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3 Phase Converter Schematic. (Miller system)

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I was going through my old Cad files and found this schematic I made a while back.
I figured that with a good write up to explain how it works to back it up it could prove useful for anyone who wanted to run three phase motors or build a balanced phase rotary converter for running three phase powered equipment off of a single phase source. It’s also known around my parts as a Miller system.

This is not the crappy low power, low efficiency, weak starting phase converters that you buy from the dealers. This is a well proven good high powered high efficiency design that can make most unmodified three phase motors run on single phase with both full starting torque and normal running power and efficiency.

I have built dozens of them and many are in daily usage applications and have given years of uninterrupted service working as both motors, phase converter power sources, and a few are now in dual use applications where they do double duty as a motor driving a mechanical load and as a balanced phase source rotary converter that runs additional loads.

3ph converter.png
The way I designed this type of system (Miller System) is rather strait forward and that allows for it to have some basic rule of thumb numbers that will get you close enough to be able get your setup working and will allow you to fine tune your system with very favorable results after that. For a basic reference point in the descriptions I will use what’s needed for converting a standard 1 HP 230 Volt 1750 RPM three phase motor.

For a typical 1 horsepower 230 volt three phase motor to work well on single phase you will need two AC motor run capacitors (C1 and C2) of around 10 micro farads each and preferably with at least a 300 VAC rating however a higher voltage capacitor works the same. The motor start capacitor (C3) is a 100 uf 250 VAC type. These values are proportional to any size of motor within reason. For example a 25 HP motor would need two 250 uf motor run capacitors and 2500 uf of motor start capacitors. However the starting current would be around 500+ amps for a motor of that size though if its load was high! Being that AC capacitors for these types of applications don’t come in individual sizes that large it will be necessary to use several smaller ones set up in parallel banks in order to get close to the needed working values.

Obviously if other voltages, frequencies, and power rating systems are used the numbers will change as well. The fact that they still follow proportionally is what’s important. 50Hz needs 6/5th’s the capacitance. 400 volts needs 230/400th's the capacitance and KW's needs 1000/746 the capacitance. The voltage reference values for tuning follow similarly as well.

These recommended capacitor values are typical values and are by no means set in stone. Depending upon the characteristics of the motor and what type of loads it powers they can vary from as low as 50% to as high as 150% of the typical suggested values. So if an exact capacitor value is unavailable just pick its nearest value, its likely going to be close enough. The recommended 10 uf per hp at 230 VAC 60 Hz with a 100 uf per HP starting capacitance just works out as the common average. Also because every motor brand, model, and speed has a different inductance and average efficiency you may need to change the two capacitors values up or down for peak power and efficiency when matching the actual motor to its load. That is why the actual values you may end with could be rather far from the typical suggested values.
This is done by reading the voltage across the L1 - L3 and the L2 - L3 lines while the motor is at its typical load range.
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The run circuit.

The two motor run capacitors go from each supply line (L1 and L2) to the third phase line L3. That creates two basic LC tank circuits which use the rotors spinning motion to create a simple phase shifting autotransformer effect in the motor itself once it’s up to speed. This is what makes it possible for the motor to run up to its full working power rating. This is also how the three phase windings are able to work at the proper phase angle relationships to each other so as far as the physics are concerned the rotating magnetic field sees three sine wave power sources with a 120 degree phase angle separation between them.

If the voltage is higher than around 115% of the incoming line voltage the capacitor values need to be bigger. If it’s lower than 90% of the incoming line voltage they need to be smaller. And always keep them equal. This is keeping the LC tank circuits tuned to near the line frequency and is what keeps the current loads between each phase balanced using that phase angle shifting autotransformer effect.

3ph converter.png
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The start circuit.

The start circuit is just a larger value of capacitance that is momentarily connected between L1 - L3 or L2 - L3. To set the direction of rotation connecting to one leg (L1) starts it rotation going one way and connecting to the other leg (L2) starts it rotation going the other way.

Obviously disconnect it from the circuit once the motor is up to speed. This can easily be done with a common potential relay (S1) that is common to HVAC applications. However these relays release voltages will likely need to be adjusted in order to prevent it from either releasing too soon or not releasing at all.
If it doesn't release when the motor is up to speed its voltage needs to be dropped. If it chatters or releases too soon it voltage needs to be raised. Most potential relays can be disassembled and adjusted internally.

For setting the basic start circuit uses a potential relay that’s rated for a release voltage range near your motors third line running voltages. That is if both the L1 - L3 and the L2 - L3 voltages are around 230 - 250 volts when the motor is working normally the potential relay should have a release voltage of 240 volts or slightly less. However lowering or raising the starting capacitance value will change the starting current draw and torque and will also affect the potential relays release voltage set point as well.

3ph converter.png
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Some additional set up notes.

One often overlooked problem that does frequently arise is that as with a large single phase motor this system also has a high starting current draw until it gets up to speed. That can create a large enough voltage drop that the start cycle may not be able to function properly despite much efforts in tuning it.

The only cure is to have large enough wire and a large enough power source to be able to feed it properly. On a 230 volt 60Hz system a typical load number is around 4 amps per HP at full running load. Starting load can easily be 5 times that though. A 40 amp circuit can easily start and run a 10 HP motor provided that the actual source can support the possible 200 amp start cycle load with out excessive voltage drop at the motor itself. Changing to a smaller start capacitor value will help limit the starting current but it also reduces the starting torque as well. This can cause slow starting which puts a heavy load on the windings and supply wire for a longer period of time.

Depending upon where the unit is being used and the length of the supply lines from the source plus the actual capacity of that power source the maximum size motor you may be able to start will vary greatly. Someone on a farm or with a large house that has a 200 amp or larger service that’s supplied by a 25 KVA or larger transformer could conceivably power a 20 hp motor without problems. However a person living in an old residential neighbor hood in town that shares one common transformer with several houses may not be able to run over a few HP without the whole neighbor hood knowing about it every time it starts!

For reference I run a 15 Hp industrial air compressor off of a 60 amp circuit and it has no problems starting all the way down to around - 20 F. But the motor is ten feet away from a 200 amp service that’s supplied by 4/0 gauge wire connected to a 15 KVA transformer 50 feet away.
So before you run out and buy that industrial monster machine you found for scrap price be aware that you may have problems getting it to go if you don’t have the actual power system capacity to start it.

Assembling one is reasonably easy and that’s why I give the information out for free! However tuning it to work efficiently and reliably is the hard part!
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Hi i had a question on your schematic in the description to use power correction caps between each phase line 7.5 mfd for 220volts where would these be installed? If my converter is up and running are they something i would put across the three output phases? I am building this to run a welder . It is based off 15hp detla-wye 1725rpm westinghouse 256 frame 220volt motor. I plan to utilize apporox 15 amp draw of of the 40 amp potential current capacity i will have available.
Iwould appreciate any help you can give. Thanks
The schematic in the first post pretty much explains the connections and the relative capacitor values as well.

I am not sure how to make the circuit connections any clearer than that.

For a 15 Hp wye connected motor the C1 and C2 values for a 220 - 240 volt motor would be roughly around 150 uf each. The welder itself may affect that number though and require a bit more depending on its design. No idea how much though.
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sir iwant single phase to three phase converter circuit for small model designing (eg:12v single phase to three phase convertor) thankyou am waiting for ur replay
my email
All the numbers you need are in the schematics and relating write up.

Understanding and implementing them properly is up to you.:)
Phase converter

Thanks for the help. The reason for the 480 single phase is the line loss on a long run and the wire cost when higher currents are required. The 480 reduces both. If the utility can supply 240 single phase they can supply 480 single phase. The application is an one hundred hp center pivot.
Thanks again,
Additionally if you have the formula that shows the voltage relationship to capacitance, I would appreciate a copy. I know the increased voltage lowers the motor current, ie the capacitor current requirement, but do not have documentation.
Unfortunately at that power level you would be far better off getting full three phase power brought in or to run a diesel, propane, or natural gas powered pump system.

At 480 volts you wont find any motor start capacitors that will work directly so a large combination of series and parallel 250 volt rated ones would be needed which will be expensive. Also at 100 HP the starting currents would still be massive even on a 480 volt source. Basically to be honest doing a 100 HP single phase electric pump system is not likely going to be cheap or practical.
Your time and money would be better spent on getting a full three phase power supply brought to the site or going with a diesel, propane, natural gas powered pump.

Just for estimations sake my rough numbers for theoretical sytem like this would require the two running capacitor banks to be around 300 - 500 uF 600 volts each and the start capacitor bank to be around 3500 - 5000 uf 500+ volts. Although power factor correction capacitors in the in the 600+ volt 50 uf size are available they are not cheap and as far as the starting capacitor bank going you would need about 30 - 40 500 uf 250 volt rated ones which is not cheap either. Add in contactors relays and wiring and I would suspect you may be into this system for well over $3000 - $4000 and I would not guarantee that it would work reliably either.

Phase conversion systems like mine work well for smaller more practical 230 volt systems (under 20 - 25 Hp in most cases) but as the power levels increase past that the cost goes up to the point its just not practical in comparison to other ways of powering things.

Sorry but thats just my viwes on this large of system.:(

I found your schematic and explanation very informative and useful. Thanks for posting them here.

Hi there. I built a system very similar to yours some time ago. I did many trials to settle on final capacitor values, finally getting a low current demand and quiet running, with well balanced phase voltages. I used a timer and relay for the start circuit. To get my balance of the phases, I made a little star connected matrix of three 68kohm resistors, and checked all three "phase" voltages using the star point as a dummy neutral. (any comments on this?). I was working with a 230V 50Hz supply and an old 2HP 1440 rpm motor. My values of capacitance were 50uF and 8uF, with a second 50uF across the other 50 for startup. This gives me a decent 230V three phase supply, but is there any simple and safe way to get a 415V 3 phase output? I have seen one on Utube, but it looks crude and unbalanced to me, appearing to have no capacitance present when switched from delta (start up) to star (run). This would save me having to uprate contactors and change trip circuits on my new lathe, and later horizontal miller.
Many thanks, OldLes.
Input voltage

I have come by a 30hp rotary converter. It was previously installed with a split single phase 480v input to give 415v 3ph output. Am I able, without too much modification, to plumb this into my workshop which has only 240v?
Any pointers greatly appreciated.
Only if it has connections for a 240 volt input otherwise you will need to have a substantial step up transformer to get it to work!
GEDW, I see it was March when I posted my question above, and since then I have analysed the circuits of a Transwave static convertor, a Transwave 10KW rotary, and a Boost 12 KW rotary. All 230 v single phase input, all incorporating a version of 230 to 415V step up auto transformer. The Boost has a simple one as described, the static has a "double" one (415-230-0-230-415) whilst the Transwave rotary has a 0-230-415, + 0-180 with a few extra tappings to allow probably for a 380v or a 415v output.
For you application, you would need a simple 0-230-415v autotransformer, BUT it would have to be rated at 25KVA, which would be capable of pulling 100 amps from your single phase supply. Is your supply big enough? You may need to contact your local electricity board regarding any special regulations or requirements.
How big an OUTPUT do you need? You could try to pick up a (say) 10 HP motor of otherwise similar specification, and reduce all you capacitors to one third of their current value. Typically in these rotaries and statics, if they need say 1,000 microfarads, they would use perhaps ten off 100 UFD in parallel, so you would simply remove six of them. If you follow this route, be sure to establish exactly the slave motor's specification. You may generally come across motors today which are marked up 230v delta connected, 415v star connected on the nameplate. Both the above mentioned convertors had 415v DELTA connected motors THESE ARE NOT THE SAME!!! Instead of 230/415v, they are really 415/720V, and all were two pole (2,940 rpm). The sort that would be used on a large fan with a star / delta startup configuration.
Since March, I have modified that UTube design I referred to, experimented with six different slave motors, and I THINK I am now at the stage where it WILL WORK!!!!
Addition to previous

I just realised, I don't know your location, GEDW. I am in the Isle of Man, which like all the UK, uses a 230V single phase system, or if you are lucky and can afford it, a 415V three phase system. If you are on the wrong side of the pond, please ignore my previous post, as I don't know anything at all about your systems.
Old Les.
I'm in Wales so the voltages apply. There is a good sized toroidal transformer mounted in this phase converter (8" by 3.5"). The input and output are 11 core and 9 core individually covered wires respectively,(could the voltage be varied by disconnecting some from the input?) and there is a smaller output to a PCB. The most power I would draw from this would be a welder of around 8kW; does it matter if a converter is oversized? There is also a larger PCB with 4 LED's on it to show 'supply voltage' from 180-240v, though I think this is to do with balancing the phases.
It's a 'Trident 30' made by RMK components from Belfast I believe. It cost me nothing so it's worth a bit of work, but I've very little knowledge beyond a vague idea of how converters work.
Because of my current interest in these machines, what would be especially interesting would be to get a circuit diagram. With that, I could maybe advise better. It only took me about 30 minutes to draw out the circuit for the Boost machine, though I must admit there is one small section (regulator) where I have had to draw a little "black box outline". If you want to discuss this for a while "offlist" until more progress, PM me and we can take it further.
I am not sure if it is OK to use a welder off an RPC. I suggest you ask more questions about that. I have a Citringham welder, with output at 50 and 80V, If memory serves it is 180 amp max output, so I guess that is 180 X 50 = 9KW, so similar to yours. However, mine is 230 or 415 V, either direct to single phase mains, or across two phases of a three phase supply. I would hesitate to connect that to two phases of an RPC. I did consider sacrificing my welder to build my RPC, as its primary is tapped at 0-230-415V, so in effect it is an auto transformer such I described earlier. I can only run mine off 230V, so, using a 16A supply, I guess that limits me to about 3-1/2KW.
I am not sure right now if you can post pictures here, but a few photos from inside your Trident 30 would be interesting.OldLes.
I am looking at building one of these for my 6hp 3phase motor to run from 240V single phase
The capacitors are no problem but I have two questions
1/ At which points exactly do the power factor correction capacitors attach?
1/ Can you point to a suitable relay on ebay for me ? I have surmised I want something like a 90-66 so the min & max voltages are around the 240 Volt mark
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