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230/400V Step-Up transformer for VFD input?

Aryan165

Plastic
Joined
Apr 24, 2017
Location
United Kingdom
Hello all,
I've been reading this forum for years and have also posted before, but I have learnt a lot and found answers to most questions by just finding existing threads. This forum is the best thing on the internet bar none!

I've been researching possible VFD and 3-phase motor options for a while to get a feel for what I can buy when I get my first 3 phase machine, as well as learning about VFD's and 3-phase in general.

I have learnt so far that:

1) single phase input VFD's exist and are built to handle extra input current (1.732 x current theoretically) for a given 3-phase power output.

2) you can use a 3-phase input VFD as long as you derate it by 50% for your desired output, given the VFD doesn't have phase-loss detection or that this can be turned off in the settings.

Living in the UK with a 240V (nominally 230V) single phase service as standard, my options (as I understand so far) for using a VFD to power 3-phase motors off only a single phase supply seem to be:

1)Buy a single phase input 240V VFD, and rewire any machine motor for 240V (if motor happens to be dual voltage), or replace the machine motor with a 240V motor (again, if possible/economical).

2)Buy a 400V (i.e 415V UK) 3-phase input VFD, derate it 50%, and supply with single phase.

The latter would require the single phase voltage to be 400V as I understand it. Please correct me if I'm wrong.
Has anyone done such a thing? Is it even possible to buy such a transformer in the UK (to step up 230V 1-phase to 400V 1-phase to supply 400V VFD)? Would it work?

I've tried every search term I can think of to find such a transformer (230/400V step-up), but all I get are ''site'' transformers designed to step up 110V to 240V for power tool use on building sites, and occasionally 400V to 230V step down transformers for seemingly the same (site) use.


I would appreciate any input form anyone that has powered industrial (400V) machines with VFD from single phase, or has any idea what I could possibly use for this purpose. I am aware of rotary converters, just trying to get an idea if VFD is a viable option for powering 3-phase machines that I might buy (think used factory/school equipment, local ads, auction finds etc).

Thanks
Aryan
 
You won't be able to feed 240 into a vfd with a 380/400 volt input. And, as you probably already know you won't be getting 380/400 out of a vfd with 240 incoming. There's no voltage step-up taking place within the vfd.

VFDs are designed a single input voltage supply because of a couple reasons. 1) the voltage/current handling capabilities of the power semiconductors 2) the voltage ratings of the DC bus filter capacitors.

There are books devoted to each of those subjects.

So a VFD with a 380/400 volt input will be expecting the dc bus voltage to range from 537-570 vdc (give or take) and when the firmware doesn't see that it considers it a show stopper and shuts it down due to low voltage.

Yes. A transformer can be installed ahead of the vfd and there will be losses which increase the power bill. Virtually any transformer can be "turned around," i.e. the secondary becomes the primary (incoming power) and vice versa. Here in the U.S. you apply 240 to the 240 volt secondary (which will actually be the primary) and if so tapped you will get 480 on the primary winding (which actually becomes the secondary). Load regulation can become an issue in some instances.

Your best bet is to find equipment with dual voltage motors and vfds with the low voltage input. They will be far less expensive due to lower cost power semiconductors used internally. Of course the wire gauge required will be double but you will still save money.
 
I told you there's no voltage step up taking place within the vfd. And that is true. But you may be reading the above wondering why the dc bus voltage is so high. The reason is the number of volts you're used to hearing about in ac circuits is being expressed as an RMS value. While the peak to peak value is 1.414 times this value. Therefore, 240 volts (rms) equates to 339.36 (340) volts peak to peak. When the ac voltage is rectified then filtered with the proper filter capacitor for the load you will see 340-350 volts DC across that cap.
 
Wayne, thank you for your reply and the info.
I am aware that there is no voltage step up happening in VFD's, and also aware of RMS and peak AC voltage. You mention putting a step up transformer after the VFD, which is something I hadn't thought of. I was thinking of stepping voltage up before the VFD and supplying 400V 1-phase to a 3-phase 400V VFD (50% derated).

I did think of the possibility of using a 400/230V step-down transformer backwards, as a step-up transformer, but I remember reading something about transformers being optimized for one or the other (step up or down)? I need to read more about using transformers like this.

Also, 1-phase input VFD's seem to be made to 2.2 KW (3 hp) max. Say I wanted to run a 4 kw motor? Would it be better to buy a 400V 3-phase VFD and step up the 1-phase voltage before the VFD input, or buy a 230V 3 phase VFD and step up after VFD? (to power 400V motors)
 
Nah... When I said install the xformer ahead of the vfd I mean install it in series with the vfd input. Not on the output.

One would think it would work on the output. I have never wired one up that way. I don't know how the inductance of a transformer would differ from a motor given the kva/watts of each were similar. The greater the inductance the harder the power semiconductors have to work.

Never had the luxury of an expendable vfd to experiment with.
 
Oh, I misunderstood. thinking about it, it would probably be best to have the transformer on the input of the VFD, so it is being fed at the line frequency all the time, as it was designed.
 
Many VFDs can be run on single phase, but NOT all of them, even with de-rating.

The 50% is not universal, although it is a pretty good guess in general.

For a shop full of stuff, it is probably more sensible to do a Rotary Phase Converter (RPC) just because you can run several machines with it. VFDs are one per machine, generally (it's a nuisance to do more than one).

A transformer can be applied before OR after the VFD, although "after" requires some thought in terms of ratings, and also is best for plain V/Hz operation. You need a 3 phase transformer setup.

"Before" still requires consideration of power requirements and transformer rating. Generally "before" is likely to be less problematic, and cheaper.
 
400V motors up to and including around 4kW are typically wired for either 230 delta or 400 star. These will work well with a 230V VFD, and are common. Above that they are usually 400 delta and 690 star.

There are also a handful of products around that allow you to use a 400V VFD from a 230V single phase input. They do this by charging one half of the capacitor bank during the positive half cycle, and the other during the negative half cycle, giving you twice the usual voltage, and feed that to the VFD's capacitors. It needs to have DC bus terminals.

This should work quite well as most '400V' VFDs are rated 380-480V to cover the US market too.

A 400V VFD will work on 230V single phase with this | Trade Me

I haven't been able to find a reputable certified source for them though.
 
Thank you JST and SomeoneSomewhere for the input. I figured stepping up the single phase 230V to 400V before the VFD might be less problematic.
I have a transformer like this in mind, just used for single phase: AT3 3,5-22/23-4 | Block 3.5kVA Open Frame Autotransformer, 220 V ac, 230 V ac Primary, 400 V ac Secondary, 3 UI 114/64 Core | RS Components

As for powering multiple motors with a single VFD, I thought of using a change over switch, with the VFD and the switch mounted in a convenient spot, and all starting/stopping done with the VFD (through control wiring/switches near each machine), the motors staying permanently connected to the change over switch (this is also only a one-man shop, so no chance of someone switching things at the wrong time). This will likely be in a small shop. I'm thinking I can start with the lathe, and connect more machines as they come, and I can always buy more VFD's. I like the soft start aspect of a VFD, and I also think the 3 phase out of a VFD is probably better than that out of a RPC? This is just my understanding, I could be completely wrong.
 
Starting a motor on VFD is definitely nicer for the motor than direct on line, whether that's from a static or rotary phase converter or an actual three-phase grid.

Buying a transformer either second hand or from a local transformer manufacturer is likely to be far cheaper than from someone like RS.

You can also essentially make a 1:2 autotransformer out of a standard isolating transformer if you connect both windings in series. That's likely to be a cheap route, though it will give you more than 400V - that's actually a good thing; you will be able to run motors at full torque up to 60Hz or so.

But I would say cross that bridge when you come to it. Not much equipment for a one-man shop is going to have a motor that cannot run on 230V.

As for multiple motors on a VFD, one issue is that you lose all the on-board protection and tuning, unless it's an expensive one that can store multiple sets of motor data - usually still limited to 2-4 motors max.

Small VFDs are cheap enough that you may as well just buy more as necessary.
 
It is entirely possible to feed your 230v mains into the secondary of an ordinary 'step down' transformer to 'step up' to the higher voltage, and doing it at higher power levels is entirely possible.

It is entirely possible to feed 400+v single-phase AC into a similar-voltage VFD, to drive a 3-phase motor of similar voltage...

This is how a third of my machine tools are driven.

My Monarch 10EE lathe's spindle is driven by a 1970's era 7.5hp Allis-Chalmers 230/460 motor powered by a 1990's era Allen_Bradley 1336 (not a 1336S or 1336F... Older... first generation)...

So it goes 240v 1ph -> 240/480 transformer -> AB 1336 VFD -> Allis Chalmers 460v motor.

It is NOT a good idea to run a VFD into a transformer, then to a motor. The inverter doesn't 'like' the addition of an reactive load between it, and it's reactive load... particularly so because the transformer really doesn't like being operated at anything outside it's designed frequency range. Look up "Q" in AC Electronics.

BTW... it's also perfectly feasible to set up your 1800rpm motor with a drive reduction, and then program the inverter for 'overspeed'... so that it's upper frequency is higher than your base frequency. IN my case, the Monarch runs about 3:1 reduction, and highest VFD output frequency is around 200hz. That makes the motor spin to (1800/60)200=6000rpm... divided by 3.5=2000rpm at the spindle. Motor spins nicely at very low speed, with gobs of torque.

The transformer I found at a junkyard, salvaged from an old highway sign. 10kva 480:240/120. I used the machine's original power control contactor to control power to the transformer. VFD is hard-wired to the transformer, and I tap across one winding of the 240/120 secondary to provide 120v power for lights and controls. Works dandy!
 
Thanks for all the info. I now realize stepping up the voltage to 400V before feeding the VFD is best. I'd love to be able to salvage a transformer from something, just don't know where to start or where to look. I know either the primary or secondary of the transformer has to be rated for at least 400V (depending on whether I'm using it as it was designed or ''in reverse''), so as an example, a 120/240V step up transformer shouldn't be used as 240/480V step up, even though the ratio is correct, putting 240V on a winding that was intended for 120 (and the same for the secondary) is bad for the transformer. Correct?
 
Ratios...

Thanks for all the info. I now realize stepping up the voltage to 400V before feeding the VFD is best. I'd love to be able to salvage a transformer from something, just don't know where to start or where to look. I know either the primary or secondary of the transformer has to be rated for at least 400V (depending on whether I'm using it as it was designed or ''in reverse''), so as an example, a 120/240V step up transformer shouldn't be used as 240/480V step up, even though the ratio is correct, putting 240V on a winding that was intended for 120 (and the same for the secondary) is bad for the transformer. Correct?

yes, use the appropriate voltage for the winding to which you'e connecting.

if its within 20-25v or so, its close enough... 10% tolerance. Its a simple circumstance of ratios...
 
Check the rated input voltage on your VFD. If it is '380-480V', you can feed up to 480V (plus tolerances) into it, even if the motor is only 400V rated - the VFD will step it down. 230:460V transformers might be more available in single phase.

Yes, you need to use the windings at the right voltage, not 240V into a 120V winding.
 
I have used a couple of options for this type of use
1- singe to 3 phase step up VFD 230v in 400v out
2- to get a cheap step up transformer I used an old arc welder i gad that had dual 230 and 400 v input options. this is quite common for old transformer type welders.What you do is wire the 230v supply to the 230v connection on the welder then you get 400v or more 2 phase output from the connections that you would normally feed the 400 v supply to.I ran this system for some time with no problems.The big advantage is of course cost as old transformer type welders can be brought for scrap value or free.
 








 
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