The last week or so, the new ‘carbon-wrapped’ motor to be used in the Tesla Model S Plaid has been everything anyone has been speaking about. It’s been touted as ‘innovative’, ‘revolutionary’, and the ‘most advanced motor in the world’, and is apparently the first ‘production electric motor that had a carbon overwrap rotor’.
I’m somewhat at a loss here. I mean, we know the drill. A non-motor-expert company person (usually either the CEO or some PR person) speaks to a non-motor-expert journalist. Both try to convey their understanding in a way they think a layperson would best understand, and the end-result is nigh-incomprehensible to actual specialists. But, this is perhaps the most extreme case so far.
Anyways, this post is about carbon fiber sleeves, as you may have guessed. It’ll briefly cover their traditional uses, and then go through some ways that Tesla might be using them. All examples in this post were computed with EMDtool.
So let’s dive in
Unlike what is implied in the news, carbon fiber wrappings have been long-used in high-speed machinery: turbos and compressors and the like.
Usually, such motors are surface-PM motors, like in the pic below. The cyan arc-thing is the permanent magnet, located on the surface of the rotor core. To stop the magnet from flying away at high speeds, the entire rotor is wrapped inside a thin-walled cylinder (black in the pic) of some suitable material.
Note the word “suitable”. There are usually no single-best solutions in the motor world, and rotor sleeves are a particularly nasty example.
A perhaps older solution would be to use a non-magnetic metal sleeve, like Titanium or Inconel. This approach has many good things. A metal sleeve is quite strong, and has very similar thermal expansion characteristics compared to the rest of the rotor (more of that soon).
The main drawback is that metals are good electrical conductors. As a consequence, strong eddy currents get induced in the sleeve. The pic below shows an example: the snapshot illustrates the eddy currents at 20 000 rpm, in SI units (so peaking at close to 10 Amps per mm-squared). From the pattern of the eddies, you can perhaps guess that they are mainly caused by the stator slotting.
These eddies cause losses and impair the efficiency. And perhaps more importantly, they can make thermal management of the rotor tricky.
Like you should be able to guess by now, carbon fiber sleeves are another option.
Carbon fiber composites are quite poor conductors (at least transversely, meaning perpendicularly to the fibers), and thus almost free from eddy losses.
But on the flip side, they are real nasty pieces of work, thermal-expansion-wise. Which they don’t do, really. They don’t expand.
The rest of the rotor does. Meaning, the carbon fiber sleeve gets stretched as the rotor expands thermally. And since carbon fiber is, as we scientist-engineers say, stiff AF, that expansion generates large stresses into it.
At the same, the sleeve must still fulfill its purpose even when the rotor is cold. Remember, it is there to keep the magnets from flying away. Meaning, it will have to sort of compress the rotor; to apply enough pressure on the magnets that they stay attached to the rotor.
And, it has to do this at the top speed, plus some margin. Meaning, it will have to cancel the centrifugal stresses from the magnet mass and its own mass, and still apply net radial force to the magnets.
This, in turn, implies that it will apply a larger pressure when the motor is stationary, commonly called pre-stressing.
All together, the sleeve must be sized appropriately to survive its own pre-stress, plus the centrifugal forces at top speed, plus the extra forces from thermal expansion.
Tesla use – IPM
EDIT: since first writing out this post, new information has
been pointed out to me surfaced, about the Plaid only utilizing interior-PM motors and no induction motor on any axle. So, feel free to just read this section, and skip the IM-focused stuff below.
Indeed, the pic below illustrates the motor structure quite nicely indeed. It has the low-V shape that the Model Y did, and like most traction-IPMs do in some permutation or another.
Importantly, we can clearly see that there is no iron bridge between the magnets, at the tip of the V. There might not be any in the inter-pole region either, near the rotor surface, although that is difficult so say for certain from the pic.
This type of structure makes sense, and is in many ways optimal for V-IPMs. For contrast, the pics below illustrate the typical IPM rotor shape, with bridges both at the V-tip and in the interpole space. Normally, these bridges are required for mechanical stability, and offer an easy path for some useless leakage flux to flow
By contrast, the Plaid motor appears to get rid of the bridges, using the fiber sleeve to hold the rotor in place. This is course eliminates that leakage flux path, and allows the rotor to spin at higher rpms.
On the flip side, the sleeve in the pic does appear to be quite thick – definitely more than the sub-mm that I first assumed. This automatically implies that the motor has a magnetic airgap (physical gap + sleeve) way bigger than the typical ~1.5 mm seen in traction-IPMs of this size. Indeed, the magnets in the pic seem quite thick, too, presumably precisely to drive enough flux over the gap.
Ramblings on torque
Now that we’re at it, the airgap length of a traction-PM motor is a curious beast. Unlike what one might expect, smaller is not better (
insert some suitable wife-joke here). Remember, a V-IPM produces torque by two mechanisms: PM-torque and reluctance torque.
Normally, the PM-torque is by far the dominant component at small to moderate loads, with a progressively larger contribution from the reluctance component at higher loads. Increasing the airgap brings both components down, initially, but the PM torque can be quite easily returned to its initial level simply by slamming in a little more magnet material.
The reluctance-torque-per-Amp is not easy to restore, though. But, if we can increase the airgap AND keep the saliency ratio (Lq/Ld) the same, we can actually increase the peak reluctance torque achievable. After all, the reluctance torque ultimately limited by the motor saturation or demagnetization risk, whichever happens first. By increasing the airgap we can still drive the machine to similar levels of saturation or flux linkage, but now with higher currents, meaning higher torque. (Of course, this all depends on the colossal IF: IF the saliency ratio and flux paths stay relatively unchanged otherwire).
Final note: I initially thought the news about a carbon-wrapped motors were not about an IPM. This is mainly because they kept rambling and quoting themselves about a ‘copper rotor’, which I find very hard to associate with an IPM. Now, it seems that there’s a simple explanation: there is no copper. It wouldn’t be the first time Musk has gotten confused in his communications.
Incidental ramblings and a shout-out
Incidentally, another approach would be to use a dovetail structure in the rotor – see this thesis for more information.
Tesla use – induction motor?
EDIT: It seems that the plaid is all-IPM after all, so feel free to skip this section.
Musk is repeatedly speaking about copper in conjunction with the carbon fiber sleeves. This would imply we are dealing with an induction motor. This would make some sense; after all Model S Plaid is said to have a similar motor configuration as in the Model Y, meaning an IPM plus an induction motor on the front axle.
Granted, induction motors don’t normally have sleeves of any kind. After all, they need to have relatively small airgaps (usually 1 mm tops, in motors of this physical size), while traditional fiber sleeves commonly start at 2 mm. However, thinner sleeves are also starting to emerge (I spoke to the friendly folks at Inometa, who can go down to 0.3 mm).
The main question here remains: why would anybody want a sleeve on an induction motor?
The published articles so far are less than helpful on this. One states that a ‘carbon sleeve rotor creates a stronger electromagnetic field than a rotor that is held together by metal’, which doesn’t really say much to me.
One possible explanation could be that having a pre-stressing sleeve around the rotor would make it possible to either have open slots in the rotor, or at least reduce the thickness of the ‘claw’ area between the bar and the rotor surface.
The following pics should clarify this. The first one is very rough model of the old Roadster motor, with eyeballed dimensions. The second one is then a zoom-up of the rotor cage, demonstrating how the rotor bars are indeed inside the rotor core.
The iron on top of the rotor bars of course offers a path for the leakage flux to flow, as can be seen in the pic below, depicting the peak torque operation.
Now, having instead open slots like depicted below, or just closed slots with reduced tip height, could yield a small benefit in terms of maximum torque. In this case, about 404 Nm instead of 395, at 30 Arms/mm^2 stator current density.
Of course, there would be tradeoffs to consider, like harmonic losses caused by the rotor slotting.
But still, this is for now the only scenario I can think of where one would like to use a carbon fiber sleeve in an induction motor: Being able to use a copper cage in the first place, at high speeds, without having to bury the cage deep inside the rotor core.
- Tesla Model S Plaid will apparently have carbon fiber retaining sleeves in their motors
The sleeves might be used on the induction motor, to allow the use of copper cage at high speeds without having to bury it deep inside the motor.
- Nope, it’s an IPM, with fewer or no bridges at all in the rotor, at the cost of a reduced mechanical clearance between the stator and rotor and/or increased magnetic airgap (clearance + sleeve together)
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