AC and DC Motor in Transport

Briefly stated, the AC motors used in electric cars are so-called "brushless DC" motors. In reality, they are 3 phase synchronous AC motors (e.g., the rotor is permanantly magnetized). The motor is customarily driven with three phase power developed from a three phase IGBT bridge, generally operating from 250-500VDC. Pulse width modulation is applied to the drive to control motor winding current, where the motor stator inductance integrates the PWM into the desired current, in the same way as a switching power supply works. True 3 phase drive in this instance is referred to as a "sine drive", and is supported very heavily by International Rectifier (www.irf.com) and others.

Alternatively, there exists the so-called "6-step" drive, where two of the three phases are energized at any one time. Back EMF from the open phase is read and is used to determine commutation time to the next of the six states of 2 out of three phase excitation combinations. Back EMF is generated due to the fact that the rotor is a permanent magnet as stated above. This system has the advantage that position sensors (customarily Hall-effect devices which read magnetic "flags" on a shaft mounted disc) are not needed for state-change commutation. Conversely, sine drives most often measure the current in two of three phases (third phase current can thus be inferred) for current control in what amounts to an AC variable frequency drive.

This technology is a relatively recent development.

When electric trains were first put into service about 100 years ago, developers were happy to use electricity in any way possible for powering of trains, especially in subways and train stations where the smoke produced from burning coal was terrible. So, brush type DC motors were used, with controllable field winding resistance to control acceleration (the speed control handle the engineer moved changed taps on the resistor, as wasteful as that was). Below a given speed the rotor windings and field windings (including series resistor banks) were connected in parallel. At a certain speed, the field was reconnected in series with the armature, producing higher speed from the back EMF typical of a "series connected" motor, much the same as used in a Dremel tool, only much larger. To this day the signals used for electric railways have acronyms stemming from parallel or series connection (S and P, if Im not mistaken), indicative of allowance of high speed, or restriction to a low speed.