tps wrote:Yep, the motor continues to spin the same direction. The direction of current flow, however, reverses. Current always flows from higher voltage to lower voltage. When charging a battery, the charger has to put out a higher voltage than the battery's voltage to get current to flow into the battery. The inverter is where this magic happens when the car is moving. You can think of it almost as having a continuously variable voltage conversion ratio between motor and battery. To provide power to the motor, it must adjust the ratio so that to motor will draw the desired amount of current from the battery. To regenerate it adjusts the ratio so the battery will draw current from the motor (acting as a generator). This is not really so much different from down-shifting the transmission in a car to get a bit of engine braking effect when going down a hill, except that an ICE engine just wastes all the braking it's doing as more heat exhausted through the radiator, whereas the inverter puts the energy (minus the conversion and charging loss which is exhausted as heat from the LEAF') back into the battery.
This is an excellent description of how a brushed DC motor controller works. The Leaf, however, uses a brushless motor which is far better for three main reasons 1) no brushes to wear out and 2) the coils are on the *outside* housing (stator) instead of on the inner rotating part (rotor) so you can get the heat out much easier and 3) you get regen for free. Brushless motors use permanent magnets on the rotor. The controller monitors the position of the rotor and simply activates the coils on the outside in sequence. When coasting, it will align the coils exactly with the rotor field. When accellerating it just adjusts the timing of the coil activation to lead the rotor (drawing power from the battery and pulling it along). For braking it changes the timing to lag the rotor causing the rotor to induce voltage in the coil - sending charge back to the battery. There's extra detail to deal with back EMF and limit regen but it's actually a quite elegant solution. 80% efficiency is very doable with good fast switching, low resistance switches - in fact almost required. If you are less efficient then that you have to figure out how to get that heat *out* of the controller.
I have a lot of respect for the engineering in the controller. Some buddies and I designed a smaller (18hp) brushless motor controller for a battlebot a few years ago and we literally blew up one prototype (and others just caught on fire) before we worked out all the kinks - there's a lot of energy moving around in that thing!