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144 Build Your Own Electric VehicleThe increase in armature current results in an increase in torque to take care of theadded load. Basically, constant, counter emf tends to remain constant over a wide rangeof armature current values, and produces a fixed speed curve that only droops slightlyat high armature currents.The shunt motor’s linear torque and fixed speed versus armature currentcharacteristics have two undesirable side effects for traction applications whencontrolled manually.• First, when a heavy load (hill climbing, extended acceleration) is applied, ashunt motor does not slow down appreciably as does a series motor, and theexcessive current drawn through its armature by the continuous high torquerequirement makes it more susceptible to damage by overheating.• Next, in contrast to the “knee” bend in the series motor torque-speed equations(Figure 5-9), shunt motor torque-speed curves are nearly straight lines. Thismeans more speed control or shifting is necessary to achieve any given operatingpoint. Series motors are therefore used where there is a wide variation in bothtorque and speed and/or heavy starting loads.• Other than having much lower startup torque, shunt motors can perform aswell as series motors in EVs when electronically controlled.• The downside is that a shunt motor controller can be more complicated todesign than a series motor controller.Field WeakeningYou can also achieve a higher-than-rated shunt motor speed by reducing the shunt coilcurrent—in this case, you place an external control resistance in series with the shuntmotor field winding. But here, unlike the series motor’s runaway rpm region at no load,you are playing with fire, because the loaded shunt motor armature has an inertia thatdoes not permit it to respond instantly to field control changes. If you do this whileyour shunt motor is accelerating, you might cook your motor or have motor parts allover the highway by the time you adjust the resistance back down to where you started.Be careful with field weakening in shunt motors. Again, these motors soon lost favordue to the inherent simplicity of a series DC motor arrangement. Heating was less of anissue, and certainly not something that drivers want to have to worry about; the seriesmotor was better suited to the application.A shunt motor is instantly adaptable as a shunt generator. Most generators are infact shunt wound, or variations on this theme. The linear or nearly linear torque andspeed versus current characteristics of the shunt motor manifest as nearly linear voltageversus current characteristics when used as a generator. This also translates to a highdegree of stability that makes a shunt motor both useful for and adaptable to regenerativebraking applications, either manually or electronically controlled.ReversingCurrent flows through the field in a shunt motor in the same direction as it flowsthrough the armature, so reversing the applied voltage polarity reverses both thedirection of the current in the armature and the direction of the field-generated flux,and does not reverse the motor rotation direction. To reverse motor direction, you haveto reverse or transpose the direction of the shunt field winding with respect to thearmature.