An EV’s speed controller is the equivalent of the carburetor or fuel-injection system in an ICE vehicle. To control the vehicle’s speed, the controller takes the energy from the battery pack and feeds it to the motor in a regulated manner. Modern controllers do this by pulse-width modulation, taking the full voltage from the battery pack and feeding it to the motor in thousands of tiny on–off pulses per second. The longer the duration, or “width” of the “on” pulses, the more electricity the motor receives and the faster the vehicle moves. Because the pulses are so tiny, the process feels completely smooth to the driver.
EVs can have AC motors or DC motors, and each needs its own kind of controller. In EVs with AC motors, an AC controller must first convert the DC from the batteries into AC before feeding it to the motor.
How does the controller know how much energy to give the motor? The potbox tells it. This linear potentiometer is a sensor that produces a resistance output proportional to its displacement or position. It responds to the driver’s foot pressure on the throttle pedal and sends a corresponding signal to the controller. The throttle pedal in an EV works just as it does in an ICE vehicle—the more you depress it, the faster you go.
The motor is the brawn of the EV, converting electrical energy from the batteries into mechanical energy to propel the vehicle. Instead of invisible electrons flowing through wires, we now have rotating components.
It’s the relationship between electricity and magnetism that enables the motor to do work. Passing electricity through a wire creates a magnetic field around the wire. By winding wire in a motor and running electricity through it, magnetic poles that repel each other are created, causing the shaft of the motor to spin.
If the EV has regenerative braking, the motor can also act as a generator. When the vehicle is coasting or braking, the momentum of the car drives the motor—rather than the motor driving the car. The magnetic fields induce current in the wires, the flip side of the process described above. The electricity flows backward through the controller (which rectifies it from AC back into DC) and into the battery pack. This process also creates drag on the motor—the “braking” part of regenerative braking, which is very similar to what happens in an ICE car when you lift your foot off the throttle in a low gear. Though it’s an intrinsic part of AC drive systems, regenerative braking is more rare in DC systems, where a special controller and extra wiring are required to allow the motor to serve as a generator.
The energy output from the spinning shaft of the motor now needs to reach the drive wheels. On a very small EV, the motor might drive the wheels directly, but with full-size vehicles, at least one level of gear reduction is necessary to reduce the revolutions per minute (rpm) of the motor to a usable speed at the wheels. A “direct-drive” vehicle will have a single gear reduction, which might be a gearbox or a belt-and-pulley arrangement. No shifting is necessary. This is common with AC motors that have upper limits of 8,000 to 13,000 rpm. DC motors with limits of about 5,000 to 6,000 rpm usually use the same kind of multiple-gear manual transmissions found in ICE cars. In EVs with manual transmissions, the clutch is usually retained and works the same as in an ICE vehicle.