3 Phase Converter Schematic. (Miller system)

DIY plans

Earlier in posts 13 and 16 I referred to a 240 to 415 converter. It started when I saw a crude setup on Utube which prompted interest on my part. I decided to see if I could make a decent working circuit myself. I spent four months of my spare time working at it last year, and finally reached a successful conclusion with which I power my Harrison L6 lathe. It is based around a 3HP slave motor, and it runs my 2HP powered lathe. You can view it here:-- https://www.vimeo.com/47988196 . It was NOT an easy thing (for me at any rate) to get it to work, and I met numerous hurdles on the way there, but at the end I knew rather more than when I started, and as a result I decided to put the info together as a set of building plans, with the complete background to the problems met and overcome, so that any competent person with an interest can build one themselves. I have started to build a second unit from new parts only, but have so much other work to do (motorbikes!) that I have yet to finish it.
I am currently offering it on Ebay, so if anybody is interested, PM me and I will give you a link to it.
I have covered the basic circuit for 230v to 230v, a 230v to 415v version with auto transformer, and then my own circuit, so it should be useful to anybody wanting to build any of these types themselves.
OldLes.

tcmtech said:

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.

Click to expand...

You don't show or explain the power factor correction caps.They are mentioned on the top right of the schematic but are not shown on the circuit

OldLes-Your Vimeo does not work on my Linux box do you have an alternative?

Vimeo clip.

April, I also use Linux (Ubuntu 10.04LTS), and it still works OK for me. Perhaps try again? Maybe you should try "downloading" the file, then play it on you box.
I just had another look at the circuit posted by Tcmtech at the head of the thread, and must say that the values he gives don't agree with any I have used, and for my original 230v in 230v out RPC I tried possibly 50 different combinations of capacitor values. HOWEVER, I should stress that I was using a typical UK version 230 / 415V three phase motor, obviously delta connected on 230V.
OldLes.

You don't show or explain the power factor correction caps.They are mentioned on the top right of the schematic but are not shown on the circuit

Click to expand...

C1 and C2 on the right hand side of the schematic in post one show where they go and paragraph two of post two explains their sizing and why.

Not sure how to make it any clearer.

tcmtech said:

C1 and C2 on the right hand side of the schematic in post one show where they go and paragraph two of post two explains their sizing and why.

Not sure how to make it any clearer.

Click to expand...

Look I am trying to be constructive here not rude but you don't explain it well
On the left side you give the calculation for C1 and C2 as 10 uf per hp and I work this as 40uf for 4 HP
On the right side you give a much lesser value for inductive loads such as a motor? and call them "power correction factor caps" ?
Are they the same thing then ?
If so which value should I choose ? Not so clear at all
Driving a 3 phase 240 V 4 HP motor I base the following values on your diagram left
C1,C2 = 10 x 4 = 40uf
C3= 100 x 4 = 400uf

Your suggested power factor correction caps are 5uf per hp = 5x4=20uf (half of the previous value)
So which is it please??

april said:

OldLes-Your Vimeo does not work on my Linux box do you have an alternative?

Click to expand...

I guess thats a no then ?

tcmtech said:

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.

Click to expand...

I'm guessing then ,my values are correct, so what is the bit on the right of the diagram there for ,and whats it about ?

http://electrical-riddles.com/topic.php?lang=en&cat=5&topic=271
This site adds some knowledge too

I'm guessing then ,my values are correct, so what is the bit on the right of the diagram there for ,and whats it about ?

Click to expand...

Post two paragraph four covers why the values can vary.

Relating to the right side notes of the schematic, that refers to external loads above and beyond just using the unit as a electric motor only. Should it be used as a rotary phase converter to power three phase devices such as other motors or equipment that has large transformers, like a welder, ideally it would need have extra power factor capacitance added to keep the phase angles close to 120 degrees and the individual phase voltages within normal operating ranges.

How much capacitance to add or take away though will depend on the load so to find the ideal values to get the line voltages to balance out will vary.

After following this thread, I feel is should delete, retract or maybe apologize for my earlier posts. Still not sure I agree with delta for low volts and wye for high volts, never seen it that way. As for 3 phase running single phase, I have seen it, but didn't know it. (Roto Phase). I'm retired now due to a debilitating accident, but I worked many years in the electrical field as every thing from designer, maintenance personnel to oil field electrician where you have to be adaptive and creative, like tying two different sized generation together, manually, to get a big motor started, then drop out the smaller generator after it's running and making 200 amp fuses out of #12 solid wire for an REN fuse.
Any way April, if I were to build what you want for my self, here how I would probably do it.
This is assuming you're working on your house and after you done, you'll just store the saw, if you're a contractor and plan to use this saw all the time, I would add a current sensing relay to drop out the start capacitor and use NEMA 4 or better enclosures because (assumed) brick cutting saws use water and a motor starting switch with over load protection.
Kinarf

kinarfi- I would suggest that maybe you have a typo as you have c1 and c2 on the same line .
Should one be on the black line?
You are right about the wet environment.

Nothing more impressive, in my opinion, than someone who admits they might be wrong.

tcmtech - I am intrigued by what goes on in the starting wire coil in the motor once the motor is running and the starting capacitor disconnected.
I saw in some diagrams that a run capacitor can be put in that third phase line too . What does that do in the motor while running?
Is there current and voltage in that wire? If so what is the phase difference.
Others do this with a slave motor between and take the 3 phase from the slave terminals so I guess there is a normal voltage produced somehow.
The two motor system seems energy wasteful to me at this point

The two stage systems lack of efficiency was largely why I started working on trying to find a way to improve the working efficiency of a normal three phase motor when running on single phase.

To me the fact that all it take to get the power and efficiency up is to have the right value of capacitance to keep the three phases balanced. After that the only stumbling block was an automatic starting system and that was just a matter of using enough capacitance to force one of the phases to be far enough out of sync to make a solid rotating magnetic field form.

To me what surprised me is that no one ever bothered to carry the concept past using just one small capacitor to make a rotary converter unit. It was really just a matter of adding more capacitance in the right places then doing a enough testing with different sized motors to find the average values needed to make it reliably replicable.


Relating to the generated third power line of course there is power going through there. The two motor run capacitors make simple LC tank circuits out of the other two winding sets and the rotating magnetic field of the rotor keeps them balanced pretty close to 120 degrees out of phase from the power input.

If you do amp and voltage measurements of between the two power leads and the third phase line you will see that if everything is setup right for that particular motors characteristics the voltages and currents in all three lines will be within the normal working ratings of the motor.

The three leads of the motor will have fairly equal currents between them and the current between the two power leads and the third phase will make up the difference for the total power going in.

Thanks tcmtech
Did my 4.6 Kw (6HP) 3 phase with this setup

Motor is in star (wye) configuration
2 run capacitors of 1 of 46 uf and 1 of 10uf
1 start capacitor of 460uf with 1 pushbutton
Single phase line in is 240 V - 2 lines ,active and neutral.
Earthed all through

Success Voltage seems balanced at about 235V each line -no load yet

Hello Folks,
I hope you will bear with me as I have not had any experience with electrickery for half a lifetime. I have an old lathe with a 5Hp Star 3 phase 415v motor. I want to run it off 240v single phase which would mean changing it to Delta. The information shows only the Star arrangement, can it be made to work in Delta and are there any other considerations to allow for this change? Any advice would be a great help, thanks.

Star or delta?

Simple answer is yes.
Remember you are in UK, this thread originates from US, so it is easy to draw wrong conclusions, even though the originator does give data for different voltage supplies.
The "plans" I currently have for sale on Ebay UK show how to get 415v RPC using a motor such as yours as the slave, but in the build up to that design, I give a circuit for a 230V delta connected motor such as yours producing 230v three phase. In fact all the commercial RPCs I have looked at were delta connected, and this star connected design surprised me. (Just shows my ignorance, I gather the star circuit for an RPC has been used for decades just like the delta circuit).
Les.

Either delta or wye winding configurations work just fine with this design. The important part is getting the two motor run capacitors sized right under the motors normal working load conditions.

As long as the phase to phase volts are within 10 - 15% of the line voltage the running amps for each phase will be well within the motors rated working range.

Now relating to common motor winding configurations that varies with every country. I have used this design with both Delta and Wye and triple Delta motors without issues.

If you are wondering what a triple Delta motor is it's most often a 9 lead or 18 lead 230 - 460 volt type found in the US and Canada. To change voltages it either uses all three Delta winding sets in parallel for low voltage or stacks one of the three on top of the other two in a series parallel combination for high voltage.

Thank you for the information, I have ordered 3 digital voltage displays so I can see the voltages at all times, I assume they will have to be wired to earth. I also intend to stick a temperature switch directly to the motor just to be really sure all is well. Just need to source a shed load of capacitors and work out all the other switches and wiring which should be fun.

Star or delta?

TCM, as I said, I have not come across the star circuit before, and all the (UK sourced) RPCs I have seen have been delta connected. However, it is the C1 and C2 values that intrigue me. The values for your star circuit for a 2HP motor motor at 240v are both 20Ufd. For a delta circuit, I arrived at C1=50Ufd, C2=8Ufd (and C3=50Ufd).
I assume your values were calculated for 60Hz, whilst mine (UK) were found at 50Hz. I arrived at these values after MANY tests, so wonder why your C1:C2 ratio is so different to mine?

Two commercial units I studied by different manufacturers ONLY had C1 capacitors, NOT C2.
A 4KW version had C1=70Ufd, a load-switchable 40Ufd in parallel as required, with a further 40Ufd C3 start capacitor.
Another, 12KW design, had C1=120Ufd, with a further 600Ufd in a circuit in parallel with C1. The circuit included a "black box" and a power thyristor module. Because of the little sealed black box, I can not say how the 600Ufd was involved, but I assume it was both available for start up (C3) and load control (C1).

Les.

From what I have been able to find many of those designs came from 50 - 70 years ago and back then getting higher value AC capacitors was very expensive so they went with as little as they could get by with and the designs, functionality or efficiency never got questioned since.

Sort of works enough and works well are not the same thing!

Now relating to using larger values of capacitance for C1 and C2 as I have covered in my first few posts that largely related to added phase and power factor balancing/correction for loads other than the device being used just as a motor. Also the size of the starting capacitor can vary greatly depending on if the motor is running a low inertia starting load like wat a pump or fan sees or a high inertial load like what a fully loaded grain auger, conveyor, or lifting hoist may see.