3 Phase Converter Schematic. (Miller system)

[april]

What I would like to know is "what is an RPC ?"
I looked for it and got this and am none the wiser
https://duckduckgo.com/?q=RPC

I see OldLes mentioned it above . Is it Rotary Phase Converter?

If people wish to use acronyms thats fine of course , but first write them out in full and then use the acronym . It helps such as me who wish to follow but have no idea what you are on about

PS I bought a three phase Compressor that needs 30 amps but I am just reselling it . I don't think I will have a go at running that from 2 phase, couldn't get a large enough 240 V supply.

 

[mk1wideboy]

What I would like to know is "what is an RPC ?"
I looked for it and got this and am none the wiser
https://duckduckgo.com/?q=RPC

I see OldLes mentioned it above . Is it Rotary Phase Converter?

If people wish to use acronyms thats fine of course , but first write them out in full and then use the acronym . It helps such as me who wish to follow but have no idea what you are on about

PS I bought a three phase Compressor that needs 30 amps but I am just reselling it . I don't think I will have a go at running that from 2 phase, couldn't get a large enough 240 V supply.

Yes, In this case RPC stands for Rotary Phase Converter.

 

[pcpellet]

How does one get a 400v three-phase output from such a system?

Other designs I have seen have the neutral input on the star-point of the motor windings (often labelled XYZ) and the input phase attached to one of the tails (point U) with a capacitor between the other tails V and W. Thus the three outputs would be from U, V, W with neutral carried through from the single phase source.

How does this affect where one might attach the run capacitors and do the values still remain roughly the same, ie about 12uf per horsepower using a 50hz supply. Also is the additional excitation mentioned between V-W above simply added to the value of one of the run caps?

The miller system as depicted works perfectly for me giving 220-220-235 three phase output with no load connected but I would like to make a 400v 50hz version too from another 5hp motor I have

Thanks for your help guys

Ger

 

[tcmtech]

THe quick and short answer would be to either use a dual voltage 230/460 12 lead motor and configure the windings to make the three phase power on one set of the winding and use the second set to boost the voltage or use a step up/boost transformer.

Personally I would go with the boost transformer method.

 

[tunedwolf]

tcm

What are your thoughts on the so called Digital Phase Converter? Have you ever built one? A commercial unit that immediately comes to mind would be the Phase Perfect range. I ask because I may well have a need for a convertor to run a CNC machine in the near future.

 

[tcmtech]

If I need a dedicated and accurate three phase output for something I just go with an off the shelf VFD unit. Most of the 5 HP and lower VFD units are designed to work with or are capable of using a single phase power source at a reduced load if not specified.

Bigger ones can too but usually need a bit of modification to their power input side first like adding more capacitance and a heavier rectifier systems and occasionally modifying any input phase sensing circuits so that they will ignore that they are no longer using three phase input power.

 

[Spottydogg]

I am currently attempting to connect up a 3-phase, 2.5kW, 3.4HP electric motor, in the Delta configuration from a 230V supply, at 60 Hz.
At 60 Hz, the motor is rated to run at 265V.
I am having trouble deciding what size of capacitor I should connect to make this motor run. I have tried 8, 12.5, 20.5, and 25 microfarads, which has made the motor turn, but too slowly and it is also causing the motor to heat up, and is tripping my breakers.
If anyone has some advice or formula to calculate the correct size of capacitor, this would be much appreciated.

 

[tcmtech]

Use two 30 - 35 uF motor run capacitors for the run set and a 250- 350 uF motor start to get it up to speed wired as shown in the schematics at the beginning of this thread.

Being you will be running it under it rated voltage expect that you might only have 2.5 - 3 Hp to work with without over heating.

 

[Mike Scoggins]

TCMTech, I have a mechanic friend who uses a 1970s-vintage 3-phase Delta-Rockwell 14-inch lathe (2 hp drive motor) in his shop along with a 3-phase drill press and 3-phase bench saw. Several years ago we installed a commercial Phase-a-Matic static converter rated at 1-3 hp to drive his equipment. This worked reasonably well for a while, although he sometimes had trouble starting the lathe. The drill press and saw, having smaller motors, always worked fine. After he blew the start cap in the Phase-a-matic, I used your Miller design to build him a rotary phase converter with a 3-phase 5-hp idler motor. Based on your formulas I used two 70 uF run caps, a 230 uF start cap, and a GE 3ARR3J3P6 potential relay (pickup voltage 162-175 volts, drop out voltage 40-90v, contacts rated for 35 amps and up to 5 hp). This worked well for over a year and a half, although he still occasionally had trouble starting the lathe up cold and would have to try it a couple of times to get it to run. The lathe has a forward-neutral-reverse switch and a Westinghouse "Life-Linestarter" Type N magnetic starter/contactor so the operating switch doesn't have to handle the full input voltage. A few months ago, he blew the start cap in the homebuilt converter as well. We reconnected the old Phase-a-matic (which I had since repaired) while I worked on the converter that I built. The old idler motor would not spin up to full RPM like it used to, so I suspected something was wrong with it and we reconnected the Phase-a-matic without it. (We had never used an idler with the Phase-a-matic.) I took the homebuilt converter back to my shop to work on it. I thought that increasing the start capacitance would help with his startup problems, so I installed two new 230 uF caps in parallel for the startup circuit. I also installed a new potential relay and two new run caps (same values as before), and put a 15K ohm 2 watt bleeder resistor across the start caps to discharge them. I have an old 3-phase Westinghouse 12-hp fan motor in my shop to use as a test bed and it started up fine with no problems, so I figured everything was okay. Meanwhile my friend blew the start cap in the Phase-a-matic again. We reconnected the homebuilt converter to the lathe, but we still had trouble with the old idler motor (which used to work fine), so I connected the homebuilt converter directly to the lathe as a static converter. The start relay did not seem to be picking like it should and the lathe motor got hot and started smoking. I suspected the start caps were not disengaging. We took the motor to a local electric repair shop and had it rebuilt, and my friend acquired a used Marathon 10-hp 3 phase motor to use as an new idler. After installing the rebuilt motor in the lathe and connecting the new idler, the lathe does not want to start up like it should and the contactor chatters and has difficulty closing. The lathe also runs backwards from the way it used to, and switching the phases doesn't seem to help; it either works or it doesn't, but the controls are now backwards. The start relay seems to be opening and closing (I can hear it click) but both the idler and the lathe motors are getting hot. I have tried swapping L1, L2 and L3 on the contactor, as well as L1 and L2 on the input, and nothing seems to work like it should. When the lathe does start up, the idler changes pitch and vibrates noisily. I can't help but wonder now if the motor repairman changed something around when he rebuilt the lathe motor. My background is in electronics engineering, not electrical power systems, and at this point I am at a loss to know what to do. Can you or any of the other experienced techs here give me some advice?

 

[tcmtech]

Have you checked your line voltages at the idler motor and at the machines it's running?

So far everything you describe sounds like a severe undervoltage problem being the voltage relays are not releasing thus keeping the start capacitors on line until they overheat and blowout.

Also the other relays on the machines chattering is a fair sign that they too are experiencing a severe undervoltage as well.

Given the 5 hp unit you designed on my formulas and circuit can start a 12 HP motor at your place without a fuss, where you likely have adequate power, but things are acting up at the other guys place with small 2 HP motors I suspect he has some supply wiring issues someplace.

 

[xdx]

Sir I am planning on adapting the Miller System on a small printing shop. Need some advise:
1. All 5 motors (10 HP in total) will be running simultaneously at a given time
2. The two 2HP main motors simultaneously requires frequent jogging for 16 hours of operation
3. Mains supply is 2-wire single phase 220v, active and neutral.

thanks in advance!

 

[tcmtech]

Shouldn't be a problem. I've used this setup to work as a rotary phase converter for a three phase welder and it endured doing dozens of start and stop cycles an hour without problems for years of regular welding work.

 

[tmdarth]

tcmtech

Do I correctly assume your "miller system" will power the working 10hp 3 phase motor? If so, what is all the talk of a RPC? It would appear to me that your system eliminates the need for the idler motor.

I would like to build this converter to power a 10hp 220v 3phase motor. From your information, I would use 100 uf capacitors for C1, C2 and 1000 uf for C3. I hope these work out as I don't have a basket full of capacitors. I have no idea of the load on this motor as it only turns an unbalanced shaft to produce vibration on a fairly large Vibratory deburing machine. My only clue is that it was built with the 10hp motor.

Am I on the right track?

 

[tcmtech]

Yes it will power a typical 10 HP motor directly provided the primary power source is capable of supporting it. I have a 15 Hp motor on my shop compressor that been run this way for about 10 years now and has never had problems.

RPC is Rotary Phase Converter which is what this design is of sorts. Th only difference is this design can either work as a dedicated phase converter to supply three phase from a single phase source or it can work to allow a three phase motor to be powered directly from a single phase power source without giving up any of the power rating or starting torque.

 

[tmdarth]

Yes it will power a typical 10 HP motor directly provided the primary power source is capable of supporting it. I have a 15 Hp motor on my shop compressor that been run this way for about 10 years now and has never had problems.

RPC is Rotary Phase Converter which is what this design is of sorts. Th only difference is this design can either work as a dedicated phase converter to supply three phase from a single phase source or it can work to allow a three phase motor to be powered directly from a single phase power source without giving up any of the power rating or starting torque.

The motor is rated at 30amps, but your warning of starting power worries me. What would you suggest as a breaker for the 220v supply.
Also, I have not been able to find an "AC potential relay" as specified.

And thank you for this whole thing, it will save me a wad of money and be fun building it.

 

[tcmtech]

If your capacitors are sized right you should get enough power factor correction for your 10 HP motor to run off of a 40 amp breaker set. My shop compressor runs off of a 60 amp circuit without problems.


Yea they're fun to build. Even more so once you get familiar with the tricks to making them work with most any odd motor and capacitors and potential relay you can find laying around and can get into debates with know it all electricians that say it's impossible.

 

[tmdarth]

April

Are you still around? I would like to know how your converter worked under load. Did you change the capacitors?

Also where did you find your "push button" that would take that kind of load? I assume that it was for starting instead of using tcmtech's automatic start system.

 

[marty58]

You Know if Old Les concentrated less on selling his plans or motor or whatever he is trying to sell via this thread, the discussion might be more constructive.

As a capacitor in the line changes the phase angle by 90 degrees only and the phases in 3 phase motors are normally 120 degrees apart , I must admit I am at a loss to understand why this works at all. However work it does.
I suppose that 90 degrees is 3/4 's of 120 so its close enough

 

[marty58]

The idler motor is started rolling by the phase shift given by the capacitors from the single phase line. As the motor rotates the squirrel cage or wire wound rotor has currents induced into the rotor bars or wire in the rotor which then induces a counter EMF or voltage on the three phases of the motor. That is the gist of the whole circuit. Hope this helps.[/QUOTE]

 

[SjwNz]

Hi Everyone,
just joined the group yesterday after finding this thread.
I am thinking about buying a bigger lathe which is 6hp (4.5kw?) 2speed 415V 3phase motor.
I looked at different options to run it.
1 - changing the motor to 240V delta and run a variable speed drive on 240V, but was told by people who sell the drive that I will not be able to use the VSD on the 2speed motor.
2 - Get a new motor to use with the Variable Speed Drive. ( limited to 2.2kw )
3 - Buy a Rotatory Phase convertor 240V in 415V out, made by EUROTECH which is only 30mins away but costs $3000.

The guy from EUROTECH told me a little about how it works but not knowing anything about these units before I was a little lost even though I work in the electronics area.
After about a week of searching the net and finding this thread its all starting to make scene now and I am about to have ago at making me a convertor.
Because I am after 400v output I needed a step up transformer, I have three 10 to 15KVA transformers that use to live on power polls and after looking
at the 240volt windings, I found there were 2 winding in parallel, and I was very quickly able to make a auto transformer from it .
I am going to remove some of the winding's as its making 470V and only need 400V.

I have found a couple of 10hp 3phase motors for the idler, one is 415V delta, and the other 415V in Star. I don't think it matters which one I
use as long as its wire for 415V , am I correct in thinking this?? Also one of the motors has 60Hz on its name plate, will it matter if it runs on 50Hz ??.

Just need to get some caps and begin assemble and testing. I and really looking forward to working on this.
I am already looking into switching in difference cap values while the unit is running by monitoring the phase voltages
to try and keep voltages balanced, as this was what the EUROTECH guy told me his units do.

If it all works out I will post my results if anyone is interested .