Forums

For those wondering why AWD is less efficient

For those wondering why AWD is less efficient

Wilber | 2019年3月16日

Bighorn - thanks for posting. But Elon seems to be comparing one permanent magnet motor to two induction motors. My AWD M3 has induction front and perm mag in rear, so NOT two induction motors. Maybe what he says about 'decoupling' is the important part?

Frank99 | 2019年3月16日

It almost sounded like he was saying that the Model 3 AWD uses two induction motors. I'm pretty sure that I'm just reading it wrong - the general consensus is that the rear motor is the same between the RWD and AWD and is a PMSRM. Is the front motor an induction design, or a PM motor like the rear?

Frank99 | 2019年3月16日

Wilber -
I think what he's saying is that, because the front motor is spinning any time the car is moving (there's no clutch to "decouple" the motor from the wheel), there's extra friction as well as inertia in the front motor that need to be overcome causing a loss of efficiency.

beaver | 2019年3月16日

Don’t forget the weight of the motor and cabling, small but real reduction in range due to weight. If I bet the front induction motor not being able to coast is the larger driver of lower range. In plus side I think the Dual Motor has different ratio front to back so it can be more efficient at highway speeds.

Bighorn | 2019年3月16日

In the Model S, they idle one of the motors to make the AWD more efficient than the RWD. I think he’s saying they can’t/don’t decouple the combination of induction and permanent magnet pairings in the Model 3. So both are always working, rather than one going along for the ride. I guess he didn’t succeed in explaining it:)

Coastal Cruiser. | 2019年3月16日

"...Model 3 partial permanent magnet induction motor..."

Unless Elon misspoke, the sentence construction seems to have him inferring that the front induction motor has magnets too. At first blush that seems hard to believe, because the rotor on Tela's traditional induction motor is pure solid copper. Where would the PMs go? Elon must have put that wrong. Right?

However, in researching the design of the reluctance motor used on the rear of the 3 -which is augmented by magnets stuffed into the rotor- I was introduced to a research paper whereby a 30% gain in power was achieved by placing the magnets in the STATOR along with the existing electromagnets.

There have been no stripped-to-the-bare-wires teardown of any Model 3 stator that I'm aware of, so the question of whether Tesla has stuffed the stator with magnets has been unanswered (as best I know). But this is a HUGE question, and if true -if the stator on the induction motor has rare earth permanent magnets mixed in with the electromagnets- then this explains a bit more of Tesla's secret sauce that gives its motors an edge over the competition (keeping in mind that virtually all EV makers are using the same basic PMSRM Permanent Magnet Switched Reluctance motor).

So Elon may have just let slip another huge clue in the Model 3's motor design. Another clue is Elon saying:

"If we could decouple forward induction motor to avoid spin losses, then range of dual might be higher"

I'm "guessing" that there is not much in the way of "spin losses" with the conventional induction motor. The opposing magnetic fields on, say, the Model S induction motor is created only when the electromagnets in the stator are switched of/off. If the electromagnets are not turned on then the rotor sort of freewheels. Very few counter-forces to cause "spin losses" (as best my imperfect understanding makes of that term).

However, if there are permanent magnets embedded in the stator, that magnetic filed is ALWAYS ON, and the spinning of the copper induction rotor may well create "spin losses" which I'm guessing is another phrase for "Back EMF". In that case being able to decouple from that motor, like a railroad car dragging down a locomotive would be uncoupled (sort of), would help make the AWD config more efficient. Maybe. This is all my personal theory not to be taken seriously.

Bighorn | 2019年3月17日

@CC
I think someone mentioned there being six magnets in the rear motor. Don’t quote me.

Coastal Cruiser. | 2019年3月17日

@BH,

What you might be recalling is that there are 6 "poles" on the rear motor (up from 4 poles in the traditional induction motor). That would be 6 discreet locations around the motor where the electromagnetic fields are being created. ;>

I'm pretty sure from the Munro teardown that there may be more than 6 physical magnets on the rear motor, but again, we have learned they are in the rotor of that motor. I'm digging in deeper to see if I can either prove or disprove my above theory.

Bighorn | 2019年3月17日

Right—sounds familiar

gmr6415 | 2019年3月17日

@Frank99, I think you have it correct. Unless the front wheels could somehow be decoupled from the axles, so they can freewheel and the front motor and gearbox can go "idle" there will always be friction from the moving parts of the front motor and especially the front gearbox.

Wilber | 2019年3月17日

Hi All: thanks for your explanations, they help. But most of you folks are out of my league, so i wont pretend to fully understand. As i see it the 'decoupling' issue is the main factor. A little dissappointed that Tesla got cheap with the M3, so the dual drive is not as efficient as it is for the S or X. but, i will survive.
As to the front and rear motors in the M3 AWD - my description was straight from the Monroney sticker, so i am fairly confident it is accurate.