Thread: 415v three phase from 240v single phase a home made digital phase converter?

Hi Lamestllama, Guys,

Do you have the tech data for the transformer. A diagram would be nice. I know that it is an auto transformer and its rated at 8Kw.

Originally Posted by jack620

Any reason why you couldn't tie the neutral from the Y to the mains neutral?

Hi Jack,

I have no technical answer for this one way or the other.

I was being a bit more adventurous than Llama, I wanted to use the output from the created 3 phase system to trigger on a 3 phase solar inverter. Fronius told me that the "grid" I was creating did not meet Australian standards. When I questioned in what way, the original contact there had left and his replacement told me I had to use a single phase inverter.

So just thinking this through, (besides making my head hurt) if you tie the created neutral to the mains neutral one would hope that this ties the created neutral to earth potential and any out of balance voltages then show as out of balance phase voltages. something along this line - say a single phase load causes a drop of 6 volts on the phase it is connected to. Without bonding the neutral to the mains neutral the phase to phase voltage would still measure 415 volts but the phase to neutral volts would be 234 volts on the loaded phase and maybe 244 on the other two phases. The created star "neutral" point would have moved 6 volts. Bonding the created "neutral" to the main neutral may result in the usual situation of the phase to neutral volts on the loaded phase dropping to 234 volts and two of the phase to phase volts dropping as well. That would be ideal.

However..... all voltages are relational

The created 415 volt system has no real relationship to the grid due to the VFD and autotransformer arrangement, at least as I understand how VFD's work (my knowledge on this is basic, rectify the A.C. then invert again) So it is possible that loaded phase causes the created "Neutral" to drift 6 volts (as per the example above) in relation to the created system. The mains having no relationship to the created system probably doesn't care one iota. However the result is 6 volts between the bonded neutrals/earth and the created system. It will likely reduce the risk of shocks and tingles as everything should be at the same potential.

The only way to check this would be to check the phase to phase and phase to neutral volts, apply some load to one phase and remeasure. originally I considered using a remote earth and trailing lead, however, there is no relationship to earth here either. Although no harm in attempting that test I'm a bit doubtfull this will give any useful results.

It is also to be hoped that there is no relationship bewteen the created Grid and the mains grid. Should there be a relationship of some description the smoke inside the VFD and transformers may escape, and, as we all know that smoke has to remain inside for things to work.

I guess I am leery of tieing the two together without adequately testing it.


I do recall that I had decided to use isolation transformers in a delta star arrangement and run a seperate main earth to an earth stake from the star point, to mimick the normal mains system. This helped ease the headache!


btw, Please, nobody compare these arrangement to what others do in the USA, we run a multiple earthed neutral system, they don't as far as I know. That makes all the difference.

Still quite keen to hear how this goes.

Cheers

The Beryl Bloke

Originally Posted by lamestllama

I will probably use an RCD downstream of the transformer.

Hi llama,

This implies earthing, which I am guessing you will bond to the star point of the auto transformers?


The judicious use of appropriately rated varisters is likely a good call!

Don't mean to come off as kicking your ideas around, all for them, just do so safely. None of us want to hear that you or your family have got a serious shock from this.

There is an engineer in our office from time to time and when he gets back from holidays (next week? ) I will ask him about this type of system.

Cheers

The Beryl Bloke

Originally Posted by Theberylbloke

I do recall that I had decided to use isolation transformers in a delta star arrangement and run a seperate main earth to an earth stake from the star point, to mimick the normal mains system. This helped ease the headache!

I intend installing a seperate main earth to an earth stake from my star point as I cannot be sure there is no relationship between the created grid and the mains grid. That could even be VFD dependent. Still working on how to make some inductors for an LC filter that will smooth the output from the VFD so the transformer operates using a sine wave rather than the PWM output of the VFD.

Hi Lamestllama, Guys,

I wouldn't worry too much about filtering the VFD output, the transformer will do that for you. If you have a scope and know how to use it safely, you could look at the voltage and current waveforms.

I do however have some reservations with respect to safety earth connections. In theory the star point can be connected to earth ! But I doubt that the VFD passes the earth connection through. If it does then I think that your earth problem could be resolved.

Som infö about VFD waveforms here
VFD current waveforms and values

Ok so my engineer acquaintance was back at work on Tuesday. I gave him the run down and we drew the circuit out. He very quickly indentified out of balance loading as a possible issue. He expressed some reservations about connecting the star point of the created grid ( effectively it's neutral) to the mains neutral. He said he would have think about the idea and get back to me. I'll see him early next week and find out what he thinks.

One of the other issues I see here is that a 8kVA 3 phase transformer has a maximum output of about 11 amps per phase. It will pass more under fault conditions although the VFD probably won't. What do you intend to use for circuit protection? In one regard that may not be overly critical as most cables you are likely to run are rated above 11 amps and will not be damaged by a short or overload at the load end. None the less I would not feel happy running "unprotected" cables around the workshop.

I fully agree with your idea of earthing the star point.

Cheers

The Beryl Bloke

Originally Posted by Theberylbloke

Ok so my engineer acquaintance was back at work on Tuesday. I gave him the run down and we drew the circuit out. He very quickly indentified out of balance loading as a possible issue.

Out of balance loading shouldn't be an issue for the VFD as long as the currents are within each phases capacity. The output stage of the VFD simply switches the DC bus.

Originally Posted by Theberylbloke

He expressed some reservations about connecting the star point of the created grid ( effectively it's neutral) to the mains neutral. He said he would have think about the idea and get back to me. I'll see him early next week and find out what he thinks.

The reason why measuring output of a VFD is awkward gives the reason why tying the mains neutral and the centre point of the transformer together would be a disaster.
https://www.testandmeasurementtips.c...iewing-dc-bus/

Originally Posted by Theberylbloke

One of the other issues I see here is that a 8kVA 3 phase transformer has a maximum output of about 11 amps per phase. It will pass more under fault conditions although the VFD probably won't. What do you intend to use for circuit protection? In one regard that may not be overly critical as most cables you are likely to run are rated above 11 amps and will not be damaged by a short or overload at the load end. None the less I would not feel happy running "unprotected" cables around the workshop.

I see the transformer output connected to an RCD and circuit breaker.

Eric

Originally Posted by lamestllama

The reason why measuring output of a VFD is awkward gives the reason why tying the mains neutral and the centre point of the transformer together would be a disaster.
https://www.testandmeasurementtips.c...iewing-dc-bus/

I assume you are referring to this quote from the linked article:

A point to note is that neither side of the dc bus, negative nor positive, is at ground potential because of the grounded midpoint mentioned earlier. The hooking of a probe ground clip to either side, even before touching the probe tip to anything, will cause a tremendous fault current through the circuit under test, through the probe’s ground lead, back through the oscilloscope and eventually to the service entrance panel. There is a potential for costly damage. To avoid this hazardous situation, there are two possible strategies.

This passage says you can't connect the ground lead of a CRO probe to either the positive or negative DC bus. I agree that would be foolish. I doesn't say you can't earth the grounded mid point. I'm not saying you CAN, just that this article doesn't say you can't.

Greetings,

Finally had time to speak my tame engineer today.

Now that he has had a little time to look at the circuit he repeated his previous comment about connecting the star point of the three autotransformers to the main neutral was a bad idea.

This does mean that you will have to think about your earthing arrangement carefully. The earth stake at your main switchboard has an unknown resistance to the general mass of the earth, likewise an earthed star point for your created 3 phase grid, this will result in have a circuit between both Neutrals via the driven earth stakes. Due to the resistances involved this may not be a problem, although as the resistances involved could be anything from a couple of ohms to a couple of Kohms, I won't guarantee that.

Following on from this is the issue of how various equipment is currently earthed. Consider a simply flouro lamp screwed to a purlin in an all steel shed and supplied from the mains. It should be earthed as per AS3000 with an earth wire at the fitting. Said fitting will be electrically connected to the purlin and thus the rest of the metalwork of the shed, by the mounting screws. Any item of equipment supplied from your created 3 phase grid (and earthed via it) will possibly have to be kept electrically isolated from any other metalwork connected to the mains earthing system. Alternatively you could test between the two earth systems with a voltmeter to see if there is any voltage present. I'd suggest doing so under a few diffferent load conditions. I'd expect no voltage difference at no load or evenly balanced 3 phase loads. The greatest voltage difference probably will occur if there is significant single phase load. However I could be completely wrong on this.

It's probably doable to seperate the two earthing systems if need be, however it may require a fair bit of planning, implementation and testing on your part.

Cheers

The Beryl Bloke


Chris,

A schematic of a three phase autotransformer is shown about 2/3rds of the way down this page https://www.electronicshub.org/autotransformer/ In our case the terminals marked X1, X2 & X3 are connected to the 240 volt, three phase output of the VFD and the terminals marked X1, X2 & X3 provide 415 volts supply for equipment. We are looking at why you should not make connections between the star point (Marked "N") at the bottom of the diagram and a mains neutral. In theory the VFD would provide isolation and you should be able to connect the two points. In practise, I don't know enough about VFDs in general or any specific VFD in particular to be prepared to recommend anyone attempt this.

If these were isolation transformers connected in delta on the primary and star secondary, rather than autotransformers, this would not be an issue. Although I seem to recall the isolation transformer option was more expensive.


I am keen to hear how this works out.

Originally Posted by Theberylbloke

Chris,

A schematic of a three phase autotransformer is shown about 2/3rds of the way down this page https://www.electronicshub.org/autotransformer/ In our case the terminals marked X1, X2 & X3 are connected to the 240 volt, three phase output of the VFD and the terminals marked X1, X2 & X3 provide 415 volts supply for equipment.

Thanks for the diagram. I'm not sure if I'm looking at the correct diagram, but did you mean H1, H2 and H3 are connected to the 415 supply to the equipment? Otherwise you'd have the VFD output and the equipment all connected to the same terminals (X1, X2 and X3) which wouldn't achieve anything.

Originally Posted by jack620

Thanks for the diagram. I'm not sure if I'm looking at the correct diagram, but did you mean H1, H2 and H3 are connected to the 415 supply to the equipment? Otherwise you'd have the VFD output and the equipment all connected to the same terminals (X1, X2 and X3) which wouldn't achieve anything.

Sorry my bad, you are correct

Cheers