converting a safety isolating transformer to an autotransforer for 460Vac - Page 1
Forum section: electronics / general

author: IsUniqueUserNameTheIssue?
author_rank: Newbie
timestamp: May 20, 2021, 12:36:20 am
content: Hello,I have several older three phase 400v input VSD / VFD 's for three phase motors. I would like to use these on a single phase supply. All will work happily on single phase, and on 230v, but at at 230v input you get 1.73x reduced power output from the VSD. While some star 400v motors can be re-wired to delta 230V, this generally only applies to smaller motors (<750w)So, can I rewire a 2000va safety isolation transformer as an auto transformer to output 460V single phase? What problems do people expect I will run into? Am I correct that I will get a 4000va output?Caveats: This will be done inside a rittal steel enclosure with din rail mount breakers, appropriate cable sizes etc.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 20, 2021, 01:28:44 am
content: Should work, but have you considered using a voltage doubler?

author: IsUniqueUserNameTheIssue?
author_rank: Newbie
timestamp: May 20, 2021, 01:50:18 am
content: I had considered it, but to get 4000va, the caps I would need would be something like 10,000uF to get acceptable ripple? And I already have a spare 2000va isolation transformer.

author: WattsThat
author_rank: Frequent Contributor
timestamp: May 20, 2021, 02:51:22 am
content: QuoteNo, you’ll typically get 2 * 1.732 less output power with a single phase 230V input into a 400V 3 phase rated VFD. But, I think you’ve got a typo there as I seriously doubt any 400V class drive would be happy with a 230V input. Think under-voltage fault. Single phase, sure, with the derate for the loss of two diodes and the reduced bus capacitance is covered by the 1.732 derate.Some 230V VFD’s are dual rated for three/single phase input with a 50% current derate for single phase but those are the exception, not the rule.Also, beware of using breakers due to clearing times. Most VFD’s require HRC fuses and motor time delay breakers will have enough I^2t let-through to cause rapid disassembly of input diodes, chokes and bus caps. Small, higher input impedance devices can usually survive with B/L/Z curves. It’s the K curves that will let the smoke out.

author: IsUniqueUserNameTheIssue?
author_rank: Newbie
timestamp: May 20, 2021, 04:11:29 am
content: Hi Wattsthat,Thanks for the heads up re HRC fuses and clearing times. I was probably going to use C curve breakers as that is what I have seen mostly fitted with VFDs, but will look into it further. I had always assumed the inrush to charge up the main caps would be the main factor in selecting a breaker for a VFD.I have some drives which are rated at 400v 3 phase, happily running on 230v single phase for small motors (0.18kw and 0.37kw) Some of them you need to disable the low voltage warning, but others just run. More commonly I have found you have to disable the loss of phase warning to get them to run happily.The current derating from three phase to single phase is interesting. I expect the DC / output side of the drive should be unaffected. Input side I have always figured derate by 1/3rd as you only have 4 instead of 6 diodes rectifying. But I have also noticed that the input diodes tend to be huge for the current expected anyway. Probably a case of it's cheap enough to apply overkill here?I'm curious to know if anyone knows if you should de-rate more because of the differences in the smoothing capacitors. I have always thought you would have to de-rate by a lot more because of the caps as rectified 3 phase is almost smooth, while rectified single phase is lumpy as. I had always assumed this is one of the reasons why single phase input drives are costlier too?

author: f4eru
author_rank: Super Contributor
timestamp: May 20, 2021, 09:24:41 am
content: It should work.I recommend over engineering the transformer, rectifier, due to high peaks in current. Perhaps add a DC-link inductance to improve power factor ? ?

author: pwrtrnx
author_rank: Newbie
timestamp: May 23, 2021, 02:28:28 am
content: Yes you can wire it as an auto Tx to get 2x the mains voltage ~ 460Vac, as the pri is designed to handle perhaps 10A max - then the max power you can pull from the mains would indeed be 4600 VA, as the input lead would supply 10A to the "pri" and another 10A to the sec - auto Tx's are a lil bit unusual that way - rectifiing into a large cap will mean the current is very peaky so the rms would be a lot higher than the ave current, thus for 10A rms out ( at 460V ) the ave current might only be 5A, which would be 2300 watts for 4600 VA ...

author: WattsThat
author_rank: Frequent Contributor
timestamp: May 23, 2021, 03:05:41 am
content: The overcurrent protection in front of a vfd is there to prevent rupture of components and fire when things go bang. Small drives, say perhaps under 20kw or so are inherently higher impedance and therefore have lower fault currents when things go sideways. This varies by manufacturer and if they add impedance in the form of ac or dc chokes - these will indeed lower peak currents and lower harmonic content by some amount. The inrush is limited by the cap pre-charge circuits so fuses or breakers are usually spec’ed as fast clearing devices. Most low power, low cost drives avoid the additional copper and iron to save space, cost and losses. It’s always best to consult the owners manual for the proper input protection. Without following the manufacturers requirements, whatever safety standards the devices are listed to are no longer valid.With regard to running a three phase drive on single phase input, yes, the derate for the diodes is the obvious 1.732 but it’s the bus capacitors that have the larger derate since you’re going from 300/360 ripple down to 100/120 (50/60Hz). Some drives will trip on a “input phase loss” due to the extra ripple as they measure that content since it affects the output voltage stability. This matters in vector modes, not so much in your basic volts per hertz mode. This is certainly a case of YMMV since lower cost, low power drives are usually just your basic shaft turners and absolute speed regulation is not a performance concern.

author: IsUniqueUserNameTheIssue?
author_rank: Newbie
timestamp: May 25, 2021, 11:29:09 pm
content: Thanks for all the responses. Great to hear the more intricate details explained.