# Best explanation I've seen as to why Tesla switched from the induction motor to the permanent magnet motor



## CoastalCruiser (Sep 29, 2017)

Some of you know that I have posted a few articles to cleantechnica.com on Tesla's motor design. When Tesla introduced a PM motor using a reluctance core design (described in below video as an *Internal Permanent Magnet - Synchronous Reluctance machine*, or *IPM-SynRM*) and endless amount of speculation went into understanding the reasoning behind the new motor design.

Since its introduction we've pulled together a lot of pieces of the IPM-SynRM puzzle, but this animation not only lets us visualize the workings of an induction motor juxtaposed to the newer PM reluctance motor, the narrator renders a nice comparison of the strengths and weaknesses of each design. And for you motor heads, the video explains a key advantage to combining permanent magnets with the age-old reluctance design. They solved a very specific problem by going this route.


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## JasonF (Oct 26, 2018)

I would guess that the motor is designed to be powerful without adding too much weight to the car, since the battery pack is already heavy.

I did some googling, and apparently a gasoline powered drive train weighs just under 1000 lbs. The Model 3 battery is just above 1000 lbs. Each electric motor weighs 70 lbs, so a dual motor long range model would be about 1200 lbs of drivetrain. The interesting part though is that the Model 3 weighs about 500 lbs _more_ than a Toyota Camry (which is incidentally slightly larger).

The BMW 3-series, widely regarded as the closest gas competitor, weighs about 200 lbs more than a Model 3, with a similar drivetrain weight to the Camry. The only reason I point all of that out is that I realized all of that extra weight can only come from either cabin or roof glass, or from _higher quality of interior materials_. It's kind of an eye opener to how much weight that actually costs.


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## garsh (Apr 4, 2016)

JasonF said:


> I would guess that the motor is designed to be powerful without adding too much weight to the car, since the battery pack is already heavy.


That's certainly a big part of it. The PM motor is also physically smaller than an induction motor for the same power.
One of the biggest benefits is that Tesla's PM design is more efficient than an induction motor. It's worth the higher cost of the PM motor if you can get the same range with fewer battery cells.


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## JasonF (Oct 26, 2018)

garsh said:


> That's certainly a big part of it. The PM motor is also physically smaller than an induction motor for the same power.
> One of the biggest benefits is that Tesla's PM design is more efficient than an induction motor. It's worth the higher cost of the PM motor if you can get the same range with fewer battery cells.


I remember when shopping for pool pumps years ago that the more horsepower the AC motor was, the more torque it had (for more pumping power) and the thirstier it was for power because it had more windings. The variable speed ones changed that dynamic a little bit by converting the incoming power to DC and then sending it through a variable frequency drive, except a slower moving motor has less torque, so the motors had to get even bigger and heavier (even more windings) in order to run slower with the same amount of torque.

So pool pumps managed to somewhat solve the efficiency part, but they didn't solve the weight or cost of build issues - you have to pay for all of those extra copper windings! That's where the PM motors seem to have filled the gap - plus added a little more efficiency on top of that.


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## CoastalCruiser (Sep 29, 2017)

garsh said:


> It's worth the higher cost of the PM motor if you can get the same range with fewer battery cells.


Interesting. Would be interested in anything you have on cost differences. As best I have it the Tesla PM motor is less costly to produce than their induction motor. That is why it was chosen for the less expensive Model 3. Reluctance motors in general are a low cost design. There is the cost of the rare earths of course. But also, Tesla uses a copper rotor (more elaborate than illustrated in the animation video) which is a costly item to produce. JB Straubel did an interview with motor.com explaining the process. It's a shame but the link to the article is dead now. This is how I summarized JB's input on the copper rotor in the induction motor at the time:

_"Tesla uses a copper rotor for its motors in lieu of aluminum due to copper's much better conductivity. It's a more difficult and costlier process, though. The copper must be heated to a molten state and then poured into a die with an iron/steel core (known as die casting). Aluminum melts at 660°C whereas Copper melts at 1000°C. The copper must be poured without melting the die or the core. Only recently was this process perfected. The result is an efficient motor which is ~20% smaller than an equivalent aluminum motor._ "


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## iChris93 (Feb 3, 2017)

CoastalCruiser said:


> Interesting. Would be interested in anything you have on cost differences. As best I have it the Tesla PM motor is less costly to produce than their induction motor. That is why it was chosen for the less expensive Model 3. Reluctance motors in general are a low cost design. There is the cost of the rare earths of course. But also, Tesla uses a copper rotor (more elaborate than illustrated in the animation video) which is a costly item to produce. JB Straubel did an interview with motor.com explaining the process. It's a shame but the link to the article is dead now. This is how I summarized JB's input on the copper rotor in the induction motor at the time:
> 
> _"Tesla uses a copper rotor for its motors in lieu of aluminum due to copper's much better conductivity. It's a more difficult and costlier process, though. The copper must be heated to a molten state and then poured into a die with an iron/steel core (known as die casting). Aluminum melts at 660°C whereas Copper melts at 1000°C. The copper must be poured without melting the die or the core. Only recently was this process perfected. The result is an efficient motor which is ~20% smaller than an equivalent aluminum motor._ "


I think this may have changed sometime after the 3 was introduced.


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## JWardell (May 9, 2016)

There's a lot of good videos in the Learn Engineering channel explaining a lot of motor fundamentals. Definitely worth a subscription.


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## CoastalCruiser (Sep 29, 2017)

iChris93 said:


> I think this may have changed sometime after the 3 was introduced.


Just be aware that in that video Sandy is comparing the _front_ motor of the model 3, which is an induction motor (which he termed as being expensive), to the PM motor of the Y. ;>

EDIT: Oh I see. He is showing off the front induction motor of the Y also. I see your point!


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## garsh (Apr 4, 2016)

CoastalCruiser said:


> As best I have it the Tesla PM motor is less costly to produce than their induction motor.


In general, induction motors are by far the simplest motor design, and therefore the least expensive. But another issue with them is that the rotors get hot from the induced currents. Because of this, Tesla's induction motor design becomes complicated by the cooling requirements for the rotor (the moving part of the motor). So that probably makes Tesla's induction motor more expensive than your average industrial induction motor.

I've never seen information on the actual cost of Tesla's motors. I'm guessing that you could buy a copy of Munro's report to see what they've been able to estimate.. But there was this article where Tesla's motor designer Konstantinos Laskaris is quoted:

Tesla's top motor engineer talks about designing a permanent magnet machine for Model 3
​_So, as you know, our Model 3 has a permanent magnet machine now. This is because for the specification of the performance and efficiency, the permanent magnet machine *better solved our cost minimization function*, and it was optimal for the range and performance target._​​_Quantitatively, the difference is what drives the future of the machine, and it's a *tradeoff between motor cost, range and battery cost* that is determining which technology will be used in the future._​
I take this to mean that even though the PM motor itself costs a bit more, the combination of weight savings, space savings (to have more room for batteries), and efficiency improvements allowed them to reach their range goals with a smaller battery pack - the increase in motor price is more than offset by the decrease in battery price.


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## Feathermerchant (Sep 17, 2018)

As to comparing EV to ICE car weights (like Camry vs Model 3) you should also consider performance. The Camry has a lot less HP than the Model 3 so a fair comparison would include more weight for the Camry to adjust for what would have to be a larger engine and transmission.


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## msjulie (Feb 6, 2018)

Did I understand the video correctly; my 2018 has an induction motor upfront but in 2019 the front went to the reluctance perm mag type? 

My car is clearly 90-something-big% of the time running rear motor only (scan my tesla app) so would this mean that newer cars split the duties more?


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## garsh (Apr 4, 2016)

msjulie said:


> Did I understand the video correctly; my 2018 has an induction motor upfront but in 2019 the front went to the reluctance perm mag type?


The front motor is still an induction motor. Tesla just changed the design of the rotor a bit.


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