Solid copper rotors in AC induction motors

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jonescg
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Solid copper rotors in AC induction motors

Post by jonescg » Sun, 02 Jun 2013, 20:55

I've seen plenty of discussion on the interweb about solid copper rotors for induction motors. Now, when I read 'solid copper' I was expecting a length of copper tube with a wall thickness of say 20 mm and a shaft somehow supporting the tube about it's axis. A crude example of how this works is shown here in the "Cola can rotor"
http://www.youtube.com/watch?v=LcDvKyNPn4A

As it turns out, they are still using squirrel cages, with copper bars shorted at the ends and laminated steel in the middle. The only real difference is that the motor isn't wound with sliprings.
http://copperalliance.org/motor/

My question is, what is the problem with using a tube of solid copper supported by a shaft? Are the eddy currents too high such that the efficiency drops?

I want to build a rotating magnetic field using permanent magnets one day, and use it as an 'analogue' version of a stator coil. You can then put rotors of different configurations in to test their efficacy. It would make a great display I think.
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Solid copper rotors in AC induction motors

Post by BigMouse » Mon, 03 Jun 2013, 03:05

jonescg wrote:My question is, what is the problem with using a tube of solid copper supported by a shaft? Are the eddy currents too high such that the efficiency drops?
The squirrel cage is shaped like it is so that there are "loops" through which current can flow. These loops define the "poles" on the rotor, in a matter of speaking, and allow the magnetic core to concentrate the field within them. A continuous tube of copper (or any other conductor) wouldn't have defined current path nor would it be possible to concentrate the field in a useful way relative to the rotating magnetic field of the stator. It would likely spin, but torque developed would be very low.

As I understand it, copper rotors have the advantage that they reduce I2R losses in the rotor. Other than that, they are functionally identical to a standard cast aluminium rotor.

No squirrel cage motor uses slip rings.
Last edited by BigMouse on Sun, 02 Jun 2013, 17:06, edited 1 time in total.

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Solid copper rotors in AC induction motors

Post by acmotor » Mon, 03 Jun 2013, 03:17

Also, the shorted turns formed by the cage bars are slightly spiralled to reduce torque ripple. The cousin, PMAC usually contains a spiral in the stator slots.
The rotor still contains laminated iron between the ali or copper conductors.
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Solid copper rotors in AC induction motors

Post by jonescg » Mon, 03 Jun 2013, 03:18

Nicely put! Thanks!

Now if someone could come up with an axial flux induction motor... that isn't in prototype stage Image
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Solid copper rotors in AC induction motors

Post by acmotor » Mon, 03 Jun 2013, 03:24

You could consider a spinning wheel power meter, though they were considered eddy current drive.
Or a magnetic levitation train / track.
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Solid copper rotors in AC induction motors

Post by peskanov » Mon, 03 Jun 2013, 12:47

An old book I have (1949) says that, for IM, a simple iron rotor (without squirrel cage) can be built, and sometimes WAS built. It does not specify laminated or one-piece.
The book says the drawback is that speed would change too much with the load; but that's not a problem today using controllers.

Maybe they were talking about hysteresis motors? Hysteresis motors use just a ferromagnetic alloy like iron-cobalt in a simple cylinder shape.

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Solid copper rotors in AC induction motors

Post by jonescg » Mon, 03 Jun 2013, 18:37

That's what got me thinking about it. If you drag a strong magnet past a block of aluminium or copper you can feel it being dragged along. Now create a rotating magnetic field electrically and watch your tube of conductive rotor turn. I can see that a laminated rotor with a slight twist is going to be a fair bit more efficient, but my curious mind says 'why?'.

I'm keen to build an interactive display for the AEVA on how AC motors work using some interchangable rotors and stators.
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Solid copper rotors in AC induction motors

Post by Richo » Tue, 04 Jun 2013, 21:23

There are already axial flux induction motors.
They are used in high efficiency fans.
Problem is they are difficult to build thus not cheap.

There was interest in using magnets and an ali disk for the brakes of the land speed record jet cars.
Brake brings the magnet in next to the disk and eddy currents slow it down.
The advantage was it could never lock the brakes - no spin no back emf.

So the short answer is NO but the long answer is YES.
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Solid copper rotors in AC induction motors

Post by acmotor » Tue, 04 Jun 2013, 21:57

Just massive temperature rise ! Image
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Solid copper rotors in AC induction motors

Post by jonescg » Tue, 04 Jun 2013, 23:40

I popped into Lodestone Industries to enquire about magnets. I want to build a little demo motor with interchangable rotors and stators. The idea is to create a rotating magnetic field using a simple source - rotating magnets. So no, the stator won't be a stator, but the next step in the logic is to create a rotating magnetic field using poles and coils.

This way the user can appreciate the input vs output, and thus get some idea of efficiency and the relationship between speed, torque and power.

Multiple rotors (squirrel cage, solid copper, permanent magnet) and multiple stators (4 pole, 8 pole, 12 pole, 10 turn, 5 turn) would allow the user to see what changes in what way when certain variables are changed. We could even have an 'analogue' AC generator (a rotary switch) and a small three phase AC generator on hand to make the stator work.

Maybe it's all too much work, but I think it would be really handy for AEVA, schools, SciTech and the like.
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Solid copper rotors in AC induction motors

Post by acmotor » Wed, 05 Jun 2013, 05:01

Rather than rotate a 'stator' of magnets why not just set up a VFD and a wound stator ? If you've introduced electromagnetism, use a compass if you like, then drag people into 3 phase.

A physically moving (not magneticallly moving) stator is going to be a bit hard to explain your way around ? Image and may confuse rather than clarify.
OK depends on what audience you are targetting.

Me thinks if folk can't grasp the idea of a magnetically moving only stator then you have a massive job on your hands.

Good on you for planning the demo. Perhaps it is as much to expain AC motor to yourself as anything ? Image

BTW, did the guy at loadstone have his fingers bandaged as usual ? He's always pinching them between the super rare earths.
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Solid copper rotors in AC induction motors

Post by jonescg » Wed, 05 Jun 2013, 05:23

Oh it is absolutely to help me understand it as well as others. No better way to learn a concept than to teach it! I'm pretty thick, so I like to have lots of different experiences to really cement my understanding. I broadly understood the concept of an AC motor for many years, but it never really sunk in until it was described in terms of a rotating magnetic field which drags the rotor with it. Then to have three phases doing it for you... Tesla was a genius pure and simple.

The other benefit of the rotating permanent magnetic field is that it gives the user the chance to 'feel' the torque differences by using one hand to move the field and the other to load the rotor. It also serves as an example of an outrunner motor; if an internal wound stator could fit inside it (and the rotor comprised of 6 magnets).

For sure, you would also have a variable frequency 3 phase generator on hand for the latter parts of the demonstration, but maybe even a rotary switch to get the punters thinking in terms of 'alternating current'?

Little steps - I'm shocked at the poor understanding of science in the general public. I was at the UWA expo where a woman asked me what seeds do (!)

I mostly chatted with sales/reception at loadstone. I got a few ideas in mind now.
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Solid copper rotors in AC induction motors

Post by acmotor » Wed, 05 Jun 2013, 06:53

Seeds ? they are the things you genetically modify right ? Image
I know what you mean though.
And the same people believe in totally human induced global warming. Because they understand it all or because they have no idea ? Image

You could make one emotor demo and test an audience of plebs ?

   Image
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Solid copper rotors in AC induction motors

Post by Johny » Wed, 05 Jun 2013, 13:17

The way I start to describe an asynchronous motor is to ask if anyone has ever seen the experiment were you drop a cylindrical magnet down an aluminium tube and it takes a long time to go through. No one ever says they have done or seen it though.

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Solid copper rotors in AC induction motors

Post by coulomb » Wed, 05 Jun 2013, 14:05

For the rotating magnetic field, I was thinking you could do an animation of a wave passing a series of balls-on-sprints. You start zoomed out, so you clearly see the wave moving left or right, then zoom in and dim all but one ball-on-stick, to show it's just moving up and down.

Then you imagine just three of these, then you arrange these three in a circle.

I don't know how to do the zooming or even the animation, but someone here must have the skills or know someone to ask or even to pay a little.

I suppose even better would be some sort of mechanical apparatus that did the same thing, but in real-life (TM).

Just a suggestion. I do think that the rotating magnetic field is a tricky one for beginners.

Another thought: three electromagnets at 120 degrees to each other, with variable DC current, and a compass between two of them. You vary the strength of the current with two pots (maybe big lossy rheostats to keep it simple and less "magic") to show the compass needle moving from one pole to the other. Maybe you put it in the middle and use a third pot to move it another 120 degrees. Then you give up and switch over to a magic box that moves the compass smoothly in a circle, revolving once every 5 seconds or so (0.2 Hz). You have LEDs or similar across the electromagnets, so you can see the brightness changing in a sine wave (if viewed by itself), or see the brightness moving in a circle from a distance.
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Solid copper rotors in AC induction motors

Post by jonescg » Wed, 05 Jun 2013, 16:32

Great ideas Coulomb. I got the inspiration for the rotating magnetic field by the Lenz effect - you know, dragging a strong magnet past a copper bar and feeling the bar get dragged along with it. If you can drag the copper along with a magnet, then you can turn this into a rotating force. Then the next step is to replace the rotating magnet with a series of electromagnets.

The problem with lots of science-fair style models of motors, is that they are almost too simple, and look nothing like the motors we use in our EVs. As far as I know, there isn't a single two pole AC motor painted red and blue anywhere in use. Plenty of lossy universal motors though. SO I want to build the learner's knowledge quickly up to multi-tooth stators so they can see the advantages and disadvantages of synchronous and asynchronous machines. So they can feel the torque and efficiency advantages of PM versus IM.

Anyway, I hope to come up with some simple plans when I;m not doing all the other stuff I probably should be doing :D
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