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According to Elon's tweet today, the Model 3 does indeed have a switched reluctance motor. Here he is describing the motors for the front and rear of the dual motor Model 3:
"AC induction front & switched reluctance, partial permanent magnet rear. Silicon Carbide inverters in both. Performance drive units are lot sorted for highest sigma output & get double the burn-in."
According to Elon's tweet today, the Model 3 does indeed have a switched reluctance motor. Here he is describing the motors for the front and rear of the dual motor Model 3:
"AC induction front & switched reluctance, partial permanent magnet rear. Silicon Carbide inverters in both. Performance drive units are lot sorted for highest sigma output & get double the burn-in."
@BluestarE3
No slop. A pure switched reluctance motor has no permanent magnets. Adding magnets to the rotor can help in a few ways:
Increased torque from low stator fields
Regen without needing an excitation field
Smoothing of torque ripple (although a high speed DSP controller can vary the stator field to reduce ripple on its own)
So at low power, less field is needed (efficiency). At high power, it goes more toward the SR mode of operation which allows greater field strength than the magnets produce (this may reduce back EMF allowing more power at the top speed end) Old article
It’s probably worth mentioning though that modern permanent magnets in EV motors can use a lot less rare earth metals of the kind that people typically worry about than was the case just a few years ago.
Magnet makers have figured out how to unevenly distribute the metals to the edges of the magnets where they are needed to protect against demagnetization due to extreme conditions etc.
Speculating here, so big grain of salt:
A pure SR motor can be as powerful as you want (well, till the rotor saturates). So the amount of magnet is purely a design trade off decision. I'd put it around where the cruise power level lives so it is normally PM with the minimal amount of stator current. If the 3 pulls 275W/mile at 60 MPH = 16.5 kW = 22 HP equivalent PM, or it could be based on desired regen level, so double that.
Again, total guess.
Speculating here, so big grain of salt:
A pure SR motor can be as powerful as you want (well, till the rotor saturates). So the amount of magnet is purely a design trade off decision. I'd put it around where the cruise power level lives so it is normally PM with the minimal amount of stator current. If the 3 pulls 275W/mile at 60 MPH = 16.5 kW = 22 HP equivalent PM, or it could be based on desired regen level, so double that.
Again, total guess.
I should have been clearer. By performance I meant the usual consumer facing metrics, not just power. I'm under the impression that partial SRM was used to mitigate ripple.
Speculating here, so big grain of salt:
A pure SR motor can be as powerful as you want (well, till the rotor saturates). So the amount of magnet is purely a design trade off decision. I'd put it around where the cruise power level lives so it is normally PM with the minimal amount of stator current. If the 3 pulls 275W/mile at 60 MPH = 16.5 kW = 22 HP equivalent PM, or it could be based on desired regen level, so double that.
Again, total guess.
Is the amount of magnet required linearly proportional to power ?
I'm probably way off here, but it suits my ignorant self to imagine the SRM as a hybrid of the original AC induction design and a typical 3 phase PM AC motor.
I should have been clearer. By performance I meant all consumer metrics, not just power. I'm under the impression that partial SRM was used to mitigate ripple.
AH. Regarding the torque ripple argument, it seems the inverter's waveform could be pre-distorted to counter the ripple. In its favor, it could be that using magnets reduces the dynamic range needed to produce a smooth torque output, thus increasing the maximum ripple free motor output.
For pure PM, I believe so, at some stator field level, the stator field will eclipse the permanent magnets setting max torque. In the extreme case, the stator can demagnetize the rotor.
I've designed and programed BLDC motor drives, but haven't had to deal with the physical motor design beyond data sheet specification/ selection.
Some highlights from this article. First, a description of the rotor in a switched reluctance motor (SRM):
"An SR motor is simple and rugged. The rotor consists of stacked steel laminations with a series of teeth. The rotor requires no windings, rare earth materials or magnets of any kind."
Next, here is the reason a SRM can operate at 100% rated torque indefinitely under stall condtions without overheating, and why an AC induction motor can't (a major advantage for SRMs in heavy-haul applications, or for repeated drag racing sprints):
"Lower rotor losses are especially relevant during start-up where, in the SR motor, the rotor losses are no greater than when the motor operates at its rated condition. This permits a virtually unlimited capability for prolonged operation in the stall condition and for repeated starting under full-load. Such performance is often not possible with conventional drives because of the large electrical losses they experience on their rotors and the subsequent rotors heating under such conditions."
This stall performance is so important, the author says it twice:
"SR motors also offer performance advantages in motion control. An SR motor can produce 100% torque at stall indefinitely. The reason is that the rotor produces no heat at stall. Rotor bearings stay cool as well. Only the stator coils get warm, and they can be cooled via fins on the stator housing, or by other conventional means."
Any guesses why the Roadster 2 was able to do 0-70 mph wind sprints all night long at the Semi reveal event without overheating? A P100D could never do that, maybe 4 hard launches before it's overheating and the electronic nanny is backing off the power. The Roaster did that, wot, about 50 times in a row that night?
Anyway, the whole article is well worth a read, so I won't quote it all here. But as a palate cleanser, consider watching this short video on how to make a home-made SRM. The salient point is how two different magnets are laminated to create a switchable magnet: yes, you read that right: An SR magnet is switchable with electricity, but is not an Electromagnet.
A switched reluctance magnet can be turned on or off electrically, but is not an electromagnet. It does not require electric current to produce magnetic flux. The electrical current is only used as a pulse to switch the laminated SR magnet between its on/off states (yes its freekin' *magic*, until you understand the physics).
Enjoy (a la prochaine, until we meet again). Pay special attention to the 'chalk-talk' explanation of SRM theory beginning at 1:52 into this video: (I'm not going to spoil it for you, you're gonna be so surprised)
No spoilers! You have to grok this yourself, or you'll never believe it!
"An SR motor is simple and rugged. The rotor consists of stacked steel laminations with a series of teeth. The rotor requires no windings, rare earth materials or magnets of any kind."
Pay special attention to the 'chalk-talk' explanation of SRM theory beginning at 1:52 into this video: (I'm not going to spoil it for you, you're gonna be so surprised)
If I understood the whiteboard discussion, the alnichol (sp) material switches polarity when subjected to a field even briefly. Stated just as a fact without much explanation. This leaves the magic in that magnet but I suspect there is more to it. Essentially a polarity switching magnet. Seems close to perpetual motion unless the energy needed to make the switch is high. Not sure how this works for regen.
The smaller of the two motors in the 2nd generation Chevy Volt (which is rated at 48 kW) uses iron-based permanent magnets in the rotor instead of the rare earth magnets used in the 1st generation Volt.
The resulting motor’s performance and efficiency is not quite as good but it’s primarily used as a generator when the gas engine is running or together with the larger PM motor during harder EV mode acceleration.
I am trying to separate some things . I see in the video the lamination of the two magnets and I see in the Munro video the lamination of 4 magnets Munro describes as a Halbach array - a relatively new discovery. I am not certain if any of these laminated magnets are fundamental to switched reluctance motor as developed originally in the 1800s. Seems like these laminated magnets are new developments that are enhancing the original design.
If I understood the whiteboard discussion, the alnichol (sp) material switches polarity when subjected to a field even briefly. Stated just as a fact without much explanation. This leaves the magic in that magnet but I suspect there is more to it. Essentially a polarity switching magnet. Seems close to perpetual motion unless the energy needed to make the switch is high. Not sure how this works for regen.
Oh, you're asking good questions! So this video shows two different magnets laminated together. They are arrangemed with two end caps that are not themselves magnetic, but are magnetically permeable (like steel).
One of the two magnets is made from material with high permissivity, and the other magnetic material has low permissivity. So when a pulse of electricity is sent through a coil wrapped around this composite magnet, the coil briefly generates an electromagnetic field with penetrates one of the magnets, but not the other.
The result of this pulse is the lines of magnetic flux for the composite magnet are expelled from the interior path flowing through the steel endcaps, and become external to the composite magnet where they can apply their force to another object like the steel rotor of a SRM.
So its not quite a perpetual motion machine, because it takes energy to displace the magnetic flux from a path inside the magnet to one outside, and more to switch back. That's what the drive inverter has to do many times per second to create continuous rotational torque in the motor shaft (doing so cleverly with fast electronics and good algorithms minimizes torque ripple).
But the pulse of electrical energy required to move the path of magnetic flux from inside to outside (switch the magnet on/off) is less energy than would be required to continuously energize the electromagnet in an AC motor like the Model S/X.
That's why the Model 3 LR motor is 90% eff. vs the Model S/X at ~80% efficient. And its also why the AWD Model 3 with its AC induction front motor is less efficient than the RWD version with it's single SRM motor.
I am trying to separate some things . I see in the video the lamination of the two magnets and I see in the Munro video the lamination of 4 magnets Munro describes as a Halbach array - a relatively new discovery. I am not certain if any of these laminated magnets are fundamental to switched reluctance motor as developed originally in the 1800s. Seems like these laminated magnets are new developments that are enhancing the original design.
Original switch reluctance motors have no permanent magnets, only a permeable core and electromagnets. Same type of thing as a relay, solenoid, or the big crane magnet in junk yards (esp in movies). The stator makes a field, and the rotor aligns to it (repeat and offset to spin)
car cabling, superbottle, supercharger, GF1, battery pack, over the air updates, safety, quickness, sales method, fair pricing for all, mission/vision, continuous improvement [i.e. no model years], in dash vent system, driving experience, gas station avoidance. Not sure which is the most valuable. [autopilot still a work in progress - it is for everyone]
