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PMAC vs induction motor for model 3
- Thread starter No2DinosaurFuel
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- Model 3
Okay, I'm missing something here. From everything I've seen Tesla's Induction motors are much lighter than PMAC motors of comparable peak power - not that anyone else in the industry is actually building production cars with motors of comparable power so far. So how is the fact that they've been slicing weight out of the car evidence of a PMAC motor?
Thomas Edison
Active Member
This is a 2013 white paper from the Copper Industry trying to convince Hybrid Engineers to switch to AC Induction over PM motors.
http://www.coppermotor.com/wp-conte...CA-EV-Traction-Motor-Comparison-v1.8-Eng1.pdf
It's an easy read. But here's the Cliff Notes:
- AC Induction weighs 40% more at 50kW sizing and needs a battery 7% larger, which you can assume to be 7% heavier.
- A 200kW design with an 800lb battery assy, would lose roughly 156lb by the changeover. That is huge weight savings when engineering a modern car.
JRP3
Hyperactive Member
I am waiting until the model s has AWD with the new pmac motors and a higher capacity like 120kwh or more and supercharging v3 which can enable 80% in 20 minutes with. That is when I will upgrade my model s. I think that is when EV will be in parity with gas car. That model s should have well 400 miles range assuming around 250wh/miles efficiency because the heavier car compared to the model 3 with legitatmate 120kwh pack.
But I suspect the 10% -> 80% in 20 minutes charging will take a longer to develop.
But I suspect the 10% -> 80% in 20 minutes charging will take a longer to develop.
omgwtfbyobbq
Active Member
I could be.
Is this an example of a hybrid PM/induction motor?
Patent US9419504 - Hybrid induction motor with self aligning permanent magnet inner rotor
If it is, I could see some manufacturer doing something similar with an electronic limited-slip differential that has two power inputs instead of a single input. In that situation, the torque difference could be mediated by both motors and the electronic LSD, which should make for very smooth power delivery and braking.
hacer
Active Member
That invention seems to put a lot of extra mechanical junk in the motor to avoid needing position sensing on a PM rotor (which is pretty trivial) and electronics to drive the motor with respect to position. It might be useful in the case where you don't want electronic control of the motor (e.g. fixed frequency operation) but otherwise it is the worst of both worlds. I can't image it ever being used in an EV.
JRP3
Hyperactive Member
An interesting read. As you say, it's written by a non-profit that appears to be trying to convince people, and they use an actual PM motor but create their own strawman induction motor. I haven't heard of this particular group, so I'm not aware of a bias, but in creating the model of the induction motor they certainly have the opportunity to do so.
I have an especially hard time swallowing their assertion that induction motors are ~5% less efficient at peak and in a much worse efficiency range during road testing, because my X is 30% heavier and 10-15% larger in every dimension than my Volt was - and gets the same EPA ratings, despite the Volt having a PMAC motor and the X an induction motor.
If the Tesla motors are ~10% less efficient in the load range being tested as the article implies, Tesla would have to lower both the rolling resistance and the drag coefficient by at least 20% to come close - and the Volt is already a very slippery car running on specially made LRR tires. Tesla engineers can and do work minor miracles, but that just isn't happening.
Tesla and Elon said the original S motor weighed 70 pounds on many occasions - that's where I got my impression about relative weights, because you can't get anywhere close to that weight with anywhere close to that power in PMAC motors that are available for EV conversions...
This is probably the most competitive motor I've seen - liquid cooled, using the technology introduced for the 2-mode hybrid and souped up from there - and it weighs 150 pounds, while being limited to 210 HP - half the power from twice the weight (though I am confused, because they also offer a double stack version of it, with a 420 HP rating from dual controllers and specs that say it's twice the length - but only 180 pounds. I'm missing something, but either way it's not equal to the power to weight ratio of the Tesla.)
It's not apples to apples, but when I went looking for the weight of the Volt drive motor(s), Google gave me this - in the little box on the bottom left, it appears to say that the 4ET50 assembly weighs 374 pounds wet, including two PMAC motors with 166 kW of total output, three clutches, a planetary set, and a final drive/differential.
For comparison, the author of this post claims in a comment that the complete RWD S drivetrain weighs 135 kg (302 pounds) - and that includes the drive inverter, which isn't a part of the 4ET50 weight, as well as the motor, gears, and differential (and the Tesla hits 2x the max combined power of the Volt motors in the Performance versions.)
According to this post and linked Google doc, the Leaf transaxle with motor is 218 pounds, though that apparently includes the A/C compressor? That's for an 80 kW solution, half of the Volt and a quarter of the Tesla...
Edit: I didn't expect to find anything on the Bolt yet, but I was surprised to discover this article, which says the (drive unit?) total weight is 76 kgs (~170 pounds), for 150 kW output. Much closer, but still not equalling the Tesla Power to Weight, let alone exceeding it by 40%...
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There are two things you have to keep in mind here, one is that efficiency and power density are always a tradeoff in electric machines and that there is peak power and peak continuous power. This 6 phase PMAC motor produces 260kW continuously, while only weighing 50kg:
https://www.siemens.com/press/en/feature/2015/corporate/2015-03-electromotor.php?content[]=Corp
Essentially it's all a tradeoff, but the PMAC just is more efficient at lower speeds, which helps city driving range and therefore overall driving range and can be built more compact, given the same power demands and efficiency requirements.
If you want to, you can break it down to first principles. A motors torque depends on the strength of the magnetic field in the air gap, between the rotor and stator. Since a permanent magnet can create a more powerful magnetic field, than a copper, or aluminum winding, the PMAC will be more powerful. And since there will be no losses in the rotor windings, it will be more efficient, except in field weakening, where both machines behave differently when it comes to the stator.
Which kind of Motor/Generator/Units do they use on the 2016 Audi R18? If my information is correct, it's PM.
They pay >$10,000 to lose a pound of weight. Everything possible is carbon fiber, even the transmission case. Titanium bolts are hollow. All parts are hand benched to reduced weight. 476HP of electric motors + 5xx horsepower of diesel power gives you over 1,000 hp in a 900kg (2000lb) package including the lithium battery and enclosed armored safety cockpit.
If induction motors are so light in the 400+ HP arena, why didn't they used them?
Or check the Porsche 918, Ferrari LaFerrari, or McLaren P1.
None of them had access to SpaceX's magic lightening powder.
I'm too far out of my wheelhouse to make an intelligent speculation about why any of them made those choices - assuming they did, since I don't have access to that information either. I'm just pointing out that the situation doesn't appear to be as simple as your comment and that presentation made it sound, and somehow Tesla has been able to develop an Induction motor package that's lighter than the PMAC competition and seemingly of comparable overall efficiency for the last couple generations.
Maybe they've switched directions with the 3 - I certainly wasn't expecting it, but given there is some limited evidence supporting the idea I'm not dismissing it out of hand. It was certainly unusual that the entire industry was going in one direction and Tesla in another (though they have plenty of precedents for that in other matters, both business and technical...)
hacer
Active Member
I guess you will be surprised to learn that permanent magnets are initially magnetized from a current in a copper circuit that produces a magnetic field significantly in excess of the residual pemanent field that it (the permanent magnet) will have thereafter (or until it encounters an electromagnet stronger than itself which may very well demagnetize it - one of the limitations of PM motors is that you must limit the stator fields to prevent demagnetizing the magnets!). So much for your non-principle.
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hacer
Active Member
My guess is to avoid the primary drawback of high-power induction motors: cooling of the (rotating) rotor. To be honest, that is the Achilles Heel of Teslas as race cars. They can put out high power, but not for very long because the motor and probably a lot of other components are not designed for continuous high-power output.
insaneoctane
Well-Known Member
Although the PMAC motor is still speculation, the longer it goes undenied by tesla, the more likely it is true. I suspect they worry the announcement might further cannibalize S/X sales.
insaneoctane
Well-Known Member
No. But, I suspect Telsa doesn't want to take another S/X owner and give them yet another reason to feel the M3 did something better....
Yea, true. But the rotor of most induction motors is aluminum, so the magnetic field must be weaker. But theoretically you are still right, since the magnetic field must be tree times as strong to magnetize the magnet.
But the real first principle I meant was the rotor loss. The higher potential for power is just something that exists because we can't replicate such a strong magnetic field with a coil, given the same physical space. But that's not given by any physical law, as you mentioned correctly.
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