### Author Topic: (AC power) Converting single-phase into 3 phase: What are my options?  (Read 5990 times)

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#### Brumby

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #50 on: September 23, 2018, 03:28:34 pm »
It's been stated before that messing about with the internals of this "black box" is completely out of the question.

It should also be repeated that there is a known weakness with the neutral line inside the box - and since messing about inside is out of the question, then any solution has to be sympathetic.

You only need to think about the basics regarding current in the neutral to appreciate the need for a 3 phase solution....  Say you have 3 circuits that each draw 20A.  Use a single phase solution and the neutral current is 60A.  If your neutral is taxed at 40A, then running everything from a single phase is just not a long-term option.  Use a 3 phase solution and the neutral current is zero (for a balanced system) - but even when unbalanced, the neutral current can stay well within its limit for quite a range of unbalance.

And just to repeat - it is some part of the neutral line within the black box (that we cannot modify) which is the limiting factor.

The solution must not be a single phase one.

#### NiHaoMike

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #51 on: September 23, 2018, 03:46:36 pm »
You only need to think about the basics regarding current in the neutral to appreciate the need for a 3 phase solution....  Say you have 3 circuits that each draw 20A.  Use a single phase solution and the neutral current is 60A.  If your neutral is taxed at 40A, then running everything from a single phase is just not a long-term option.  Use a 3 phase solution and the neutral current is zero (for a balanced system) - but even when unbalanced, the neutral current can stay well within its limit for quite a range of unbalance.
Run two phases from one hot of 120/240V and the third from the other hot. The neutral current would then be 20A with equal loads. As mentioned before, since the load is not balanced, put the phase with the highest current draw on one hot and the remaining two on the other hot to get something closer to balanced.

It's also worth noting that with non PFC rectifier loads, the neutral currents do not fully cancel with 3 phase, but do largely cancel with split phase. Therefore the split phase idea would be even less of an issue. I would say wire it up split phase and monitor the neutral current to determine if it's acceptable.
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#### Beamin

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #52 on: September 23, 2018, 03:59:42 pm »
Is this some sort of milling machine with CNC and motor controllers? Hence all the different power supplies?

I have a hard time seeing and often miss things or if a thread has more then one page.
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#### drussell

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #53 on: September 23, 2018, 04:54:33 pm »
I'd go for the 3phase powerline installed to the building by the power company .

cvanc has to come up with a solution in less than 2 weeks

I believe that when the OP said:

I've got maybe 2 weeks to propose a solution so there's a bit of time to kick this around but not a lot.

OP was trying to say that while he needs to propose a solution in the next week or two, that does not necessarily mean it can be implemented in that timeframe, though I am sure that it will become a "sooner, rather than later, please" affair shortly after that.

Since the power company hasn't even responded yet, there is a good chance that it could potentially be many months before that could be installed, if it is even within the budget.  If three phase is already in the building, it will be possible and cost-competitive with other proposed solutions, probably in the range of $5000 to add a minimal 3-phase service. If 3-phase is not currently in the building, the price instantly becomes tens of thousands of dollars. If it were in a remote location (unlikely), it could be$100,000.  Once you get into multi-tens-of-thousands of dollars, it may be more cost effective to change locations than to bring 3-phase power to that building.  Without knowing anything about the building or its location, I cannot speculate further, nor can the OP until better information is received from the client.

Any operation that is running an old telecine machine like this (OP, what is this, a MkIII or something?  ) is either doing something very specialized, or more likely trying to save money by using older equipment and that may well make the cost of bringing in 3-phase unattractive.  The OP knows more about the overall situation that I do, though.

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and i don't think the proper rewiring of the machine for 2 or 1 phase operation can be done in this timeframe due to the complexity of the existing installation inside the machine and the problem of accessing it and moving this really big , heavy and fragile heap around without crashing it .

For single phase, 120 volt operation, absolutely.  Not only would that require a complete re-wire of the main power distribution in the machine, it would be an impractical thing to do.  Why would you load down one side of your 120-N-120 with a single 60-90 amp load?

For split single phase, 120-N-120 operation, that remains to be seen.  While I certainly understand the OP's hesitation about attempting any modifications, and appreciate the fact that this position may well not change, it is still an option, and the main distribution inside the machine does not have to be accessed or touched in any way to achieve.

If we assume that there are 7 power supplies on each of the three incoming phases, the OP would likely only have to be able to access the power input wires to three or four of those supplies that are on any one of the three phases.  New wiring could be brought directly out of the unit and connected externally to whichever phase had the lower loading.  Cost?  Maybe a couple hundred dollars in supplies, absolute maximum, plus the OP's time to get to those three or four supplies within the unit.

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The rotary converter seems to be out of the question because of size / weight and noise issues .

Size, weight and noise are actually relatively minor concerns with this approach.  I am more concerned with that kind of unit's ability to provide anywhere close to balanced voltages.  Not only do they typically NOT provide anywhere near a balanced voltage output, the mismatched and time-variant loading of this unit complicates matters further.  It will be virtually impossible to statically balance the capacitor arrangement to have anywhere near proper balanced power.  What may work fine for a motor in a machine tool is not necessarily appropriate for a fragile telecine machine that expects clean power.

I also think that harmonic currents are going to be a huge issue with a rotary converter.  Clipping of the wave is going to be severe unless the motor is huge and the cost of 25+ HP motors is not insignificant, nevermind the capacitor bank, etc.

A third option is a motor-generator set.  THIS has the potential to work well enough, provided the generator is sized appropriately to handle the horrible power factor without completely losing the tops of the waves.  Far superior from an electrical perspective than the rotary converter but, again, it is going to have to be vastly oversized from the nameplate rating for a resitive load to be able to handle capacitor-input rectifiers clean(ish)ly.

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Gluing the 3 phases together to 1 phase is a nice idea

No, actually that is a terrible idea.

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... - i wonder how big the current impulse will be when you turn on 2 dozen big power supplies at the same time . . .    and i'm not sure this nice setup with the 3 audio power amps will be able to handel the startup current either ( that is where the rotary converter will work pretty good )

A rotary converter would just have severe voltage drop for short time while the capacitors were charging.  That isn't really a good or bad thing.  A motor-genset would have the same issue, but that in and of itself wouldn't cause any problems.  There are far bigger problems with a rotary converter.

As for the electronic APS-3720 AC power supply / regenerator doodad shown above, the available output current depends on what the original amplifier capabilities are and how they have been modified.  The original specification for the design requirement given to me was for a 3-phase, 400 Hz, 115 volt, 20 ampere supply, later amended to be variable from approx 200-600 Hz for flexibility in testing once I offered them the option of it being variable.

My initial testing was done with a QSC RMX-2450, since I had one on hand anyway, and it worked so well that I have kept using that same circuit in subsequent units, even though I had initially thought it might need to be beefier.  The current limiting is adjusted to start to come in at about 25A and short circuit protection trip at a bit over 30A, IIRC.  I would have to check my notes.  There will be distortion of the sine wave by that point, but it will do it, and there is a distortion indicator on the front panel of each phase's LPA to indicate that condition.  I can start my 4 HP air compressor (wired for 120 instead of 240) off one phase.  It starts slightly slower than it does on MAINS power (takes a couple cycles to get to speed) just like it does on a generator (even wired for 240), but it works fine.

Even if it were an issue, this apparatus can easily ramp up the voltage to provide soft-start if required.  I don't recall if I ever tried a 2400W+ load that was that "nasty" like a huge rectifier-capacitor battery charger load simulation, perhaps I should characterize that, but all sorts of DC power supplies have been run from the APS-3720.

The one time that there actually was an issue with powering a DC power supply was shortly after delivery of the first unit.  It ended up being an interaction between the active power factor correction in the load (the DC power supply) and the feedback loop in the LPA setting up very high frequency oscillations in the LPA output stage and frying resistors in the snubber networks.  A design change was made on my end, the units updated and the resistors replaced.  There have been no subsequent issues powering any loads.  (I tried everything I could think of for load testing, from air compressors to microwave ovens, universal motors like power tools and vacuum cleaners, overloaded it with halogen lights and boiling water in kettles.)

I might just have to set one up for more torture testing, though, now that this thread has piqued my interest.

In any case, even more robust amplifiers can be chosen if necessary.  The modular topology of the whole apparatus makes alterations like that possible.  The APS-3720 shown above is capable of pulling over 40A from each of the three phases on the AC input.  Some of that goes to heat, obviously, but there is a whole bunch of surge current available to run just about anything I could imagine to throw at it.
« Last Edit: September 23, 2018, 06:53:03 pm by drussell »

#### Urs42

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #54 on: September 23, 2018, 06:07:16 pm »
I did hear that someone i know is using a modified three phase online UPS to generate three phase power from a single phase, i'm not sure what exactly he did, i think that he connected all three phases on the input to the same phase, i guess that some more modifications are needed.

#### Fryguy

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #55 on: September 23, 2018, 07:52:48 pm »
drussell - i know that gluing the 3 phases together is absolute BS and will never work without some modifications to the machine - this part was meant as a joke !

If your power-amp-rack can push that initial current without the overload-protection shutting it down - nice work !

How much would it cost to get a rack like that (working at cvanc's specs) ready to go and how much time would it take ?
« Last Edit: September 23, 2018, 09:13:07 pm by Fryguy »
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#### G7PSK

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #56 on: September 23, 2018, 09:02:14 pm »
If you really need a good 3 phase supply you need a motor generator as rotary converters don't produce even power on all phases and can lead to motors overheating, been in that situation myself and ended up with a diesel gen set as the requirement was for 40 KVA and I had somewhere to install the power plant.

#### sourcecharge

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #57 on: September 23, 2018, 10:11:16 pm »
Again, thank you everybody.

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #71 on: September 24, 2018, 03:35:45 pm »
I saw a AC source before, their really nice bits of kit, but they defiantly do have a high end look to them .

I think you have one per section and you adjust the phase shift between them. The ones I saw were meant to change in frequency and voltage and phase, so you can do military testing like 40-400Hz/90-300V AC on all three phases.

yea, looks like a chroma unit is what I saw. they look good.
« Last Edit: September 24, 2018, 03:41:59 pm by coppercone2 »

#### drussell

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #72 on: September 24, 2018, 03:45:59 pm »
It looks correct from the viewpoint of a delta connected load which doesn't know there is a common mode voltage at 90 degrees and a delta-star transformer would allow you to connect unbalanced star loads.

It's quite elegant really, it's much easier to see when you draw out the vector diagrams. Note that it cannot produce simultaneously exactly 208V phase-phase and 3x120 degrees from a 240 V split phase supply, to get to 120 degrees it has to add enough 90 degree voltage to push the phase-phase voltage up a bit. If this matters the extra voltage can be taken out again in the delta-star transformer.

Indeed, that is a good explanation of what I was trying to say.

It would work, and I expect it would regulate reasonably well even with varying loads on each phase but you need to add the transformer on the output and you will need to adjust the voltage to get close to 120 to N, so will probably need a transformer with multiple taps on one side or the other or a custom transformer.

#### drussell

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #73 on: September 24, 2018, 04:14:22 pm »
How many times have we seen a similar problem from someone with the 3 phrase needs and all of the commercially available products are either electrically noisy, audibly noisy, or both. drussell, it sounds like you have had some design experience in creating a power supply that can create a 3 phase system, and although I really liked the 3 phase audio amp idea that was discussed, this does not address large 3 phase loads.

Why not?  Three phase loads run just fine from an APS-3720.  The amplifiers have been modified to be completely isolated isolated and the appropriate side of the output from each is tied together to form a neutral.  If I set the voltage controls on the front panel to be 120-120-120 you will get 208 volts across any two legs.  Three phase motors run fine on it.  I tested an unloaded 3-phase motor on them (had no good load available), I tested my 4HP compressor across two legs, with and without additional loads and some of the door units the customer originally ordered the design for have large 3-phase 400 Hz fans and things like that in them.  They all work fine.

The load doesn't know what is powering it and the amplifiers don't know what they're powering and don't care about the power being drawn "120 degrees off" between amplifiers.  They don't know the difference between that and a reactive loudspeaker load.

With appropriately sized amplifiers, that type of solution should work for the OP.  If I can rig up a test load with the same kind of harsh, wave-hacking effects as his telecine machine would be presenting, I could verify that and change that "should" to a "certainly would".  His load might be right on the edge, power wise, of what the pictured 7200 watt RMS rated model can do (like an inverter, it will do about double that for an instantaneous surge) but using something like the QSC 4050HD amplifiers as the outputs should allow 30A continuous.

In any case, I'm not sure what you mean by "large three phase loads" wouldn't work.

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It looks to me that if someone could design and build a 25-30 KVA wye with neutral unit that is electrically quiet and reasonably priced that could be driven by a 240 VAC single phase power, this would be a win-win system that would be very popular among small shops. I have seen this problem over and over again where, either 3 phase was not available at the power poles at a location, or it was deemed too expensive to implement because the power company wanted an arm and a leg to place 3 phase service. This is always the problem for a smaller shop.

That's exactly what I designed the amplifier based unit for.  A cost-effective unit for a small aircraft engineering company here in Calgary at the little municipal Springbank airport hangars.  They happen to have three phase, of course, they just needed it to be 400 Hz.

It has proven to be satisfactory in service.  At least satisfactory enough its use is written into test procedures for the US Military for those systems, hence the need for them to buy more units for use at various places around the world.

I have thought about building ones into a custom chassis of some sort instead of supplying in rack form but have never got around to that.  Airdyne loves the current rack mount design and have some mounted in military-grade flight cases with a complementary Lambda 0-60V 50A DC supply to be able to simulate aircraft power anywhere they want.[/quote]

#### Brumby

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##### Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #74 on: September 24, 2018, 05:27:10 pm »
And from my read, that white paper makes no mention of phase angles other than for utility supplied power - and phase angles were the one parameter that I was looking for all through the paper.  As such, I can't see where you get the 120º input phase angle for their illustration.

Page 11, first sentence:

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A 10 HP model DPC-10 Phase Perfect® digital phase converter was connected to two legs of a 208V grounded wye three-phase service.  The converter generated the third leg voltage."

My apologies.  I walked right into that one as well.  Not having a good day.

However, I find it difficult to reconcile that setup with the statement "Balanced three-phase power from a single-phase source."   Wait ... scratch that.  My cold clouded head has some maths bouncing around - and I think I need to sleep on it before embarrassing myself any further.

Smf