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techdirections March 2017 : Page 17

thereby dispensing with the hefty radiator, cooling fan, water pump, and associated plumbing. Better still, by being able to tolerate temperatures that cause permanent magnets to break down, an induction motor can be pushed (albeit briefly) to far higher levels of performance, like testing 0-60 times or climbing a steep hill. Hybrid ve-hicles like the Toyota Prius or the Chevrolet Volt have to use their gasoline engines to get extra zip. Pure electric vehicles such as the Nissan Leaf depend on gearboxes to generate the extra torque for ardu-ous tasks. By contrast, the Tesla uses just one gear—such is the flex-ibility of its three-phase induction motor. When you take another look at how these motors actually work you can see how the similarities can lead to confusion. They both have rotors that are free to spin and align themselves with the elec-tromagnetic forces when poles of the individually paired windings are electrified. In a three-pole motor, when pole pair A is excited, the rotor (either a magnetic material for induction mo-tors, or permanent magnets as in a DC brushless motor) will align itself north and south with that pole. As that pole is turned off and the next pole is excited the rotor will rotate to align itself with the newly applied electromagnetic field, and so on in succession through the poles until 360° of rotation is ac-complished. The difference here is that the in-duction motor applies AC voltage to each of the windings in succession, and the DC brushless motor applies pulsed DC to each of the windings in succession—the speed of the rota-tion depending on the frequency of the applied voltage to each of the poles—the higher the frequency the faster the rotor spins. When you look at Fig. 6, the similarities are very evident. The similarity of these motors is impossible to deny, as are their differences. This is also true when the motors are used for more than just driving the wheels. If the AC supply voltage of an induction mo-tor is removed, the result is no drive force and no battery charging regeneration effect—the motor will freewheel. This is not true of the DC brushless motor. Both motors are driven by a variable speed frequen-cy inverter (one AC and one DC). Regenerative braking is possible by reducing the supply frequency. The synchronous speed applied to the stators is less than the motor reversing the rotating wave within the motor, the motor wants to turn in the opposite direction. This does not regenerate power for the bat-teries but the magnetic field in the motor spinning in the opposite direction simply acts as a brake on the spinning rotor, and the excess energy is dissipated as heat. The DC brushless motor is capable of doing this, but excess heat build-up runs the possibility of damaging the Fig. 6—Comparing induction and brushless DC motors speed. During regenerative braking, the motor is generating more volt-age than it is using and that excess voltage is fed back to the batteries. AC motors can be microprocessor controlled to a much finer point and thus can regenerate all the way down to a stop, whereas the brushless DC motors regeneration capabilities tend to fade off at lower speeds. The final advantage of the AC induction motor is in dynamic brak-ing. This is done by reversing the rotating wave within the stators. By expensive rare earth magnets within its rotor. All in all, both motors are quite good at doing what they were de-signed to do. The DC brushless motors have been around a little longer in this application and have a strong foothold in the industry, but that foothold is slipping as the technology to control the induction motor has improved. Tesla Motors has a policy of releasing its technol-ogy to other manufacturers so that they too can enter into the world of the plug-in electric vehicle. aUtomotive tecHnoLogy 17 Images courtesy Electropaedia

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