Tesla Sand Bagging the Model 3 Again?

Then things have changed since you were in grad school. Tesla has found that various motors from the same manufacturing process can result in one second less in 0-60 times, already verified by third parties with sophisticated instrumentation.

 

Just tested at 3.18 seconds.

 

Not from motor differences unless their motor manufacturing process is absolute garbage (which it isn't). Batteries, yes. Motors, no.

 

Tesla disagrees with you. Argue your thesis with Tesla, I'm just telling you that Tesla believes differently and has demonstrated it with proof.

 

If their motors vary that much in production then they have probably the worst motor manufacturing process in the entire motor industry.

I don't believe either one is true.

 

Tesla Model 3 Performance rips 0-60 mph in blistering 3.18 seconds on 100% battery state of charge

Elon Musk announces Tesla Model 3 AWD, performance Model 3 specs

Same cars, same motor configuration, but car with selected motors (performance version) is one second faster 0-60 probably due to the worst manufacturing process in the entire motor industry. Time to short some Tesla stock!

 

I couldn't find anything in either article about selected motors.

 

I ran across an interesting article and thought you might enjoy although it is short on technical details.

Sources:

• Tesla's Chief Motor Engineer Talks About Model 3 Motor Tech
  
• Charged EVs | Tesla’s top motor engineer talks about designing a permanent magnet machine for Model 3

Laskaris: Understanding exactly what you want a motor to do is the number-one thing for optimizing. You need to know the exact constraints – precisely what you’re optimizing for. Once you know that, you can use advanced computer models to evaluate everything with the same objectives. This gives you a panoramic view of how each motor technology will perform. Then you go and pick the best.

With vehicle design, in general, there is always a blending of desires and limitations. These parameters are related to performance, energy consumption, body design, quality, and costs. All of these metrics are competing with each other in a way. Ideally, you want them to coexist, but given cost constraints, there need to be some compromises. The electric car has additional challenges in that battery energy utilization is a very important consideration.

This is the beauty of optimization. You can pick among all the options to get the best motor for the constraints. If we model everything properly, you can find the motor with the high-performance 0-60 constraint and the best possible highway efficiency.​

There are times when 'perfect can be the enemy of good enough.' If a motor is made too expensive due to the materials cost of permanent magnets versus a radically efficient battery, then use a cheaper motor and ship product. But over time, materials change and better designs come along. We saw this with the changes in the rotor magnets from 'tangent' to the stators to the current "V" configuration. More power from a smaller package saves.

Now if a high-temperature, superconductor, rotor could be made, it would be an awesome machine. Induction motors suffer an (I**2)R loss that is trivial at low speeds but it exponentially worse at higher power. Now if the cooling problem can be solved, probably with a permanently on, cooling system, that would be impressive. If there were some type of EV vehicle that tends to be ON almost all of the time with an 80,000 lb weight challenge . . .

Bob Wilson

 

I could find anything ! First Tesla Model 3 Performance rolls down new assembly line - Autoblog

"The Performance model, which is separate from other dual-motor Model 3s, starts at $78,000 and gets specially selected motors front and rear for all-wheel drive."
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You're welcome.

 

First of all, that wasn't one of your original links. Second, this quote from your original links makes a lot more sense:

"The dual-motor setup will feature motors on the front and rear axles, and Musk said one motor would be tuned for longevity and the other would be tuned for performance."

There's almost no variability among motors so there's nothing to select from. But there are tons of ways to "tune" them (I could probably name close to ten off the top of my head).

 

Best to argue your point with Tesla, because they appear not to agree with you and are proving it on the track.

 

Well, pick one:

• Tesla "tunes" their motors, rather than selects them.
• Tesla has the least repeatable motor manufacturing in the entire motor industry.

In reality, motors have almost no variability because they simply follow Faraday's law, which is one of Maxwell's equations. It's just a law of nature (Ampere-turns). Since turns are the same and current is controlled, all the motors of the same design perform the same, with only a tiny bit of variability caused by tiny second-order effects.

 

Have you looked at the Toyota "field weaking" tweak?

My understanding is they somehow weaken the magnetic field to reduce the back EMF so the motor has a higher rpm limit compared to the available voltage. But I've not seen a good explanation of how it works. Mr Google isn't much help.

Bob Wilson

 

Field weakening is a common technique to increase the speed of the motor without increasing voltage. I've used it, and even once purchased at 2.5MW motor/drive system that field weakened from 1,200RPM to 2,250RPM. That was an induction machine (like on the Model S and Model X) so I'll explain how that works.

In an induction machine, you keep the field strength (the strength of the field in the air gap) constant by keeping the ratio of voltage to frequency constant (called "constant v/f ratio). In the machine I bought mentioned above, that was true from 0 to 1,200RPM but from 1,200RPM to 2,250RPM the voltage was held constant. That means the v/f ratio dropped as the RPM increased, and thus the field strength also dropped (hence "field weakening").

The result of this field weakening is that the "pull-out torque" of the machine decreased as the RPM rose. Fortunately, the pull-out torque of a normal induction machine is usually 3-6 times the rated torque so even the decrease didn't result in an inability to maintain rated torque. Further, the whole point of my field-weakening approach was to be able to hold power constant (not torque) as RPM increased, thus allowing me to use a far smaller motor/drive system than I would have if the torque had to be maintained all the way up (2.5MW versus 4.7MW), which is the normal way to cover the range I needed. That saved something like $600,000.

Doing this in a PM system is harder because the magnet strength is fixed when you manufacture the magnets (or so you hope - if you get them hot and apply a lot of field you can demagnetize them). There are two basic approaches (three if you count mechanical approaches). One is to use a secondary rotor coil that is arranged to opposed the magnetic field of the magnets. The other one is to apply reactive current to the stator in the right way so that the fields interact in such a way as to oppose the rotor field. The idea here is usually to be able to increase rotor speed while minimizing the increase in voltage that naturally comes from increasing the speed while the field strength and coil count are fixed. In my case, where I usually deal with generators, the idea of using reactive power is to increase voltage at the low end by increasing field strength and decrease voltage at the high end, which combine to increase speed range.

 

Is this close enough?

• pwm - the pulse-width modulation used at low speeds
• sir - the regular power cycle
• weak - the power cycle with more off time, partial power. It could shift left or right to avoid peak reverse EMF.

I'm a visual guy so seeing the wave forms helps me understand. So if the "weak" state shifts the power pulse to the left or right, it would avoid the peak back EMF. In fact, a double-humped square wave would do it with leading and trailing power pulse and the middle left out.

Thanks,
Bob Wilson

 

It's all PWM, all the time (can't make a sin wave from DC without it). Reactive power means the current and voltage sin waves (assume pure sin waves are generated to understand this easier) are out of phase. At 90 degrees all the power is reactive, meaning no real power is transferred. At 0 degrees, all the power is real.

Try this. Plot two sin waves, one for voltage one for current. Make their relative phase adjustable. Also plot their instantaneous product (the instantaneous power) and look at it over a few cycles. When the phase is 90 degrees, the power will be negative just as much as it's positive indicating energy is just bouncing back and forth and no net energy is being transferred.

 

If you take a given cross section of conductive material (like copper) and divide it into a variety of pieces, you can calculate how much power you can carry with each number of pieces (you need two pieces for single-phase, two pieces for two-phase, three pieces for three-phase, etc.). It turns out that dividing it into three pieces gives the most power transferred, hence three-phase is how we transmit bulk power and how almost all large motors work.