Induction Motor control?

I just came to this forum looking to get some discussion started on how hard it would be to re-purpose an odrive for induction-motor control… but if you dont mind I drop my thinking here, as I think its very interesting to consider the question of how far one could take the vison of odrive as a general-purpose-three-phase motor controller.

While I have never implemented either a brushless or induction controller myself, the only differences that I am aware of are of a software nature. But let me enumerate my assumptions, because maybe they are wildly off.

  • As far as the bulky hardware components are concerned, we are looking at the same three-phase-full-bridge plumbing.

  • Control-wise, induction motors can be run with a similar kind of field-oriented control algorithm. In a nutshell, the only difference is that you throw some magnetisation current in there with a little phase offset.

  • Implementing sensorless control requires measuring certain voltages. I can imagine hardware wise its basically the same idea as sensorless control for a brushless motor… keep an eye on the resulting voltages of the phases so you can build an internal model of the relevant internal dynamics of the system… and the rest is a software problem. But then again I have never really looked into the specifics of this at all.

On the flipside:

  • For a motor of similar dimensions, induction motors will have much higher inductance, given that there are none of these reluctant neodidlium magnets to push against. That should be all fine as far as changes in the control algorithm is concerned, but I have no idea if that also forces subtle changes on the hardware topology as well, such as to prevent current from rushing and spiking in unwanted ways?

  • Induction motors tend to run at lower electrical frequencies than synchronous motors. So that should be fine, hardware wise?

  • Many induction motors you will find in the wild are wound for different current/voltage ratings than the odrive. The typical bldc that odrive targets is tuned to the constraints given by the lithium battery, whereas induction motors have a reputation mostly for cost-efficiency in industrial settings. Of course that should be solvable by rewinding the motor, but it does raise the question; why do I care in the first place?

Well partially it is just an interesting question, but partially I also believe induction motors are under-rated for control-applications. That is, they cannot really compete if you care both about power density and efficiency, as you would in a drone. But for low-duty cycle robotics type applications, where you do not care that much about efficiency, but where you still want to get all the peak torque you can squeeze out of your saturated flux path for when your contraption does need to move, a high steel-to-copper-ratio induction motor is pretty much ideal.

If you think a 1kw induction motor needs to have the proportions of a medium sized pig, and needs an electronics control box the size of a fridge, thats no more true fundamentally than that the price of a 3kw ‘PMSM’ (suckers jargon for bldc amirite) needs to be the same price as a small car. I am very thankful for odrive for calling bluff on the latter, but I think induction motors also have a bright future ahead of them once they manage to shake off their steampunk aesthetic, and analog-era memes about low startup torque finally die out.

The economic reality at the moment is of course that there isnt a wide selection of really cheap induction motors flooding the market at the moment exactly, so I will not try to argue that there is a huge immediate untapped market for odrive here to implement such a feature. But if it really is as easy as I hope it might be, and almost purely a matter of implementing some variations of the same type of control algorithms, then that is an enticing possibility for me. And if odrive does manage to position itself as the general-purpose three phase controller… well enginerds would be probably be twice as likely to buy one just because of their appreciation for the fact that you just brought a little bit of order to the universe, and made something seemingly disjoint and convoluted, more elegant and reusable.