December 15, 2014

Turning An Outrunner Stator Into A Really Bad Induction Motor

Because why not.  A few months ago, Jamison brought a pile of small brushless motor stators down to MITERS.  I decided last night I should turn one of them into a tiny induction outrunner.  Basically, I replaced the permanent magnet rotor with an aluminum can shoved inside a steel can.  It's as simple as an induction motor can get;  it doesn't even have a squirrel cage rotor.  Although a rudimentary squirrel cage could be easily made by milling slots in the aluminum can.

Here's the stator.  Nice and densely wound, and it even has string holding the windings in!  Slightly above a hobbyking-grade stator.

I turned an aluminum can for the rotor, leaving as small an airgap as I could.  Something around .25mm

I turned a steel can, which the aluminum can is press fit into.  An aluminum end cap was attached to a steel shaft with a set screw, and was then also pressed into the steel can.  The rotor's shaft is made from fairly soft steel, and the motor has no can bearings, so any real load on the can will make the rotor rub the stator.  Definitely not made for anything other than testing purposes.  

I secured the rotor in place with  a bushing and too-big shaft collar:

Instead of commutating based on some input (hall sensors, back EMF, etc) like you would with a permanent magnet motor, you can just blindly shove three-phase into the windings of an induction motor to get it to spin (although to get decent performance more clever controls are required).

To drive the motor, Bayley and I hooked it up to this monstrosity, with some of his own sine-wave drive code.  I think the motor controller to motor ratio is a little off here:

It actually turns!

Unsurprisingly, it's a pretty terrible motor.  It took about 10 amps to get it spinning, and it produced so little torque it could easily be stopped with a finger.  I think bumping up the frequency would help, but the the motor controller brick used can't switch that fast, so the sine waves would have become much less sine-wave shaped.  It would be interesting to see how powerful and torque dense an induction motor this size could be made with some actual thought put into the design.  Time to learn more about induction motors...

1 comment:

  1. I want to make a rotomax 150 cc motor. How would you make the rotor that you made more stable. Do you know where I can get a rotor and shaft like you have?

    I think a bearing is what stabilizes it?