Field weakening can let you triple motor speed for a given battery voltage. I don't know if they do field weakening or not, but that's relatively easy to do with a DC motor, if it's connected in such a way that the stator can be separately excited. But the rated RPM is probably not with field weakening enabled.
Not all controllers do this yet. In some cases it's hard to program correctly, in other cases you need extra hardware. For a DC or wound rotor synchronous AC motor, you need PWM to separately control the field strength. These are the easiest. For an asynchronous motor, you need a power converter to transform to higher current at low speeds, and for a permanent magnet motor it's possible to program the motor controller to partially cancel the rotor field.
I attempted to explain in more detail what's going on inside the motor/inverter earlier here and here.
This is the torque and power curves for the Fluence:
They improved the motor controller for Zoe:
The second part of this curve is the ideal curve, and as close to the ideal motor as you can get. This is the perfect tradeoff between torque and speed, and it can't be improved mechanically no matter what you do.
The first part of the curve, where torque is constant, is what many want to improve by adding a gearbox. But if you can push the constant-torque region higher, then you can gear the motor taller - you only need as much torque as you can transfer to the ground anyway.
I believe that for a high performance vehicle, your money, mass and space would be much better spent by improving cooling, transforming down to increase the current, buying higher-current batteries, more motors, etc instead of increasing mass and using up valuable space by adding a transmission.
As I said earlier, there is another aspect to this, and that is efficiency. The efficiency peak of an electric motor is usually somewhere slightly below the first half of the constant-power region. Taller gearing will move it towards higher speed. That might not be ideal, as some range will be lost when driving at low speed. But the sweet spot is rather large, and placed at about 1/3 to 1/2 of maximum speed, so a car geared for 300 km/h would be close to the sweet spot at 100 to 150 km/h. Also, high speed driving is the most problematic for range, at low speeds drag is so much lower that range is much less of a problem.
But for maximum efficiency you might actually want to add a transmission, for example for electric trucks or buses. That would positively impact motor efficiency, but the gain is partially offset by a negative effect on overall efficiency and acceleration due to more mass and slightly higher transmission losses. Weight and space are less critical for trucks and buses.
Example efficiency map from this article. Note that despite the hyperbole of that article, the "substantial" efficiency gains are on the order of 5-10%, and no part of the plot except almost at standstill falls below the 75-80% bracket. The people at Vocis design and sell transmissions so they obviously want EVs to have one too.
(how do I get rid of that attached image at the bottom?)
Not all controllers do this yet. In some cases it's hard to program correctly, in other cases you need extra hardware. For a DC or wound rotor synchronous AC motor, you need PWM to separately control the field strength. These are the easiest. For an asynchronous motor, you need a power converter to transform to higher current at low speeds, and for a permanent magnet motor it's possible to program the motor controller to partially cancel the rotor field.
I attempted to explain in more detail what's going on inside the motor/inverter earlier here and here.
This is the torque and power curves for the Fluence:
They improved the motor controller for Zoe:
The second part of this curve is the ideal curve, and as close to the ideal motor as you can get. This is the perfect tradeoff between torque and speed, and it can't be improved mechanically no matter what you do.
The first part of the curve, where torque is constant, is what many want to improve by adding a gearbox. But if you can push the constant-torque region higher, then you can gear the motor taller - you only need as much torque as you can transfer to the ground anyway.
I believe that for a high performance vehicle, your money, mass and space would be much better spent by improving cooling, transforming down to increase the current, buying higher-current batteries, more motors, etc instead of increasing mass and using up valuable space by adding a transmission.
As I said earlier, there is another aspect to this, and that is efficiency. The efficiency peak of an electric motor is usually somewhere slightly below the first half of the constant-power region. Taller gearing will move it towards higher speed. That might not be ideal, as some range will be lost when driving at low speed. But the sweet spot is rather large, and placed at about 1/3 to 1/2 of maximum speed, so a car geared for 300 km/h would be close to the sweet spot at 100 to 150 km/h. Also, high speed driving is the most problematic for range, at low speeds drag is so much lower that range is much less of a problem.
But for maximum efficiency you might actually want to add a transmission, for example for electric trucks or buses. That would positively impact motor efficiency, but the gain is partially offset by a negative effect on overall efficiency and acceleration due to more mass and slightly higher transmission losses. Weight and space are less critical for trucks and buses.
Example efficiency map from this article. Note that despite the hyperbole of that article, the "substantial" efficiency gains are on the order of 5-10%, and no part of the plot except almost at standstill falls below the 75-80% bracket. The people at Vocis design and sell transmissions so they obviously want EVs to have one too.
(how do I get rid of that attached image at the bottom?)
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