
:vs_OMG: gotcha, makes so much sense, can't believe I didn't get that, thanks
 

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Discussion Starter · #9 ·
Google and YouTube teaches a lot.

Sent from my VS995 using Tapatalk
could not find an explanation as to how, just that it did, figured asking on here someone would have an answer to be helpful and could ask a very specific question and get a very knowledgeable answer
 

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Some of the oldest VFD's, and optionally yet today, totally skipped the AC input and just connected to a common DC bus.
 
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The way I explain it in my class is that to the VFD, the AC input is simply the "raw material" from which the DC is made. Think water into an ice maker; the ice maker doesn't care if the water comes in through 3 pipes or one. The VFD doesn't "care" if that raw material comes in as single phase or 3 phase (or ABC vs CBA rotation for that matter). All it does is make DC from it.

What DOES matter between single and 3 phase input however are two issues:

1) How the VFD derives it's CONTROL power it needs to operate the printed circuit boards. SOME drive use an AC input Power Supply board on the drive that takes it's power from two of the 3 incoming lines. So if you feed single phase to that design, you have to make sure your single phase lands on the correct two terminals.

2) When rectifying the AC to DC, there is "ripple" in the DC after the rectifier. The VFD then has capacitors that smooth out that ripple, otherwise the transistors may misfire. With a single phase input, the ripple is MUCH higher, so you need a lot more capacitance to smooth it out. In addition, the capacitors have to work harder and therefore get hotter. The general rule of thumb then is for the capacitors to have TWICE the rating for single phase input as for 3 phase input, IF the drive also employs a DC bus inductor to help with the smoothing. If no BC bus inductor is present, as is the case on small cheap drives, the capacitors must be de-rated by 65%, OR 50% + the ambient temperature must be de-rated by 35%; i.e. a drive rated for 40C operation must be kept at 25C if only de-rated 50%. This by the way is the #1 killer of small drives, because there is a LOT of misinformation out there about this de-rating issue.

It should be noted though that MANY of the VFD manufacturers have a version of their small (3HP and below) 240V drives that are ALREADY designed for single phase input, meaning they have factored in the capacitance needed for the rated ambient. On many 3 phase-only drives this is never the case, especially at 480V, so what some mfrs do is to use a Phase Loss detection scheme that will not allow the drive to operate on single phase input. If you cannot override that trip, you cannot use those drives on a single phase input.
 
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