04-12-2012, 02:15 PM
Electric vehicles – Electric Transmission
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Advanced electric drive vehicles such as hybrid-electric vehicles, plug-in hybrid electric vehicles, fuel cell electric vehicles, and pure electric vehicles, require power electronics and electrical machines (PEEM) to function. These devices allow the vehicle to use energy from the battery to assist in the propulsion of the vehicle, either on their own or in combination with an engine. Figure 1 is a simple diagram of an electric system drive, where the inverter (power electronics) takes direct current (DC) electricity from the battery and converts it to alternating current (AC) electricity and sends it to the motor. The electric motor (electric machine) uses the AC current to create torque (mechanical power) to power the wheels for propulsion. Note that the arrows point both ways; this is because the electric machine can also act as a generator and help the vehicle slow down when coming to a stop. This sends energy back through the inverter and into the battery to recharge it.
Transmission
A gearless or single gear design in some EVs eliminates the need for gear shifting, giving such vehicles both smoother acceleration and smoother braking. Because the torque of an electric motor is a function of current, not rotational speed, electric vehicles have a high torque over a larger range of speeds during acceleration, as compared to an internal combustion engine. As there is no delay in developing torque in an EV, EV drivers report generally high satisfaction with acceleration.
The gearless design is the least complex, but high acceleration requires high torque from the motor, which requires high current and results in Joule heating. This is because the internal wiring of the motor has electrical resistance, which dissipates power as heat when a current is put through it, in accordance to Ohm's Law. While the torque of the electric motor is not dependent on its rotational speed, the output power of the motor is the product of both the torque and the rotational speed, which means that more power is lost in proportion to the output power when the motor is turning slowly. In effect, the drivetrain becomes less efficient the slower the vehicle moves.