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View Full Version : 415v three phase from 240v single phase a home made digital phase converter?



lamestllama
31st Aug 2018, 05:19 PM
I don't have three phase at my place and getting it would be expensive. With that in mind I set up an experiment that has been a few months in the making (I had to wait for Kryn to bring me a crucial part from Melbourne). The picture shows a 240V VFD (only 1.5kw here) connected to an 8Kva three phase autotransformer.
375129
The autotransformer was originally designed to take three phase 415v and output 230v three phase but I run it in reverse. Setting the VFD to ramp up to 50hz at 240v over a few seconds avoids an over current situation when switching the thing on . I connected my unmodified Colchester Bantam to the output of the transformer and was happy to see it run with direct online starting without any issues. As expected the inductance of the transformer avoids any over current situation at the VFD when the motor starts.

As mentioned this was using only a 1.5kw VFD I intend getting something around the 4kw mark so as to more fully utilise the capacity of the transformer any suggestions here would be welcome.

The voltage out of the transformer even in reverse is around 400v so close enough.

Anybody see any problems using this as a phase converter?

Vernonv
31st Aug 2018, 05:26 PM
Cool. Interesting concept.
So the VFD is already running (at 50hz) when you switch on the motor?

Petip
31st Aug 2018, 05:34 PM
For interest, have a look at Dynoguy on YouTube, l think he does a similar setup
https://youtu.be/UA9-jXlw4r4

lamestllama
31st Aug 2018, 05:50 PM
Yes it is running at 50hz before the motor gets turned on. It has to run at 50hz so that the output is 400v so that the contactors in the lathe will work as normal. The plan is not to have to modify machines before they can be used.

OxxAndBert
31st Aug 2018, 05:51 PM
Looks like a nice idea to me. I've seen those 3 phase step-down transformers around either cheap or free and it looks like it will give you the advantages of a phase converter that you just just plug 3 phase 400v gear into without the issues of a rotary converter.

Steve

lamestllama
31st Aug 2018, 05:54 PM
For interest, have a look at Dynoguy on YouTube, l think he does a similar setup
https://youtu.be/UA9-jXlw4r4

He puts the transformer before the VFD. I put the transformer after the VFD for two reasons. I have a machine that needs a neutral which I can get from a three phase autotransformer connected in Y. The other reason is the dampening effect the transformer has on the noise from contactors.

BaronJ
31st Aug 2018, 08:26 PM
Hi,

If you vary the VFD frequency a little, you should be able to find the sweet spot for the transformer output.

lamestllama
31st Aug 2018, 09:23 PM
Hi,

If you vary the VFD frequency a little, you should be able to find the sweet spot for the transformer output.

Hi BaronJ,

Sweet spot in what way?

BaronJ
1st Sep 2018, 05:28 AM
Hi,

If you measure the output voltage, you will find that there is a point where the voltage increases or falls with changing frequency. The sweet spot for that transformer will be where the voltage peaks. The better the quality of the transformer the shallower the voltage peak will be.

So for instance if at 50hz you get 400 volts and at 60hz you get 405 volts, but at 70hz you get 395 volts, then 60hz is closest to the sweet spot. The reverse is the same. If the voltage rises at 40hz then the transformer favours the lower frequency.

You may not see much of a change in which case you have a good transformer. Do it under load as well the change will be more marked. The difficulty making measurements under load using the motor is that the load will change slightly with motor speed.

Let me know what you find.

BobL
2nd Sep 2018, 09:32 AM
Cool idea but I'd like to see what happens when the motor is switched on or off under load. OK this is is unlikely to happen except in an emergency but I don't think the VFD will like it much as the transformer may not be able to protect the VFD.

The way to get around this would be to wire emergency switches to turn off the VFD and I do mean cut the mains to the VFD.

Are you using any of the other features of the VFD?, eg speed and braking controls?

I can see it being useful on 415V delta machines with built in motors.


As mentioned this was using only a 1.5kw VFD I intend getting something around the 4kw mark so as to more fully utilise the capacity of the transformer any suggestions here would be welcome.

Does this mean you intend running more than one machine from the VFD? I would be be interested to see how that goes.

I like Baron J's idea of tweaking the frequency to improve the transformer output but bear in mind that using V'F mode VFDs motors develop less power below 50Hz so there m ay not be any gain. I'm not sure if vector drive characteristics will transfer though a transformer. BTW Vector drives have to be tuned to a specific motor so this feature can't be used in a multi motor situation anyway.

Theberylbloke
2nd Sep 2018, 01:48 PM
I have thought of this setup in the past. I assume the VFD supplies 3 phases at 240 volts with no neutral? If I have that wrong please correct me.

Your created "neutral" has no relationship to earth. A consequence is that any out of balance loading may cause a voltage rise on your "neutral". The VFD complicates things a bit because I don't know enough to know how it will affect the circuit, however that rise in voltage on your neutral may translate to a voltage between your "neutral" and any structure that is bonded to the general mass of the earth e.g. the structural steel of your shed or an earthed machine frame. Thus there is a risk of shocks and tingles. How much the voltage on the "neutral" could rise is determined by the magnitude of the out of balance load and the circuit impedance.

A second issues arises if you are intending to use this to create a 415v 3 phase supply to several machines. How will you protect the individual circuits or will you rely on the VFD. The VFD can't see a fault to "neutral/earth" as anything other than an overload for faults past the autotransformer. Thus if you put yourself between a phase and neutral the result is likely a rather grim llama. I would check the fault loop impedance, after checking that your fault loop impedance meter will not place damaging loads onto the VFD in the process of testing.

The transformer inductive impedance is a two edged sword. If load suddenly disappears from the secondary side of a transformer while the core is energised, the collapsing magnetic field will very likely induce high voltage spikes, not at all good for your VFD.

I am actually very keen to hear how you go with this. I'm in a similar situation with the cost of 3 phase supply and this is an option I have been looking at.

Cheers

The Beryl Bloke

BobL
2nd Sep 2018, 02:25 PM
I have thought of this setup in the past. I assume the VFD supplies 3 phases at 240 volts with no neutral? If I have that wrong please correct me.
Correct.


The transformer inductive impedance is a two edged sword. If load suddenly disappears from the secondary side of a transformer while the core is energised, the collapsing magnetic field will very likely induce high voltage spikes, not at all good for your VFD.

That's what I was referring to in an emergency situation. Now I'm not sure if even turning the VFD off at the mains is a good idea as an emergency either.

jack620
2nd Sep 2018, 08:57 PM
Any reason why you couldn't tie the neutral from the Y to the mains neutral?

lamestllama
2nd Sep 2018, 09:46 PM
OK this is is unlikely to happen except in an emergency but I don't think the VFD will like it much as the transformer may not be able to protect the VFD.

I am thinking of adding some circuitry to get around this possibility.



Are you using any of the other features of the VFD?, eg speed and braking controls?

No just simple V/F mode with acceleration at startup at a rate that stops the initial inrush from causing an over current fault.



I can see it being useful on 415V delta machines with built in motors.

I intend using it on a geared head drill with two speed motor and on my colchester bantam which also has a two speed motor.



Does this mean you intend running more than one machine from the VFD? I would be be interested to see how that goes.


I certainly do and like you I am interested to see what happens.

I am currently working on a low pass filter between the VFD and the autotransformer.

lamestllama
2nd Sep 2018, 10:29 PM
I have thought of this setup in the past. I assume the VFD supplies 3 phases at 240 volts with no neutral? If I have that wrong please correct me.
Correct.



Your created "neutral" has no relationship to earth. A consequence is that any out of balance loading may cause a voltage rise on your "neutral". The VFD complicates things a bit because I don't know enough to know how it will affect the circuit, however that rise in voltage on your neutral may translate to a voltage between your "neutral" and any structure that is bonded to the general mass of the earth e.g. the structural steel of your shed or an earthed machine frame. Thus there is a risk of shocks and tingles. How much the voltage on the "neutral" could rise is determined by the magnitude of the out of balance load and the circuit impedance.

Again correct.



A second issues arises if you are intending to use this to create a 415v 3 phase supply to several machines. How will you protect the individual circuits or will you rely on the VFD. The VFD can't see a fault to "neutral/earth" as anything other than an overload for faults past the autotransformer.

I will probably use an RCD downstream of the transformer.



Thus if you put yourself between a phase and neutral the result is likely a rather grim llama. I would check the fault loop impedance, after checking that your fault loop impedance meter will not place damaging loads onto the VFD in the process of testing.
Not the only way of becoming grim during this experiment. But I do thank you for your input. I have thought about the issue of voltage between earth and neutral and if I can't resolve it safely then I will simply do away with the feature and modify the one machine I have that utilises it.



The transformer inductive impedance is a two edged sword. If load suddenly disappears from the secondary side of a transformer while the core is energised, the collapsing magnetic field will very likely induce high voltage spikes, not at all good for your VFD.

The VFD should have back emf protection but I am not against using some judicially placed metal oxide varistors.




I am actually very keen to hear how you go with this. I'm in a similar situation with the cost of 3 phase supply and this is an option I have been looking at.

Thank you for the input it is good to have ideas kicked around a bit.

BaronJ
3rd Sep 2018, 01:50 AM
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.

Theberylbloke
3rd Sep 2018, 10:10 PM
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

Theberylbloke
3rd Sep 2018, 10:21 PM
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

lamestllama
6th Sep 2018, 12:54 AM
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.

BaronJ
6th Sep 2018, 08:08 PM
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.

BobL
6th Sep 2018, 09:03 PM
Som infö about VFD waveforms here
VFD current waveforms and values (http://www.woodworkforums.com/f271/vfd-current-waveforms-values-185557)

Theberylbloke
6th Sep 2018, 09:41 PM
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

lamestllama
6th Sep 2018, 11:23 PM
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.



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.com/variable-frequency-drives-viewing-dc-bus/




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

jack620
11th Sep 2018, 09:01 PM
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.com/variable-frequency-drives-viewing-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.

Theberylbloke
19th Sep 2018, 08:45 PM
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.

jack620
20th Sep 2018, 12:19 PM
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.

Theberylbloke
20th Sep 2018, 07:35 PM
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