With all the hype around Tesla's new Roadster it's time to remind what is the basic principle behind the electric motor’s design that makes it so much more superior over the traditional combustion engine in terms of acceleration.
The main reason for these outstanding acceleration numbers is the electric motor’s ability to produce the maximum torque already at zero rpm, while a traditional internal combustion engine’s torque usually tops at fairly high rpm. But what is the cause behind this instant toque?
At zero rpm, when the current is applied and flows through the electric motor within a magnetic field, a force rotating the armature will be generated. The more current is applied the higher the motor’s rpm rises increasing the vehicles speed accordingly. The torque production is instant due to the fact that during low rpm no back EMF (electromagnetic force) is generated and so the losses in the torque production are minimal.
The back EMF starts to affect the vehicles torque curve when the speed rises as this EMF, which tries to oppose the changes in the magnetic field, is caused by the rotation inside the internal magnetic field. Because of the EMF the overall force being applied to the wheels is the difference between the voltage applied to the motor and the opposing EMF. The back EMF is the reason why many manufacturers prefer the hybrid layout which gives the electric motor’s peak torque already from the start of the rev range but also keeps the car accelerating further when the speed and back EMF rise causing the torque produced by the electric motor to slowly decrease.