Mitsubishi VFD question - single phase input possible?

Hi all,

I obtained a Mitsubishi 5.5kW VFD during a work clear-out, and I am trying to work out whether it would accept a single phase input or not. It was previously used with 415V 3 phase supply in a factory environment, but the [generic] manual I have found also gives wiring instructions for single phase input (i.e. use terminals R and S for 1~, rather than R, S, T for 3~).

Link to manual - this should go to the right page...

As this is a 400V model I would assume a single phase input would not typically be used, but I have an autotransformer in my rotary phase converter to get 415V single phase, and if I could run the VFD directly from the transformer it would simplify the set-up a bit!

The VFD is a Mitsubishi FR-A140-5.5K-ER from about 1994, I believe.

Any advice gratefully received - I guess at worst I could connect it to the single phase 415V supply and see whether it powers up! It clearly would expect to see 415V between phases, so a single phase input would not exceed this...

Many thanks in advance

From looking at the manual it does indeed infer that single phase can be used. I have a couple of inverters that are fed through R & S but will also take 3 phase via R, S & T. They work no problem.

regards

Possibly, use an external rectifier feeding a whole bunch of external smoothing capacitors.

The only real difference between 1ph and 3ph input is turning the volts into DC.

The main effect of using single phase will be that the peak current through the rectifier and the ripple current in the smoothing capacitor will be higher. Since Mitsubishi has said that it can be done, they must have specified enough capacity in the devices for this to be Ok,. It should not need any external devices.

John

I have an ABB inverter that is nominally three-phase but also capable of single-phase operation. The downside is that you seem to need to downrate it somewhat when running off one phase. These things work by, in crude terms, taking the input AC, rectifying it, and charging a big capacitor which gives a more-or-less smooth DC voltage. This provides feed to the inverter circuitry which generates the variable-frequency three-phase output. Running off three-phase with a bridge rectifier at the input, the capacitor is topped up at 6 times mains frequency (each half-cycle of each phase). However, with single-phase input, you only get one-third as many top-ups (hence comment above about increased rectifier current, by about three times). That means that between top-ups when the capacitor is being discharged into the inverter circuit, its voltage falls further than if fed by three-phase. If this dip between peaks exceeds some amount, my VFD trips with a "phase input error" - it rightly attributes the problem to a missing phase at the input. It works fine with my lathe except that it won't quite handle the load at the highest lathe speeds (it's a variable-speed lathe with expanding pulley arrangement for speed control).

Mind you, in my case it's aggravated by the fact that there is a crude voltage doubler on the input to go from 240V to 480V to feed the VFD which effectively halves the input frequency, so the downrating in your case might not be too bad.

Just for information, most of the Chinese inverters used in home-built CNC routers and the like with the ubiquitous 2.2KW spindle motors have exactly this kind of single-phase/three-phase input option and almost all of us using them run them very happily off single-phase.

A 1994 inverter is probably getting ready to let the magic smoke out of its electrolytic capacitors, although running it up for half and or more every 6-12 months.extends life.

Neil

It may be worth setting parameter 19 to 230V and running the motor in delta, if the input of 230V can keep the DC link stable then the motor may develop full power..

Mike

Regardless of whether it will work or not, when was it last powered up?
As Neil mentioned, there is always the possibility of magic smoke escaping.

This was the advice I received from one particular manufacturer's tech helpdesk.

Access to a variac is useful, but it needn't be big enough to supply the inverter under full load, as this is just about applying an increasing supply voltage to re-form the capacitors.
I made the enquiry originally having released some magic smoke

Hi
Please see below guide only may not be suitable for all inverters
your decision to try it
applying low voltage should not damage the unit if anything it will display low voltage.

I think advice on long term storage of a VFD will be found in reputable manufacturers literature. This came to our attention as our stores held extensive stocks of spare drives but as they are very reliable the turnover could be very slow. When I retired a plan to do a regular power up on drives in stock was being put together. The process you have described above was much like the recommended one for our drives. As we held spares for breakdowns it was critical that drives were ready to go, just install and load the parameter backup and go.

Mike

Fluke publish a handy little guide to diagnosing voltage issues on VFD units :-

**LINK**

https://www.fluke.com/en-gb/learn/blog/motors-drives-pumps-compressors/how-to-measure-output-voltage-from-a-vfd-to-a-motor.

De-mistifies things quite nicely.

Often forgotten that the DC bus voltage is technically 1.41 times the RMS input voltage. So a 400 volt VFD for a star connected motor has approaching 700 volts inside and a 230 volt one for delta connection approaching 350 volts. So to properly go from normal single phase to 400 three phase the voltage has to be near enough trebled and capacitor storage great enough to supply the motor without too much voltage drop. A tall order. No wonder things have to be de-rated.

Clive

If a VFD hasn't been used for some time as has been mentioned the electrolytic capacitors may need reforming. However this can be done at a fairly low voltage, say 12v, rather than the full 440v.

I have several inverters (8) , that have 3 phase inputs and are all working on single phase either at 240v or via a autotransformer at 440v for motors that can't be converted to delta. They are now all over 20 years old and show not signs of any issues.

The down side of working single phase is that, as mentioned, the DC link voltage will droop on load as the ripple will be higher than when working on 3 phase so at probable less than 50% loading the inverter will run out of steam and possibly flag an under voltage warning neither of which I've found to be damaging.

CS

1) What 5.5 kW 230 V supply do you intend to use?

2) My own VFD warns against not using it for a year or more - the capacitors do not like it.

Hi All, thanks for the various bits of information. I should have thought a bit more carefully about the operation of a VFD really, as it's fairly obvious that a single phase input could be rectified to DC, albeit with the potential to retain a few ripples.

I think I'll set it up to start gently and give the capacitors a chance to recover, though it's probably possible to replace these if absolutely necessary. I have a very large autotransformer that can be persuaded to give quite a range of voltages so I'll start small and work my way up. At the end of the day this was a freebie so it's not a calamity if it doesn't work, but it would be a real bonus if it does as it would mean i don't need to run an idler motor.

For a power supply my electrician is about to hook up a 40A radial circuit from the RCD-enabled distribution board, which will do for checking this out. This supply is ultimately intended for my rotary phase converter, but that is another project entirely (I may post on that elsewhere, as it's been an interesting journey of discovery!).

The VFD is inside a steel cabinet with specific fuses for the 3~ inputs on RST, so there is an additional layer of protection for the wiring and device there. I think I'll just give it a go with single phase to terminals R and S at incremental voltages from 12V to 415V and see how I get on.

Many thanks again for the various bits of information, Simon.

I trust that if a socket is to be provided, consideration has been given to compliance with 553.1.201 of BS 7671.

Yes

Well, to close this out it seems that this model of VFD does not work with a single phase input. I painstakingly went through increasing voltages to try to condition the capacitors, but at 415V single phase there was still no sign of life!

Many thanks again for the help - much appreciated.

Probably a fault in the unit rather than use of single phase. I'd expect any simple 3 phase VFD to at least power up without any load when powered from a single phase.

Robert.

Robert, you were absolutely right! I decided to delve deeper into the manual and it turns out there would originally have been a couple of jumpers to connect the control circuit to the input power. These had been removed to enable the alarm system to stay live in the event of a trip, and though I disconnected all extraneous wiring I hadn't twigged that I needed to bridge a couple of connections.

So using 420V single phase it powers up and gives the 'right' information on the screen. I'll try it with a motor under no load in the next few days.

Thanks again for the tip, potentially saved the unit from the skip!

I'm not actually sure what you're getting at here, from the relevant section of BS7671:2018:

Operation of a VFD is not "household and similar use", so the paragraph is irrelevant and fitting a BS EN 60309-2 compliant "CEEform" interlocked switch-socket would represent a compliant approach, as per the wording of 553.1.3

Simon

Before trying with a live load take another deep delve into the manual to verify whether there is parameter setting associated with removing those jumpers to enable an outside active alarm in the event of trip.

Over 15 years ago I obtained a used Mitsubishi VFD to dive my shaper and recall having to do a bit of parameter fiddling to get it up and running. I have vague memories of the guy who sold it to me saying it came out of a complete system with some jiggery pokery to handle some stuff outside of the VFD. It's possible he told me that he had sorted the connections so it was ready to go but I'd need to set the parameters for the motor. Incorrect screen information and non running motor sound familiar.

It's been good as gold once I got it going tho'. Mitsubishi build good inverters but the manual needs careful reading. The one with mine is very complete but terse.

Clive