Looks like the scandal about Toyota's brakes in cars like the Camry has spilled over to their hybrid Prius models. Electric cars brake a little differently than regular fuel-driven cars. Aside from mechanical-hydraulic brakes, electric cars also use something called regenerative braking, which every electrical engineering student who takes up subjects in motor control and design should be aware of.
The principle is simple. If you move a magnet over a wire in a certain direction, current will flow. Inversely, if you run a current on a wire that is coiled, it will become a magnet. And if you put two similar magnetic poles together, they will repel. Put opposite poles together, they will attract. So that is the basic principle that governs a motor, to convert electric current into rotation motion.
Remember those toy DC motors where if you attached them to a battery, they spun in one direction. Reverse the polarity of the battery and it will spin in the opposite direction. On a toy DC motor without a load attached to it, because there is almost no mass attached to the rotor (no mechanical load, low rotor inertia), it can reverse direction almost instantly.
When an electric car (like the Prius) reverses the polarity of the battery going into the motor, it will -- like its toy counterpart -- attempt to reverse the direction of the spin of the motor. But a car with passengers, has significant mass. And if that mass is moving, it will -- as Newton said -- have inertia, so it will not automatically follow the reverse spin of the electric motor.
Once this happens, the mechanical torque (rotating force) induced by the car's mass (with the passengers) still attempts to spin the electric motor forward, while the reversed polarity of the battery to the motor attempts to spin it backward. So in effect, the mechanical forward torque is battling the reverse electrical induced torque. This is like holding on to the fan rotor of your electric fan and trying to stop it from spinning. If the motor and battery are properly sized, the car should brake and stop. Once that happens, power should be removed from the motor otherwise the car will move in reverse.
The reason it is called regenerative braking is because while the car is still moving forward and the electric motor is attempting to brake it, the rotor (moving part of the motor) is spinning in reverse of where the battery (and the motor) wants to take it. Because the rotor also has magnets, spinning it will result in a reverse current that charges (not discharges) the battery.
It is like gaining gasoline back in the tank when you brake your car, if that analogy makes it simpler for you.
In principle that is how it works, although it needs to be augmented with mechanical-hydraulic brakes, which in the case of the Prius are ABS (anti-lock) brakes. Unfortunately, according to published reports, the problems with the regular Toyota cars like Camry seems to have spread into the Prius. Whether these are overreactions on the part of the Prius car owners remains to be seen.
Read the original article in BusinessWorld here