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148 Build Your Own Electric VehicleThe application of a motor in a vehicle doesn’t need to contend with multiplesources of power. Universal motors are typically used in appliances, and not much atthat, given what happened to the world during the oil crises in 1973 and 1979. Efficiencybecame a watchword, and a NASA patent was eventually put into the public domain.This resulted in the “Green Plug,” which is now standard equipment in all new motordrivenappliances. These motors shouldn’t be put in EVs of any sort. There are bettersolutions.AC Electric MotorsNow that you’ve met DC electric motors, it’s time to meet the motor you encountermost often in your everyday life—the AC electric motor. The great majority of ourhomes, offices, and factories are fed by alternating current (AC). Because it can easily betransformed from high voltage for transmission into low voltage for use, more ACmotors are in use than all the other motor types put together. Before looking at ACmotors and their properties, let’s look at transformers.TransformersIn its simplest and most familiar form, a transformer consists of two copper coils woundon a ferromagnetic core (Figure 6-7). The primary is normally connected to a source ofalternating electric current. The secondary is normally connected to the load. When achanging current is applied to the primary coil, a changing magnetic field common toboth coils results in the transfer of electrical energy to the second coil.The other aspect of transformers useful to you (highlighted by Figure 6-7) is that anequivalent circuit of a transformer can be drawn for any frequency, and you can studywhat is going on. This is useful and directly applicable to AC induction motors.AC Induction MotorsThe AC induction motor, patented by Nikola Tesla back in 1888, is basically a rotatingtransformer. Think of it as a transformer whose secondary load has been replaced by arotating part. In simplest form, this rotating part (rotor) only requires its conductors tobe rigidly held in place by some conducting end plates attached to the motor’s shaft.When a changing current is applied to the primary coil (the stationary part or stator), thechanging magnetic field results in the transfer of electrical energy to the rotor viainduction. As energy is received by the rotor via induction without any direct connection,there is no longer a need for any commutator or brushes. Because the rotor itself is simpleto make yet extremely rugged in construction (typically, a copper bar or conductorembedded in an iron frame), induction motors are far more economical than their equallyrated DC motor counterparts in both initial cost and ongoing maintenance.While AC motors come in all shapes and varieties, the AC induction motor—themost widely-used variety—holds the greatest promise for EV owners because of itssignificant advantages over DC motors.These solid-state components have resulted in AC induction motors appearing invariable speed drives that meet or exceed DC motor performance—a trend that willsurely accelerate in the future as more-efficient solid-state components are introducedat ever-lower costs. This section will examine the AC induction motor for its torque,speed, reversal, and regenerative braking capabilities—the factors important to EVusers. Other AC motor types not as suitable for EV propulsion will not be covered.