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146 Build Your Own Electric Vehicle• Regenerative Braking—A compound motor is as easily adaptable as a shuntmotor to regenerative braking. Its series winding gives it additional startingtorque, but can be bypassed during regenerative braking, and its shunt windingallowed to give it the more desirable shunt generator characteristic. Manual orsolid-state electronics controlled, compound motors are usable for and adaptableto regenerative braking applications.Permanent Magnet DC MotorsWhen you were first introduced to the DC motor topic, permanent magnets were usedas an example because of their simplicity. Permanent magnet motors are, in fact, beingincreasingly used today because new technology—various alloys of Alnico magnetmaterial, ferrite-ceramic magnets, rare-earth element magnets, etc.—enables them to bemade smaller and lighter in weight than equivalent wound field coil motors of the samehorsepower rating. Rare-earth element magnets surpass the strength of Alnico magnetssignificantly (by 10–20 times), and have been used with great success in other areassuch as computer disc drives, thereby helping drive down the production costs. (SeeFigure 6-4.) While commutator and brushes are still required, you save the complexityand expense of fabricating a field winding, and gain in efficiency because no current isneeded for the field.Permanent magnet motors approximately resemble the shunt motor in their torque,speed, reversing, and regenerative braking characteristics; either motor type canusually be substituted for the other in control circuit designs. But because modemmaterials can support higher levels of magnetizing force—the H factor in equation2—the much smaller armature reaction of permanent magnet motors greatly extendsthe linear characteristics of conventional shunt motor speed/torque curves down tozero speed. This means that permanent magnet motors have starting torques severaltimes that of shunt motors, and their speed versus load characteristics are more linearand easier to predict.Brushless DC MotorsWith no brushes to replace or commutator parts to maintain, brushless motors promiseto be the most long-lived and maintenance-free of all motors. You can now customtailorthe motor’s characteristics with electronics (because electronics now representFi g u r e 6-4 Permanent magnet DC motor (Cour tesy of Zero Emission V ehicles of Australia andwww.electric-car s-are-for-girls.com).