I was wondering if anybody could tell me if it is possible to shift the phases of a household 240VAC line to match that of 3 phase power. I have built a rotary phase converter, but one of the problems with rotary phase converters is that leg 1 and 2 are 180 degree phase while leg 3 is 120 degrees. Can anybody give me any advice?

-Adam

 

Hello,

Do you have a schematic of what you have upto now ?

I have posted a 3 phase oscilator in another thread:
http://forum.allaboutcircuits.com/showpost.php?p=122512&postcount=10

Greetings,
Bertus

 

What is the power requirement?

If the project is for experimentation or a small device, under 50 watts or so, it is usually accomplished from batteries which then run three variable frequency synchronized 3 phase oscillators, as in the link bertus gave above. The variable 3 phase are used in brushless motors for R/C Aircraft, and many other fun things.

If the load is over 1/2 HP, the expenses would be large for a solid state circuit.

Which method did you use for your DIY RPC? Did you start with a Delta or Wye connected motor?

 

Thanks for the reply both of you. The power requirement is very large. At-least 25A for now. It will be used to drive a large milling machine with 3 phase Yaskawa servos, and 5HP spindle. The motor I started with is a 10HP Baldor 3450 RPM. I got it from a friend for free. For right now, this is the method I am using:

I use a piece of tie-down strap wrapped around the motor shaft, pull it hard to get the motor spinning, then hit the start button and the motor revs up and runs. I want to display the phase angles on my o-scope, but my scope cant fit the whole wave on the screen and its only 2 channels. Any ideas on how I can fit the waves on the screen while were at it? lol.

-Adam

 

For the scope - Safety!

I'd suggest a set of 100:1 CAT III Oscilloscope Probes, but those aren't cheap.

Slightly more affordable method, relatively useful in the amplitude domain will show Time Domain correctly and safely for phase diferences, but not Accurate for Voltage peak measurements! Need 3 220V single phase to 24/48VAC, or other < 100V secondaries transformers, with one end of each secondary ground for scope.

With that setup, you could then view the now isolated lower voltage between ground and the outputs. Even if you only have a 2 channel scope, you would want to add the three step down transformers for a balanced load.

Right now, if you are using "Standard" Probes, there really isn't much insulation between you and lethal voltage/current.

 

The converter needs to be 10%-20% larger than the intended load using a great design, some say 33%. So, to start a 25HP mill, you would need a 30HP+ or larger Delta Wound RPC.

I don't know of ANY designs offhand that are DIY and 25HP. That is (over) an average household's power link (roughly 15kW) in a single unit (21kW after converter losses).

I'm sorry I can't be of more help. The big power units have been tweaked for efficiency by the manufacturers, and in addition are CNC clean, listed as not a fire hazard, etc. Due to those points, I could only recommend a reputable manufacturer. You mentioned servos and spindle, so I'm assuming it is a CNC unit, which adds a bit more to the price of a commercial unit.

Metalworking groups/magazines may have some extra information/better solution or answer, and may end up costing as much as a new one, or a lightly used one after all is said and done.

Others here may have more input though. It's late and my math isn't quite right. Maybe this whole time it was a 2.5HP motor you were looking at.

 

I never thought about the transformers. Thats a good idea.

As for the power, I mentioned 25 amp load. The milling machine that I am building has a 5 horsepower motor for the spindle and 3 motors for the gantry. The 3 servo motors total 6.2 horsepower. so thats 11.2 horsepower total for the machine. I know that the 10 HP motor that I am using is not big enough for now, but I just need something to do testing with and will be upgrading to a larger motor later.

As for power, I am able to put up to a 125 amp breaker in my panel, which happens to be right above my converter.

-Adam

 

but one of the problems with rotary phase converters is that leg 1 and 2 are 180 degree phase while leg 3 is 120 degrees.

A properly designed rotary phase converter has all three phases 120 degrees apart. For the power levels you require, it might be better to shell out the one or two thousand bucks for a 15hp factory-made unit.

 

I have looked into those high quality phase converters, but like you said they are 1 to 2 thousand dollars. This is just for a hobby machine, it wont be making me any money for awhile, if ever. I mostly just want to be able to power these servos that I have, but I have heard that they dont like messy power and tend to fry the power stages when thats what they get. I think that with 2 different phase angles, they wouldn't last long.

-Adam

 

OK, I am back to working on this project. I have found out that I am able to power the servo drivers with a large 300VDC power supply. The only thing is, I still need to create a 3 phase power supply to create 208 to 230VAC to truck the drives into thinking that they are still being powered off of 3 phase power. The drives have a phase detection circuit in them, and will not enable the motor without 3 phases hooked up.

So my question now is, How do I create an inverter stage to go along with the circuit that bertus posted? The load is very small, I dont suspect to need more then 1 amp to trick all 3 drives. O, Just as a note, the drives logic is powered off a separate single phase 220V input, so the 3 phase doesn't even do that.

Anyone have any advice?

Thanks

-Adam

 

It might be easier all round to buy a single phase motor.

One of the first projects I ever worked on as a young engineer was a 3-phase lathe speed controller, fed from a single mains phase. The 3-phase outputs were generated digitally, and switched by MOSFETs the size of my fist. There's a reason these units are expensive.

 

I cannot just buy single phase motors, these are high precision top dollar servos. The motors new are between 3 and 5 thousand dollars each and the drivers new are between 4 and 6 thousand dollars. Im basically sitting on 21K dollars worth of servos. There not new, about 7 years old, but I had a chance to hook them up to 3 phase power temporarily, and they are incredible.

Darren, Would you happen have the schematics for your motor controller? Would you be willing to share?

-Adam

 

Dose anybody have an idea of how I can use Bertus's circuit to drive some kind of inverter circuit? I just need an idea of how to drive the power stages.

-Adam