Thursday, August 29, 2013

Why tesla motors uses AC induction motor instead of a DC motor?

DC motors are conventionally known to provide higher torque making them an obvious choice for electric automobiles. However, they are not used in pure electrics (as opposed to hybrids) for a certain reason.

Conventional DC motors are not used due to their maintenance problems with the brush and commutator mechanism. Instead brushless DC motors are used. Came across this comparison on the tesla blog saying why AC motors are preferred over brushless DC. The main reason is the ability to control the magnetic field B which is proportional to V/f as opposed to using a static magnetic field in the stator in brushless DC and the other aspect is cheapness (lack of a permanent magnet in the rotor of a AC motor as opposed to brushless DC).

Excerpt from this blog:
Both DC brushless and induction drives use motors having similar stators. Both drives use 3-phase modulating inverters. The only differences are the rotors and the inverter controls. And with digital controllers, the only control differences are with control code. (DC brushless drives require an absolute position sensor, while induction drives require only a speed sensor; these differences are relatively small.)
One of the main differences is that much less rotor heat is generated with the DC brushless drive. Rotor cooling is easier and peak point efficiency is generally higher for this drive. The DC brushless drive can also operate at unity power factor, whereas the best power factor for the induction drive is about 85 percent. This means that the peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive.