Digital CNC phase converter build

Hi all

I am looking at three phase conversion options for my workshop I have looked at rotary converters

and I have also looked at static and digital conversion options what I'm wondering is does anyone have a digital converter from the only manufacturer that makes them. Drives Direct is the name.

If so how well does it work and what is your experience with it?

I'm looking at getting 3 phase from the grid (extremely expensive) and also looking at getting a rotary converter with digital phase balance. from the USA.

The only other option I can come up with is to design and build a digital phase converter on my own, i have a lot of experience in electrics and electronics but cant figure out exactly how the digital phase converter works. The basis of it must be a VSD with Harmonic filter in front, and Sine Wave filter on the output side. And a bank of capacitors on the output to take the start current. But apart from that im not sure what else. And those bits alone in no way could justify the cost they charge. Seeing one would help or any ideas any of you may have.

Best place to start is to describe what you are trying to do. Are you intending to run a 3-phase motor from a single phase supply, or create a substitute for a utility 3-phase supply.

I'm not convinced that capacitors on the AC output will help with start current.

Personally I went with having a proper 3-phase supply installed at home.

Andrew

Description of a commercial system here.

Description: The system is controlled by a small micro-controller, specifically a digital signal processor (DSP) which can measure voltages and feed controlled pulses into the switches, in addition to performing high-speed calcu-
lations. The DSP is constantly monitoring the system voltages and current to insure that the input current is
sinusoidal, and the output voltage is also sinusoidal. The output voltage can be made equal in magnitude to
the input voltage to an accuracy that is primarily determined by the measurement accuracy of the DSP.

Looks like the microcontroller measures the input phase and calculates when the third phase should be, switching pulses from a DC source as necessary to charge a capacitor as necessary to maintain the instantaneous voltage. I don't know how to keep the output clean without an expensive filter, but I guess it can done digitally at high frequency.

gnuradio is good for exploring DSP concepts and methods. (It's a flowgraph modeller.) The actual DSP would have to be fast enough to keep up : some microcontollers have DSP capability.

Dave

Hi andrew,

Ive got a 480v split phase supply,

What i want to be able to run is a press brake, cnc plasma, cnc router and synergic welder. Possibly more in the future.

The 3 phase utility install is 42k plus vat.

Just looking at other options to try and create 3phase power.

Looking at a company called phase perfect in the USA, they have a similar 480v split phase system, so thats another option.

So how many kva do you want or need ? Had you considered a diesel powered alternator ? I built one with peugeot 1905 and a 20 Kva alternator worked very well. I also have a 25Kva unit. Drove an 11kw motor and a 5kw together also drove a 400A mig welder. The best the grid can give round here is 2ph. Just a thought. Noel.

First consideration has to be how much three phase power do you need? Closely allied to that is the question of what can the existing single phase connection provide?

After that, I guess you will have to look at the cost and merits of the different approaches.

1 A static converter (Transformer plus inductors and capacitors) will work and allow you to use more than one machine from it, but the phase to phase angles will not be ideal .

2 A rotary converter, similar to the above plus an idler motor will give better output phases but will waste a bit of power keeping the idler motor turning. The idler motor provides a bit of flywheel effect, helping with the surge when you turn a machine on.

3 The various sorts of electronic inverter will provide excellent output waveforms and so on, but may cost a little more. You have the choice of providing one big one that all the machines connect to as if it was an ordinary three phase supply, or little ones on the basis of one per machine, which give the advantage that you can use the inverter as a variable frequency drive.

4 Generate your own three phase as per Noels suggestion. Perfectly feasible but has its own set of troubles like noise and complexity.

5 Pay through the nose to have three phase installed. This always seems to cost far more than the job really warrants, I get the impression that suppliers around the world would rather you didn't have three phase.

You can't add capacitors to the AC output of an inverter to smooth the supply or to provide for starting surges. On electronic inverters, such capacitors would go inside on the DC bus and you would not want to try to modify the design. There will be small capacitors on the output to help suppress interference, and you probably don't want to change those either. On a rotary inverter, the idler motor provides a flywheel function.

My own preference is to use a VFD for each machine, which gives the advantage of variable speed, but for some machines this means getting inside to alter the wiring, eg to make sure that a coolant pump or a fine feed motor is getting what it needs rather than running off the variable frequency supply. Some may not be comfortable modifying their machines in this way. I think with the relatively low cost of VFD's these days it is quite a good way to go.

John

Hi John, thanks for the reply,

As mentioned above i have a 480v supply, so would like to get a converter big enough to use most of that.

With that in mind, i would like to get maybe a 40HP unit, which would give me close on the limit of what i can get.

The two main options i am now looking at is

1, Making my own system, with a 100HP VSD, sine wave filter and a isolation transformer.

2, Looking at the 480v phase perfect system from the USA which would also need an Isolation, Delta to Star transformer on the output side to bring the voltage down to 440.

The one bit i dont quite get about the Phase Perfect system, is how the system works. It seems to work like a refined Rotary. in that i mean it uses the two legs of the 480 and only artificially makes the 3rd one.

With that in mind, the center tapped transformer providing my 2 legs of 240 at the moment is center tapped, meaning that the 2x240v i have are 180 degrees apart, but the phase perfect system takes those two legs past as they are and put's the 3rd leg at 90 degrees to them. Which they say equals a balanced 3 phase sine wave at 120 degrees.

I just cant figure out how that is acheived like that.

Looking forward to your reply,

Robert

OK, I was thinking in terms of the usual model engineering scale of things, where a Bridgeport is a big machine, you seem to be looking at quite a bit more power than that.

The two phases of your existing supply being at 180 degrees doesn't actually matter, you just need a third phase at about the right phase angle and voltage and things will work. I stand to be corrected here, but if you had a star connected motor, I think the centre tap of the transformer would not be an actual neutral point and so should not be connected to the star point of any motor. Think about it like this, if you had a 480V three phase supply and measured the Voltage between two phases and looked at it on a scope, it would look just like what you see with the single phase supply. So it is true that your converter just needs to provide one new phase at 90 degrees. (We could do vector diagrams to show this....) But this will be effectively a delta connected supply, with no actual neutral point. If you want one you can provide a delta to start transformer at the output although for 40 hp, say 30 KW or so that would be a big and expensive transformer

If there is a rotary component, eg an idler motor, then what you get out will be not too bad. I've actually run a 3 HP 460 Volt motor on single phase, with the 240 Volt supply being stepped up by a transformer and applied to two of the motor connections. It did not self start of course, but a kick on the pulley in the right direction would get things going. I never measure it, but had I needed to run another three phase motor, connecting it to the three phases of that motor would have provided a converter.

With electronics of course it is possible to do this sort of conversion at any sort of power level you like, so long as your wallet can stand the pace. AC to DC and back again is pretty standard for long distance and undersea transmission, and they work up into the 100's of Megawatt range. But I don't know what is commercially available in the sort of size range you want. It won't be cheap, but then the transformer and idler motor won't be either.

Hopefully it is redundant to say that any system will need to be done properly and in accordance with any local regulations, this sort of stuff can easily kill you or burn your house down!

regards

John

That's my understanding too, except I'd say 'it doesn't matter much' rather than 'it doesn't matter at all'.

The paper I linked in my first post covers the subject quite well; it covers the pros and cons of the various systems reasonably well. All the methods have disadvantages, such as the gross inefficiency and noise of motor-generators, wobbly phase imbalance of static converters, inefficiency and slightly better imbalance of idler motor systems, the harmonic distortion of VFDs, and the not true 120° 3-phase produced by the Phase Perfect system (which is probably the same as the Direct Drive solution.)

Of the various offerings VFD and Digital Phase are probably the best:

• VFDs get the phase angles right and allow speed control and other electronic advantages, but are essentially one-per-motor, which gets tricky when a machine has more than one 3-phase motor, or a dual speed motor. Sharing them between different machines is not recommended. Even so this is the easy answer.
• Digital Phase gets the phase angles wrong, which probably doesn't matter, and - most important - the system allows multiple motors to be plugged in to the same unit. At the moment, they're industrial (5kW and up) rather than consumer units, and priced appropriately. Just the thing for a guy with a 480V split-phase supply, who can't afford to upgrade to 3-phase.

As Robert's requirement looks industrial rather than home electrics, I'd be inclined to get in an expert, and/or cough-up for a plug-in commercial unit. Keep it simple.

Rolling your own is likely impractical:

• How good is your understanding of the techniques required? DSP? Microcontrollers? Power electronics at tens of kilowatts? Safety, Regulatory and Insurance requirements?
• Sourcing and paying for the components. A commercial seller keeps cost down by buying wholesale and claiming back VAT. Home builders pay retail prices and tax on top. That's balanced by not having to pay someone else to do the work, but bad news if it takes a month or two to get the business up and running. No point in DIY if it costs more than borrowing the money.

VFDs make a lot of problems go away. The downside is one per machine, the upside is they can be installed one at a time and are available at consumer market prices. They're not electrically controversial in the sense converting 480V twin phase to 3-phase is. Provided the VFD (in a protective box) can be plugged into an existing single phase supply, it's not necessary to get a qualified electrician in.)

Dave

Hi Dave,

I must say, to be honest the only reasonable option i see in front of me is the phase perfect system. It is plug and play and although the phase angles may be slightly off, it seems by all accounts to still do the job and has many reviews to that effect.

thanks

Robert

Hi,

The Phase Perfect system would be better called Phase Imperfect. They do not say anywhere on their datasheet what the output phase relationship is, but from the description it is clearly not 120 degrees or balanced. Two are 180 degrees apart (the incoming line) and the third is 90 degrees from these. Unlikely to give smooth torque from a motor desigined for 120 degrees. Also note if any of the machines have a single phase 480V load interanally you will have to ensure they are across the incoming phase. If any require a Neutral connection You will have to add a delta star transformer. This is more cost.
I would not bother with it for machine tools. Probably passible for basic prime movers but even things like pumps could suffer damage from the torque variation caused by the mismatch beteen the mechcanical angular pole spacing and the electrical supply phase.

Individual VFDs per machine (even multi motor ones as long as you keep the "speed" at 100%) is likely to be a better solution. Or get the advantageof variable speed and fit a VFD per motor.

What country are you in?

Robert G8RPI

Edited By Robert Atkinson 2 on 20/07/2021 15:51:40

The operating principle is described in this patent. It's quite clever and not as Robert describes. There's a published IEEE paper also in 2019 22nd International Conference on Electrical Machines and Systems (ICEMS) (This may be difficult to access.)

Leaving aside the question of the patent, reproducing this at home looks very difficult! Not only is there the question of hairy power electronics, but the real-time control software looks quite tricky.

I agree with others who suggest separate VFDs per motor, which would turn out probably cheaper than either a 3 phase supply or one of these hairy beasts, or one of the more primitive static or rotary converters. I think you have one or two machines that are particularly problematic, the press brake and welder? Don't let the peak power requirement for these impose unrealistic costs and complexity on the rest of the solution.

Edited By John Haine on 20/07/2021 16:10:16

Edited By John Haine on 20/07/2021 16:14:45

Would a 3 phase diesel generator 5.5kVA from Machine-mart be suitable

Hi Robert,

Im in the UK, in the Shetland Islands to be exact.

The VFD option per machine is plausible for anything motor driven, but doesn't give me the option to power the 3 phase plasma cutter, or a welder for that matter, so thats why i had discounted that. Not only that but the press brake alone, the main motor on the one i am looking at is 15kw. So it would need to be a 30KW 480V vsd to give me any chance of powering it, and it has a VSD / Soft start built into it, so im not sure how they would see each other.

So again, i do think my only real 2 options are a full workshop converter, or paying SSE to install the 3 phase.

I have also looked at Genis as mentioned by Maurice, but the cost of a geni big enough to work these machines and how much they would cost to run and maintain, over 5 years i would be better off getting the 3 phase installed.

Would a Motor-Generator Set such as **LINK** be suitable ?, thats if they are able to build one that is able to work with a 480v split phase supply.

Have you enquired about a wind powered unit ? I understand there is always a good wind blowing in that area.

Emgee

You seem to require a three phase supply in your workshop to run several machines when the only input is single phase. This would need a large converter, which would be expensive and might cost more than individual VFD's for each machine. Also, individual VFD's can be programmed to suit each machine for motor start and stop characteristics and speed control, plus forward and reverse if needed.

Having a three phase supply from the mains would seem to be the best long term solution for such a large workshop.

Edited By old mart on 20/07/2021 18:26:09

The VFD's would work for all but the plasma machine, so not the best option.

As david said above an idea of a Motor Generator may be the best option. Or the Phase Perfect is the one i have to say i am leaning towards just now.

Hi John,

Thanks for the link. It's certainly not as originally described and has some key components not shown in the installation manual. That said the Patent i terrible and does not reveal anything about how it actually works. Presumably it is using PWM and the output capcitors (not shown in manual circuit) are doing a lot of the work. Thi is supported by that that they have to be reguarly checked / replaced.
It's whats not in the patent and specification that would bother me. No output voltage spec other than about the same as the input. No output power factor range. No minimum load. No step load change response. Some of the patent claims are a bit broad e.g. basically any circuit with a phase controlled input bridge and polarised storage capacitor would breach it. Lots of prior art to get around that one. The patent does not mention PWM or if they don't use PWM how they generate the waveforms. All in they must have some pretty advanced math to generate the required waveform for zero to unity PF over 0 to 118kVA. I've recetly been working with some custom <50kVA converters and the supplers of those have struggled to meet specification even with full 3 phase bridges and limited load power factor.

It's also does not appear to be suitable for UK use. Standard is 60Hz and there are no claims for any safety or EMC approvals. They have the ELT mark on parts of documents but only their variable speed drives are listed and then to very limited tests

https://ramuk.intertekconnect.com//WebClients/ITS/DLP/products.nsf/vwSearch?SearchView&Query=FIELD%20ListHead%20Contains%20phase%20perfect%20or%20FIELD%20CatCode%20Contains%20phase%20perfect%20or%20FIELD%20Title%20Contains%20phase%20perfect%20or%20FIELD%20ProductInformation%20Contains%20phase%20perfect%20or%20FIELD%20ProductInfo%20Contains%20phase%20perfect&SearchOrder=1&SearchMax=1000&SearchWV=FALSE&SearchThesaurus=FALSE&SearchFuzzy=FALSE

If any of the machines to be powered have a neutral connection (5 wires) then a delta/star transformer will be required.