One motor per wheel
The notion of having one motor per wheel in an electric vehicle like a Tesla has been mooted and debated. The consensus seems to be that the pros (electronic authority over torque vectoring – no mechanical bits required, eliminating the weight of a differential) are outweighed by the cons (additional cost of motors, gear trains for each motor, possible weight gain with extra motors).
A trend?
The original Model S had one electric drive motor. The D models S has two electric drive motor. The next Roadster will have three electric motors.
Is it crazy to think Tesla will introduce a four motor car at some point? Do two motors weigh more than one motor making the same total horsepower?
Braking bad?
One overlooked advantage of having a motor at each wheel is that it may eliminate the need for heavy and complex friction braking systems. This article on electromagnetic braking for electric vehicles from Electric Vehicle News makes a compelling point:
“Bullet trains use electricity to brake 16 car 640 tonne trains from 320 km/h at a controlled and predictable deceleration rate.
The challenge remains that EV motors put out power (acceleration) at a much higher rate than they can take back in regeneratively (deceleration) largely due to lithium battery charge rate limitations. This might be a problem that goes away with the concerted effort to improve EV charge times and associated battery technology, or it may be addressed with a capacitor (like Mazda uses in its i-ELOOP system).
Principles involved
The trend from a single electric motor to two, three or four is Principle 1 – Segmentation at work. Using those motors to serve as brakes is Principle 6 – Universality. Eliminating mechanical systems in favour of electronic control (vector control differentials, friction brakes with hydraulic modulation for antilock and traction control) is Principle 28 – Substituting of Mechanical Systems with Electromagnetic Systems.
