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Shopping for electric cars, I had a thought: how do dual motor, AWD drivetrains work? On a conventional car there’s one engine distributing power to both via a shaft and gearing, but it would seem on the EVs the two axles are not locked with gears and a central shaft? If this is the case, how is it ensured that both axles move in sync? Is this high-precision control systems or is there a mechanical linkage that isn’t depicted in drawings of transmissions like Teslas?

A Tyshka
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    The sync mechanism is the friction with the ground. Even with a conventional drivetrain, the only time all wheels are turning at the same rate is when moving straight ahead on a flat surface. Also, keep in mind that the throttle is a torque control, not a speed control. – Dave Tweed Jan 15 '22 at 13:13
  • Engineers have gone to great lengths trying to mechanically unsync the front and rear axles, using various flavors of differentials such as Torsen center diffs or planetary drop boxes. But limited slip is needed, so the emotors will need some sense and control mechanism to manage the torque split between axles. – Phil Sweet Jan 15 '22 at 13:51
  • Also, there are dual emotor setups that don't have independent axles, so what you are really asking about is independently driven axles. – Phil Sweet Jan 15 '22 at 13:54
  • I would be astonished if the engineers have not designed feedback loops to equalize torques (assuming either two or four-motor designs) so as to minimize stress on the tires as well as the drive axles. Similarly, a software version of "limited-slip differential" is relatively straightforward. – Carl Witthoft Jan 18 '22 at 14:53
  • BTW, Tesla in particular turns off the smaller motor in dual-motor cars when the power demand (acceleration request or operating speed) is low. – Carl Witthoft Jan 18 '22 at 14:55

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I don't know if there is a sync system but if there is a sync system it probably would be in the form of speed monitoring on the motors, not a mechanical linkage).

However, I do think you're overestimating how much out of sync they can get. Motors moving too fast (i.e. faster than the vehicle) will load up more and slow down, while motors moving too slow (i.e. slower than the vehicle) will be loaded less and speed up. Since both motors are on the same vehicle and the vehicle can only move at one speed, the motors find an equilibrium amongst themselves where their speed is the same.

For example, most robots with independent wheel motors usually don't have any explicit measures taken at all. You just send the same PWM duty cycle to both motors (assuming they are identical motors). I don't even think about it.

The only thing you really need is to make sure the throttle to both motors is the same so they share the load equally, but that's super simple and not syncing.

DKNguyen
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Note that EVs with multiple motors have special control circuits which kick in and limit the current to any motor whose tire has broken grip with the ground and is spinning, so it doesn't rob the other motors of electrical power. This means that a multimotor EV must actively monitor all the wheel RPM's and have a spin detect system tied into the motor controllers.

In the absence of spin conditions, it is sufficient to place all the motors in parallel so they all see the same voltage- turn their individual wheels at the same speed. In a turn, where the inside wheels turn slightly slower than the outside wheels, they will draw different amounts of current, which the parallel connection will automatically manage.

niels nielsen
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  • "... it is sufficient to place all the motors in parallel so they all see the same voltage ..." What kind of motors are you assuming here? (I have no idea. I'm just wondering.) – Transistor Jan 15 '22 at 21:37
  • permanent magnet DC. – niels nielsen Jan 16 '22 at 02:50
  • I think this answer has swapped current and voltage. Loosely speaking, a PMDC motor has speed vary with voltage and torque vary with current. For other types of motors, speed control techniques will vary, but generally more torque will need more magnetic field and thus more current, however that is achieved by the controller. – Kevin Reid Jan 17 '22 at 20:01
  • @KevinReid reid, will edit. -NN – niels nielsen Jan 18 '22 at 05:16