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A Tesla Model S for example has a single speed gearbox in between the motor and the rear axle of the wheels. What's the purpose of that gear? Why can't the motor be mated directly with the axle instead?

With internal combustion engines I understand that a multi speed gearbox is required because a combustion engine can't deliver maximum torque from standstill nor for long in its RPM range, but since EVs can, what type of performance does a single speed gearbox enhance (more torque, speed, energy efficiency or something else)?

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    Gearing is needed because the motor rpm range is much higher than the wheel rpm range. To efficiently make use of the entire motor rev range a gearing is needed. Furthermore, in practice an electric motor is surely not able to deliver the same torque at all speeds, and surely not the same power. Smaller electric vehicles still use a gearbox. Tesla, in the american way, just opted for a monstrous battery and oversized engine to cope with the problems. It makes for a low top speed comparative to its engine power. – Bart Mar 23 '18 at 10:22
  • I believe this is a duplicate question, but I cannot seem to find it straight away. – Pᴀᴜʟsᴛᴇʀ2 Mar 23 '18 at 13:42
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Tesla's motors have peak power in the 5-10k rpm neighborhood. They need to gear it down so that the power is utilized at wheel speeds in the 1-2krpm neighborhood. (extremely round numbers folks..)

Don't confuse the fact that electric motors have nearly constant torque with constant power

  • Hi, thank you for your comment. Have you got a source where you can refer to regarding that peak power at 5-10k rpm? – CRTx Mar 25 '18 at 18:16
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It may just be for packaging reasons. I could not quickly find the gearing, but it likely doesn't matter much. Taking a look at the photo here shows the motor set back on the subframe, it looks like this allows the half-shafts a straighter shot to the differential, and then to the motor. enter image description here

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The reason is simple.

To have lots of power in a small motor package, you want high RPMs. The size of the motor dictates the torque. To have lots of power, you can either increase torque (thus increasing motor size and therefore things like costs of rare earth materials in magnets if the motor is a permanent magnet motor) or increase RPMs (which is essentially free). Based on this, it should be clear the way manufacturers have maximized power is maximizing RPMs. Gearboxes then convert the high RPMs to lower RPMs and higher torques.

However, wheel with a circumference of 2 meters or so will require 1000 RPM to reach 120 km/h. That's about as fast as most people go in their cars. Electric motors can turn at over ten times this speed.

1000 RPM is very low speed for an electric motor. These electric motors spin at RPMs much higher than internal combustion engines because the airflow restrictions aren't there: electric motors don't require air like internal combustion engines do.

The motor-generator 1 (MG1) in my 2016 Toyota RAV4 hybrid produces its peak power at 12500 RPM (source)! Much higher than typical internal combustion engines.

The same problem makes electric bicycles fundamentally difficult to build. The cyclist pedals at very low RPMs, the wheel rotates at slightly higher RPMs but electric motors want to spin at much, much higher RPMs. Most manufacturers have put the motor in the wheel hub because RPMs are higher there than at the crank spindle. But this is still suboptimal. Far more optimal would be to change the gearing to favor high RPMs at the crank spindle, and remove the cranks and pedals, replacing them entirely with the electric motor. But without cranks and pedals it wouldn't be an electric bicycle then. There is fundamental incompatibility between the gearing required for pedaling, and the gearing required for electric motors. About the only reason why these hub motors work is that they typically are limited due to legislation to 250 watts max.

The same reason also means washing machines typically aren't direct drive but rather belt drive. The belt drive allows high RPMs at the motor. Some manufacturers make these direct drive machines (e.g. LG), but there is no good reason for their existence apart from marketing reasons.

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