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Induction motors as servo motors

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(@eelco)
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I think its awesome that solo also is able to control induction motors.

Which has started me thinking (and trying to convince others) that induction servo motors are an underrated idea. Of course the general idea is that BLDCs are awesome, (and then there are boomer memes about induction motors having low startup torque that I still see being propagated in this day and age of digital control where we dont just plug everything directly into the grid).

Im wondering if anyone here has hands on experience on the matter, or interesting theoretical arguments to bring to bear on the matter.

The basic tradeoff between induction and permanent magnet motors is that permanent magnets give you rotor currents for free (or at the price of neodymium). They do not dissipate current in the rotor, nor or you dissipating current in the stator in order to induce that rotor current. For a motor primarily concerned with continuous power output per kg, this is obviously a massive saving that the same number of kg of induction motor with a similar cooling setup cannot compete with, as we see affirmed empirically.

 

But if my use case demands I instead intend to maximize my 0.1s peak torque pulse, per kg of motor? And ive got a smarter controller than just plugging my induction motor into the grid?

Permanent magnets limit your operating temperatures, and your operating flux densities. So in theory an induction motor can have a higher peak torque, if you manage to saturate your iron; which is a limit you are generally are advised to stay away from with permanent magnets. At least as long as your thermal window allows, and if your electronics can keep up with the current rating, you should be able to squeeze out the iron-limiting torque per unit area of air gap, no? If you try to do the math on that from the bottom up, I arrive at some pretty crazy peak Nm/kg numbers.

But if that is true, why are induction motors so rare in low duty cycle servo applications? I can not think of any. Am I missing something?

 
Posted : 14/09/2022 1:21 pm
Forum 1
(@eelco)
Posts: 3
Active Member
Topic starter
 

To answer my own question; I do think there are decent reasons why induction motors are not optimal for burst torque. While it is relatively easy to induce a high field strength in your rotor, you are doing so at relatively high inductance / low reluctance. That means there isnt as much energy stored in that magnetic loop, compared to the impressive B-H product of a PM motor. So once you are done inducing a field in one pole and move over a pole to start generating some torque, that previous field will collapse relatively quickly, limiting what you can do with it. If you want to rival the B-H product of PM magnets, youd need to move towards getting those same high fields strengths with several mm of air gap, the way a PM motor has several mm of unit-relative-permeability material in its magnetic circuit. And while nothing is impossible, thats where things will start to get a little awkward.

Thats my current understanding at least; but if history is any guide it will probably sound really dumb to me a few years down the line.

 
Posted : 20/09/2022 2:30 pm
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