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Chapter 6: Electric Motors 143use this approach, or a variation of it. Field weakening had its limitations, which driverssoon didn’t want to contend with, given the other better options available to them.ReversingThe same current that flows through the armature flows through the field in a seriesmotor, so reversing the applied voltage polarity does not reverse the motor direction. Toreverse motor direction, you have to reverse or transpose the direction of the fieldwinding with respect to the armature. This characteristic also makes it possible to runseries DC motors from AC (more on this later in the section).Regenerative BrakingAll motors simultaneously exhibit generator action—motors generate counter EMF—asyou read earlier in this section. The reverse also holds true—generators produce countertorque. Regenerative braking allows you to slow down the speed of your EV (and saveits brakes) and put energy back into its battery (thereby extending its drivable range) byharnessing its motor to work as a generator after it is up and running at speed. Putanother way, the vehicle, once running at speed, has kinetic energy. Regenerativebraking allows you to electronically switch the motor and turn it into a generator,thereby capturing the energy that would normally be dissipated (read: lost) as heat inthe brake pads while slowing down. The motor does the braking, not your brake pads.While all motors can be used as generators, the series motor has rarely been used as agenerator in practice because of its unique and relatively unstable generatorproperties.Shunt DC MotorsThe second most well-known of the DC motors is the shunt DC motor, so namedbecause its field winding is connected in parallel with the armature (Figure 6-3). Becauseit doesn’t have to handle the high motor armature currents, a shunt motor field coil istypically wound with many turns of fine gauge wire and has a much higher resistancethan the armature.Torque CharacteristicsBecause the shunt field is connected directly across the voltage source, the flux in theshunt motor remains relatively constant. Its torque is directly dependent upon thearmature current, as described by torque equation:Torque 5 K 3 f 3 I aThis shows that, in a shunt motor, torque varies directly with the current, and thestraight-line relationship shown in Figure 6-3 results. Although there is initially nocounter emf to impede the flow of startup current in the armature of a shunt motor, theshunt motor’s linear relationship is quickly established. As a result, the shunt motordoes not produce nearly as much startup torque as the series motor. This translates toreduced acceleration performance for owners of shunt motor-powered EVs.SpeedWhen a load is applied to any (but here specifically a shunt) motor, it will tend to slowdown and, in turn, reduce the counter emf produced.