Creating a 3 phase output from a single 12v AC or DC rail.

[Life4Never]

Hey guys, I'm new here but considering that I watch Dave with his videos, I have decided to ask the community for help.

I am currently undertaking a little side project for school. A friend and I have decided to try and create a squirrel cage induction motor from easy to find parts as a demonstration of induction motors to the Physics department as a year 12 parting gift. The whole mechanical design of the project is simple and pretty much set out already however the electronics is a bit more difficult, at least for us who have basic electronic knowledge. The school has power supplies which can supply a voltage of 12 volts in either DC or AC at 50 hertz. I planned to create the 3 phase output from this 12 volt source. I have looked on the web about phase shifting the wave by using RC circuits however this was a stretch with my knowlege in DC only and pretty basic, as well as that  the electronics was not really the point of this project.

The basic setup is 6 electromagnets which are paired up so there are 3 circuits. Each individual circuit must receive a separate AC source which are out of phase by 120 degrees. While I could use a 3 phase power input, I was also considering using a timer of some sort to time a burst of current at a set interval to try and create the illusion of a moving magnetic field however I have no idea if this would work or not.

So this is my problem, somehow creating a 3 phase power output from the power supply provided in order to make this induction motor work. As this is a demonstration project I hope it could be easily made and with  easily sourced parts from the interwebs.

I have considered whether the voltage was too low however I could always step it up using a transformer or I might as well try with 12 volts and if its too weak I could cheat a bit by placing magnets in the actual rotating section. I could also go with a 2 phase design as that would work as well but that would only be if the original plan is too out of reach.

Thanks for reading and I hope you can help me with a schematic of a possible circuit

Robert

[Psi]

Radio controlled planes use a 3phase controller to run the motors, which can be up to a 3kW, from a ~11V DC lithium battery pack.
They're basically a micro-controller and 6x N-channel mosfets to form a 3 output bridge circuit.

You can pick them up from hobby king for $30 or so
http://www.hobbyking.com/hobbyking/store/__24564__HobbyKing_70A_ESC_4A_UBEC.html
You would need something to simulate the servo feed (which controls the speed of the motor)
A servo tester should work
http://www.hobbyking.com/hobbyking/store/__8296__Turnigy_Servo_Tester.html

The only issue you might have is speed, the 3 phase maybe too fast for your motor.

The driver works something like this

[Rerouter]

if you want to make it nice and simple, get a quad inverter, with schmitt trigger inputs prefered, if not a seperate schmitt chip,

you hook up 3 inverters each with a schmitt on there input, in a loop, and use a resistor and capacitor on the output of each to make a time constant, ends up giving out a perfect if square three phase waveform to drive an induction motor (via some power element)

here is an example circuit, http://tinyurl.com/96zpgzd

other than the inverters and 10K/1uF time constant is what is effectivly a schmitt trigger just to try and not confuse you too far,

here is an ideal and very common chip to do just that http://web.mit.edu/6.115/www/datasheets/DM74LS14.pdf

7414 is a very very common chip which every supplier you look at should have

[Life4Never]

Radio controlled planes use a 3phase controller to run the motors, which can be up to a 3kW, from a ~11V DC lithium battery pack.
They're basically a micro-controller and 6x N-channel mosfets to form a 3 output bridge circuit.

You can pick them up from hobby king for $20 or so

The driver section is something like this

Interesting. I thought most RC aircraft have moved to brushless DC. Probably the larger planes then. I'm having trouble finding it on the Hobby King website. Could you provide a link? EDIT: DW Seen that you have linked it.

if you want to make it nice and simple, get a quad inverter, with schmitt trigger inputs prefered, if not a seperate schmitt chip,

you hook up 3 inverters each with a schmitt on there input, in a loop, and use a resistor and capacitor on the output of each to make a time constant, ends up giving out a perfect if square three phase waveform to drive an induction motor (via some power element)

here is an example circuit, http://tinyurl.com/96zpgzd

other than the inverters and 10K/1uF time constant is what is effectivly a schmitt trigger just to try and not confuse you too far,

here is an ideal and very common chip to do just that http://web.mit.edu/6.115/www/datasheets/DM74LS14.pdf

7414 is a very very common chip which every supplier you look at should have

Hmm. I like that example circuit. I'll look into this chip and see if it would work. Seeing as it switches from high to low, I require the circuit to go into negative voltage to reverse the current to switch the magnetic field. Wondering if it could do this.

[Psi]

na, most RC planes/helis/quadcopters are brushless 3phase AC motors.
Well. actually, it's not strictly 3phase. It has 3 phases but only drives 2 coils at any given time.
Using the 3rd for feedback.

Come to think of it that may cause issues if you actually tried to use one for your motor, but from what you've said you want to build a controller anyway.

Check out this tiny one.


And others..

[Life4Never]

How would these be wired internally?

The simple design would be

But seeing as there is only 3 wires going into the motor than where would the ground go?

would it be wired more like this?

Considering that red and black be the positve and negative and the blue wire be the control lead?

Actually I would prefer not to built the controller lol. This stuff would actually save a lot of time with the little time we have left before the HSC.

Well we can open one up and see the internals to work out how they work as a friend just happens to have one.

[Psi]

Yeah, they're pretty cheap. You could probably rip it apart and use just the fet driver section with your own controller.

However if its for a demo unit maybe it should be a proper full on AC 3phase sine wave driver rather than simplified DC square wave.

[Life4Never]

The electronics doesn't really matter. As long as it works. We are really just trying to demonstrate the changing magnetic field and how it causes the rotor to spin. Since basically all DC brushless motors are really just electronically controlled AC motors, I'm really wondering if the actual controller you mentioned above would work. It's not like we are attaching a scope to it.

[Rerouter]

Yes you can use both my control circuitry and his push pull driver to run the thing, though i found a better chip, a 40104, its the same thing but runs at up to 15V,

take a look at the schematic, that should run without an issue for you if you need simple,

edit: got my components upside down
also you shouldnt need to terminate the center it can be left floating

[caroper]

This may be overkill for your application but it does directly address what you are after.
http://www.doiserbia.nb.rs/img/doi/1451-4869/2010/1451-48691002195M.pdf


Cheers
Chris

[T4P]

Radio controlled planes use a 3phase controller to run the motors, which can be up to a 3kW, from a ~11V DC lithium battery pack.
They're basically a micro-controller and 6x N-channel mosfets to form a 3 output bridge circuit.

You can pick them up from hobby king for $20 or so

The driver section is something like this

Interesting. I thought most RC aircraft have moved to brushless DC. Probably the larger planes then. I'm having trouble finding it on the Hobby King website. Could you provide a link? EDIT: DW Seen that you have linked it.

All of them are brushless ... if it was a brushed ESC it would have only 2 wires not 3 wires
http://www.dealextreme.com/p/mystery-26500rpm-3800kv-brushless-motor-for-model-airplanes-and-diy-projects-11-1v-dc-18874?item=28
I got this the other day but it's insanely FAST

[Life4Never]

Yes you can use both my control circuitry and his push pull driver to run the thing, though i found a better chip, a 40104, its the same thing but runs at up to 15V,

take a look at the schematic, that should run without an issue for you if you need simple,

edit: got my components upside down
also you shouldnt need to terminate the center it can be left floating

Sorry still new to this stuff but how does the chip relate to the circuit diagram? Is it a simplified depiction of what's in the chip? Or are those triangles (ICs) placeholders for the chip but its just one imput and one out?

And on the RC controller, does it produce "AC" in square waves, really doesnt matter? If so does it contrinually shift the position high voltage from one to the next then to the next in a repetitive pattern because that is all I want.

Also

[Psi]

i think some of those model aircraft motor controllers are programmable and let you set the pulse speed.

However I dunno if they can be set slow enough for your demo motor though, I imaging it would needs several orders of magnitude less rotations per second than a typical RC motor.

[T4P]

Usually PWM with low RDS(on) MOSFETS
Only limited by the mosfet's Max (A)
A cheap ESC can be this : http://www.dealextreme.com/p/mystery-speed-controller-pentium-30a-for-brushless-motors-300-450-r-c-helicopters-34338?item=18

[Rerouter]

its a functional depiction, there are 6 inverters in the package, so the chip is broken up into those 6 function blocks to make the schematic more clear,


the RC controller produces a three phase square wave, as a sine would involve a more complex controller,

[Uncle Vernon]

Hey guys, I'm new here but considering that I watch Dave with his videos, I have decided to ask the community for help.

I am currently undertaking a little side project for school. A friend and I have decided to try and create a squirrel cage induction motor from easy to find parts as a demonstration of induction motors to the Physics department as a year 12 parting gift. The whole mechanical design of the project is simple and pretty much set out already however the electronics is a bit more difficult, at least for us who have basic electronic knowledge. The school has power supplies which can supply a voltage of 12 volts in either DC or AC at 50 hertz. I planned to create the 3 phase output from this 12 volt source.

.....................................

So this is my problem, somehow creating a 3 phase power output from the power supply provided in order to make this induction motor work. As this is a demonstration project I hope it could be easily made and with  easily sourced parts from the interwebs.

Given this is an educational exercise, why not apply the kiss theory and construct a rudimentary MG set.

[lab supply]-----[DC motor]==mechanical linkage==[3 phase motorgenerator]~~~~~3 phases~~~~~~~[3 phase demo motor]

You could add a direction switch to the motor, speed control of the DC etc.


What supply voltage does your 3 phase creation require?

[Life4Never]

Given this is an educational exercise, why not apply the kiss theory and construct a rudimentary MG set.

[lab supply]-----[DC motor]==mechanical linkage==[3 phase motorgenerator]~~~~~3 phases~~~~~~~[3 phase demo motor]

You could add a direction switch to the motor, speed control of the DC etc.


What supply voltage does your 3 phase creation require?

I have actually thought of this. This is probably the best way to create real 3 phase AC. Are there cheap 3 phase generators on the market? I guess I'll search ebay. Because if we created our own generator I doubt it would be very efficient at all. Maybe we could find an existing AC motor and turn it into a generator.

[Uncle Vernon]

Maybe we could find an existing AC motor and turn it into a generator.

You could be onto something.

[Life4Never]

Hmm been looking for compact size ac motor (tiny sized) and have not really found anything.

Do those RC motors work on real 3 phase power? If so, that would mean I could create a 3 phase output from it correct?

Well I think I'll focus on the mechanical aspects for now and get the thing together first.

[ejeffrey]

Keep in mind that a 3-phase induction motor will not work as a generator, at least not without modification.  Without current flowing through the windings there is no way to set up the rotating magnetic field.  You need a synchronous motor -- a small BLDC motor can probably be made to work, although it may not give a nice sine wave.

[Psi]

Brushless RC motors make terrible generators unfortunately.


You could generate your 3phase signal without too much work using..
- An Arduino with DAC module (3 channels required)
- 3x simple mono audio amplifiers (or 2 stereo) 

Just program the Arduino to generate 3 sinewaves 120deg apart and use the audio amp to get some power behind it.

The code could just be a 3x64 array of hardcoded DAC values.
Then loop through the array setting the 3 values and doing a simple delay.
You could add more elements but 64 is probably enough to get a reasonable sinewave


Or you could forget the DAC module and just find an Arduino library that can do PWM analog output and feed that into the audio amps.

[T4P]

Computer BLDC fans can be hacked for 3 phase output 

they are 3 phase but to shift the phases you need 0.1uF caps

[Psi]

hehe, if that's true then two PC fans back to back plus a few audio amps maybe all you need.

One fan running on 12V to spin the other one that's modded to generate 3phase.

[Uncle Vernon]

You need a synchronous motor -- a small BLDC motor can probably be made to work, although it may not give a nice sine wave.

Yep you've got it. Given the intent of the project I'd doubt a perfectly formed sine wave matters too much, the expected modified square wave would be quite sufficient to drive an unloaded motor.

[IanB]

Keep in mind that a 3-phase induction motor will not work as a generator, at least not without modification.  Without current flowing through the windings there is no way to set up the rotating magnetic field.  You need a synchronous motor -- a small BLDC motor can probably be made to work, although it may not give a nice sine wave.

It seems it can be done if you use the three phase motor in a combined motor-generator configuration. Here's a demo of such a setup:



Beware, there are a few advanced hacker skills on display here, not to mention a cheap throw-away oscilloscope