VFD to VFD?

Scott-ak

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I have a variable speed ACRA mill that uses a 3 phase to 3 phase Saftronics VS616G3 VFD. I don't have 3 phase in the shop so need to change this out and rewire the machine controls. It would be nice if there was a quick and dirty solution like feeding the existing VFD with a phase convertor VFD. Ever heard of doing that? I suppose rewiring won't be that hard. I suspect that I will have to get a couple contactors and relays to get forward and reverse. The only other feature & component I'm not familiar with is running the analog rpm display on the machines head. Anyone want to weigh in with suggestions and opinions???
 
Every Saftronics VFD I have run across will operate on single phase in.

But, you may need to derate it, so instead of 3 HP it might only be good for 2 HP. Take a look at the documentation. Feeding a VFD with a VFD would not be a good idea, pretty sure it would let the magic smoke out of one or both units.

3 HP rated single phase to 3 phase VFDs are pretty cheap also.

VFDs run fine when fed with a rotary phase converter.
 
UD, thanx, I'll have to look back at how I wired my lathe F & R.

have you ever seen a phase converting VFD feed a 3PH to 3PH VFD?
 
JD, I don't see anything in the manual about using it as a phase converter. The unit is 30 years old, the days before phase conversion with VFD was popular. I look again

I've got a hitachi WJ200 on my lathe and would probably stay with that line of inverters

I don't care for magic smoke of that kind so will take that option off the table
 
A VFD is simply a rectifier that converts the incoming AC current into DC current. Then 3 banks of 2 power transistors that fire in sequence to generate 3 phase AC current from the DC current. The VFD control board tells the power transistors when to fire allowing the controller to change the frequency of the transistors firing hence the frequency of the generated 3ph current.

Some VFD's have wiring lugs for "DC in" or "DC bus" which allows bypassing the rectifier all together and powering the VFD/motor from a large DC source.

A 3 phase rectifier is just a single phase rectifier with 2 extra diodes. Growing up our lead acid battery charger was a variac (variable transformer) with 2 legs of a 3 phase rectifier wired in. My father (an electrical engineer) had a dozen of these huge 3 phase rectifiers, I still have a few left.

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1 phase rectifier vs. 3 phase rectifier

There are a few reasons I can think of that a 3 phase VFD might not work using 1ph AC in:
1) The VFD controller has voltage detect sensors on each of the 3 incoming legs and is programmed to throw an error if it doesn't detect voltage on all 3 incoming legs.
2) A 3 phase rectifier was chosen by the design engineer with diodes JUST barely big enough to supply the needed DC current and losing 1 leg (1/3 of the rectifier) puts the remaining 2 sets of 2 diodes in an over current condition that will eventually over heat them and burn them out.
3) The DC current from the rectifier running on single phase isn't filtered enough (capacitor bank) to provide the smooth DC current the power transistors need.
I have come across #1 but never #2 or #3 in my limited VFD experience.

All the really old VFD's I ever worked with were WAY over spec'd in their design and had plenty big enough diodes in the rectifier to run on single phase (only 2 of the 3 legs powered).

3ph VFD's would run a 3ph motor on 1ph WAY back when before they were ever designed or spec'd to do so. This was just never a function that was considered in the VFD design because a 1KW Allen Bradley VFD cost thousands of dollars so nobody would ever use one for phase conversion to power a 3ph lathe in their garage.

We have big cheap MOSFET and IGBT power transistors to thank for making VFD's affordable for a home/small shop use. I also believe that there are a few IC (Integrated Circuit chips) designed to provide all the control electronics to control a VFD on a single chip and most all modern VFD's use one of these few available control chips.

The power transistor network in a VFD is designed to drive an inductive load (a motor). So while it is possible that running 2 VFD's in series "might" work that really isn't what they were designed to do. Many VFD's specifically state that they are made to only power a motor and nothing else (as opposed to powering 3ph lighting, 3ph power supply, 3ph welder, 3ph VFD, etc.).

Decades ago I tried using a big huge Allen Bradley 3ph VFD to run a motor off of single phase. I discovered that the low voltage DC current to run the control logic for the VFD came from a small power transformer that was powered from legs S1 and S2 so these were the 2 legs that HAD to be connected to the 1ph AC current. Powering S1 and S3 nor S2 and S3 would work to run the VFD on 1ph.
 
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Most modern 3 phase VFD will run off of single phase in a derated mode, a single phase input VFD is pretty much the same as in size/capacity as a larger 3 phase input VFD (usually by a factor of 1.5-2). Older VFD's did have phase loss detection on each leg, so some would not run off of single phase without some trickery to the 3rd leg. Newer VFD's use THD to shut down the VFD on the inputs, 3 phase input has lower THD and also the current draw pulses are less. Derating a 3 phase input VFD for single phase the magic number is around 1.7, but many newer VFD's this can be as high as a factor of 4. In addition you need to look at the output amps under the specific load, derating for carrier frequency, etc. You can fudge the derating a bit and also decrease the fusing requirements by using a DC choke. Most VFD manufactures indicate which terminals to use on a 3 phase input VFD used for single phase, with most smaller VFD's it is not an issue, larger VFD's the fan is sometimes run off the AC from two of the three input terminals.

Not very practical to use a larger VFD to run another one, unless you have something like a phase perfect which just generates the 3 rd leg. An RPC can be used, I have done this quite a few times, there is some derating and can be an issue if the VFD was marginal to begin with. In this case, the most practical scenario and cost effective is just to replace the current VFD with a single phase input VFD and program the outputs WJ200-022SF). I did this on a large plate roller years ago, the new VFD was 1/4 the size of the original one from decades ago.
 
It would be a lot easier with a RPC. You could simply plug your Acra into that, and run it like it was made to be run. No guesswork, no funny derated drives, no rewiring. Just full power and full functionality, plug and play.
 
It would be a lot easier with a RPC. You could simply plug your Acra into that, and run it like it was made to be run. No guesswork, no funny derated drives, no rewiring. Just full power and full functionality, plug and play.
Plus you can buy other 3ph machines as well
 
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