Can I power a vfd using another vfd

Hello there, I’ve poured over information on this forum and elsewhere online… But haven’t been able to find a specific answer to my question.

I’ve spent a lot of time over the past few years going to scrapyards, industrial wasteland’s, and buying things from government online auctions. One of my interests is motor drives and other industrial controls.

I’ve got a humble lab setup, but I’m very proud of it. I’ve repaired most of the gear myself, and have a various assortment of oscilloscopes, power supplies, and other gear.

However one thing I haven’t experimented with yet… is phase converters with an input of 460V three-phase.
I have several large drives such as this, ranging from 10 to 30 hp.
For example the one on my workbench this evening is a Telemecanique square D atv58hd in beautiful shape with very low hours.


A bit of backstory on my set up which will hopefully clarify my needs…

I have a separate 200 amp panel that I ran for my workshop, which gives me plenty of power to do most of my projects… And I’ve done a lot of testing with smaller VFD‘s and motor drives.

I guess I’ve acquired a few dozen in the past couple of months… With several more of the large 460 V three-phase input drives included in that lot… To add to the ones I already had. Trying to rack my brain and remember… But I’ve done quite a bit of experimenting with drives up to 10 hp and even 20 hp. But these were all single phase and put three phase output.

Now to my concerns…

I was curious if I could use one of my pre-existing single to three phase converter’s for the input into a larger drive... just for the sole purpose of testing the larger drive? I obviously don’t have a need for such large drives… But before passing them on to others would like to be able to test them. It’s just fun to mess with these industrial things, and I can’t ever see needing anything over five horse power for wood Shop or MACHINE SHOP.

I have several loads, including three-phase motors ranging from very small.... to 50 hp. Also have a few devices that were specifically built for testing in A professional environment. Like breaking units for three phase motors so that they can be tested while heavily loaded....along with their drives. These come in very handy… But so far I don’t have a way to get the three phase 460 V.

I built my workshop onto the back of my home, and it’s about 4 feet off the ground on big concrete pylons. So I just ran a few conduit pipes down under the floor and put my load motors under there. I’ve got a 1 hp, a 5 hp, 25 hp, and a 50 hp motor. Like I said it’s just a hobby, and a lot of times this stuff is new in the crate and costs very little and sometimes cost nothing at all. The most I pay is around $0.10 per pound, sometimes getting it much much cheaper than that. So it’s hard to pass up on a beautifully made 50 hp three-phase motors new in the crate… Even if I don’t have much of a use for such a thing LOL just fun and I enjoy learning.

I have all kinds of ridiculously large step down transformers… One of which in particular is a very modern unit (maybe five years old ) made by Square D. It is tapped to step down 460V down to 220 and 110. I guess it weighs about 500 pounds, and I believe it’s good to 45 kV. I have ran it in reverse several times, taking into account the de-rating insulation for running in reverse… As they specify in their documentation.

A few months ago I Purchased a beautiful Hammond power supply, (model BPS103A) capable of putting out 0-125 VDC and 0-240 VAC in single phase or three phase. Was hoping this would solve a lot of my power needs, it would pretty much solve all of my needs except for anything over 240 V

I completely restored the unit including new filter caps, rectifier‘s, and just brought everything up to modern spec. Did harsh insulation test on the main transformer and triple stacked variac, and I am very satisfied with the unit.

They come up on eBay from time to time and can be stolen at around $200 shipped. The shipping weight of this thing was about 150 pounds… And it’s made to an extremely high spec. I guess the seller spent most of that $200 on shipping.

I plan to power it using a phase converter, but haven’t quite worked out the specifics yet. I’m not an expert on this stuff… And have to go very slow and be very meticulous about each and every step before I move forward. It’s not worth getting hurt, as this is just a hobby. So I have to wait for my brain to catch up to my desires.

I’ve been very lucky to have people in the maker community and on YouTube… Lifelong electronics guys that have been willing to offer me their advice. But there seems to be so much VFD and motor drive info and knowledge in this forum… I figured this would be the place to ask about my VFD debacle.

If you have any info on running a larger three phase input 460 V drive from another drive that is single phase in and three phase out… I would greatly appreciate it! Also any advice on driving my large Hammond power supply using three phase drives would also be extremely appreciated!
Not sure if I can drive that power supply with a VFD and if that would be clean enough and sufficient.

If You’re in the north Georgia area, I’ve got tons of drives and transformers, test gear, and I’ll kinds of other things I would be willing to trade or even give someone in need.

Thanks for any information you can give, sorry I rambled on so long.
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Are there any VFDs sold as motor drives that won't fault if not sensed as connected to a 3 phase motor?

You can power a VFD from another VFD using the DC bus link . . . however, you should pay attention to the capacitance of the second VFD to ensure that it doesn’t damage the precharge circuit of the VFD connected to the mains.

One way to protect the drive that is connected to your AC source is to power the second VFD through a resistor on the DC link and once the second drive bus voltage matches the first, then close a contactor across the resistor and you are ready to go.

I think you may be overthinking this though. Why do you think you need a VFD to Power a larger VFD? It’s pointless. Power is power. For most drives, all you need is the right voltage, then the size relates to how much load you can put on it. So if you have a 480V VFD, you can power it with 480V single phase, meaning all you need is a transformer. I do it with little control power transformers if all I am doing is powering it up to see if it works. Mind you, SOME brands, especially older ones, use input phase loss protection that you may have to disable in programming, and a FEW of those do not give you that option. But inflexible features like that are fading out because it makes their brand unpopular, so I have not seen that issue now in a few years. Still, if you are harvesting surplus sites, you might come across those, but sometimes even with those you can “trick” them by just jumpering two phases together. That won’t help your sizing issues if trying to run a motor, but for testing porpoises it’s fine.

If you actually need 3 phase to run these larger units then I would suggest using a transformer and rotary phase converter. You already have motors and transformers so it should not be costly for you to construct.

So even if you do filter the output of the vfd with an LC filter... collectively all 3 phases are delivering a square wave at the pwm frequency, the amplitude is +/- 170v for a 240v drive and +/-340v for a 480v drive.

the wave form will look something like this
https://i.imgur.com/lVSO17M.png


you can take a standard line reactor and add a 4th or 5th leg to it like this
https://i.imgur.com/MX44Lrf.jpg
the inductance will increase (and thus the maximum current decrease) so factor that into your calculations, but then you can connect your lc filter back to the dc bus and get a clean sine wave out of the vfd.. clean being from line to "ground" (ground really being the dc bus)

otherwise with only 3 capacitors and a standard load reactor you can only get a sine wave line to line, you still have sharp pwm square wave line to ground.


anyhow, there is no guarantee that a vfd will produce 3 phases that are free of any dc offset or low frequency ac line to ground. they are only intended to drive motors line to line.. they sometimes produce weird stuff line to "ground" (ground being the dc bus)


to get clean power free of any weird stuff you really need just a standard load reactor, 3 capacitors delta connected line to line, and an isolation transformer. the isolation transformer will remove any dc offset and it will also remove the common mode pwm.


anyhow just use a rotary phase converter.