240-400V VFD to power a rotary coverter?

Since having to repair my rotary converter this week (one of the run caps decided to release it's magic smoke), I just had a thought.

Most VFDs don't like switching loads, and I don't fancy paying the price for one that is designed for switching loads. (I could get real 3 phase installed for less than the quotes I got!)

However, what would happen if you coupled one to a big idler motor, ala Rotary Phase Converter, and then used that to drive switched loads?
It would remove the need for a step-up transformer, and all capacitors. The idler motor would help smooth the output, and with the surges from switching on other motors.
The only thing I'm not sure about is how it would handle any spikes caused by switching off big loads. The idler motor would certainly help, but I'm not sure if that would be enough?
I suppose a surge suppressor could be added, or filters to block spikes, but I wouldn't know what would be effective.

Given the cost of these VFDs, I am almost tempted to buy another one to try, before I start rebuilding my existing rotary converter (it is back working, but it really needs a refresh, and made a bit safer than an open steel box perched on a shelf :hororr: )

Are you talking about a VFD with voltage step-up - 240V in, 400V out? Then drive an idler motor star-connected with load (switched) in parallel? I'm getting a bit confused about whether you could drive the idler with 240V in delta and take a feed to the load as star - but I think I must have read about that one in my "How to build a perpetual motion machine" book!

I looked at all the shenanigans with conversion and ended up buying a TransWave Rotary Converter, 240v single phase in, 415v three phase out.

Neale, you are correct with the first questions.

At the moment I have 240V to 415V (IIRC!) transformer, which then has a star connected motor as the idler.
There is a start capacitor connected via a 240AC relay connected between the star point and the false/generated leg of the star, so once the generate leg reaches 240V, the start connector gets disconnected.
There are also a couple run capacitors to tweak the generated leg voltage, and one of them melted at the weekend.
The converter was built about 15 year ago, so it's hardly surprising the capacitors finally failed.

What I was considering, was removing the transformer/relays/capacitors, then just fit an oversized 240V in 400V out VFD, and connect it to the idler motor (I would probably upsize the motor as well to give a bit more inertia). My concern is transients, both from the output of 240-400 VFD, and from 3 phase loads (my lathe runs a 400V VFD).
What I think I need to add is a sine wave filter to the VFD output.


Nick, the quote I got for 3 phase installation is around £2500 (plus the required digging + cabling), so buying a completely new converter isn't really that feasible. The main reason I don't go for mains 3 phase, is it would involve another meter with standing charge.
If this idea would work, I'd get far better 3 phase for a few hundred.

Are you running anything apart from the lathe on 3 phase? If not, I would guess there won't be much in the way of transients coming back to worry about, as the VFD will be ramping the lathe motor up and down. If you're switching normal motors on the other hand it could be a different story.

Yes.
Lathe has the VFD for the spindle, plus a 3 phase motor controlled via contactors for the turret.
Old Harrison mill with a 2hp motor is still running completely from 3 phase, as it needs the 3 phase for the power feed motor (although this machine is likely to go once I get the big mill up and running)
Plus an old tyre changer that gets used occasionally.

The Harrison mill is the one that puts the most strain on the rotary converter.

The big mill I'm still undecided on whether to make it completely 240V, or run the spindle using 400V (the motor insulation isn't rated for VFD use, so if I go VFD I need to add a filter).


I am tempted to hook up the 240-400V VFD I've got to the current idler motor, and see what happens, however I'm not entirely sure how much wiring changes I'd need to do to test things. It might be something to try once it's a bit warmer for dealing with wiring in the unheated part of the workshop. If the 4KW VFD and idler motor (I think it's a 4hp motor..) can handle starting the Harrison, then there may be some mileage in investigating this further.

If you're live switching a "passive" motor then the idler motor + a filter (series L in each phase + caps to ground on the load side) could help protect the VFD a lot. Otherwise for the Harrison, a really beefy VFD might be the answer - obviously since the VFD will ramp it up, the startup surge shouldn't be so bad.

And to update this, it works.

I was bored tonight, and with the CNC lathe still out of action so no need to worry about the unfiltered output blowing up that, I bodged in some wiring to test it using the 4.0KW 240-380V VFD I already have.
The milling machine starts far better than it would through the rotary converter, with only a very slight dim of the workshop lights.

I did want to measure the output voltage, but my multimeter really didn't like the unfiltered output as it scrambled the display, so next purchase is a suitable sine wave filter. If that works, a bigger VFD and sine wave filter will be purchased, and a more safety conscious and user friendly control system that doesn't involve a step ladder next to the open terminals to hit the start button on the VFD :barbershop_quartet_

If I'm bored again tomorrow, I might drag out the oscilloscope, just to see what kind of output the VFD is actually achieving.