Chapter 6:
Electric Motors
Regenerative Braking
A shunt motor is instantly adaptable as a shunt generator. Most generators are in fact shunt wound, or variations on this theme. The linear or nearly linear torque and speed versus current characteristics of the shunt motor manifest as nearly linear voltage versus current characteristics when used as a generator. This also translates to a high degree of stability that makes a shunt motor both useful for and adaptable to regenerative braking applications—either manually or electronically controlled.
Compound DC Motors A compound DC motor is a combination of the series and shunt DC motors. The way its windings are connected, and whether they are connected to boost (assist) or buck (oppose) one another in action, determine its type. Its basic characterization comes from whether current flowing into the motor first encounters a series field coil-short-shunt compound motor or a parallel shunt field coil-long-shunt compound motor as shown in Figure 6-3. If, in either one of these configurations, the coil windings are hooked up to oppose one another in action, you have a differential compound motor. If the coil windings are hooked up to assist one another in action, you have a cumulative compound motor. The beauty of the compound motor is its ability to bring the best of both the series and the shunt DC motors to the user. • Torque—The torque in a compound motor has to reflect the actions of both the series and the shunt field coils. Depending on whether you are hooked up in the differential or cumulative position, the shunt torque f and the series torque f either subtract to a difference figure or add together. The effect of these hookup arrangements on torque is illustrated in Figure 6-3, where the differential compound motor builds more slowly to a lower torque value than the shunt curve and the cumulative compound motor builds to a slightly higher torque value than the shunt curve at a slightly higher rate. • Speed—Similar to torque, the speed action in a compound motor will also reflect the dual variables of series and shunt field coil action. Figure 6-3 shows the speed curves. One of the initial benefits of the compound configuration is that runaway conditions at low field current levels for the shunt motor and at lightly loaded levels for the series motor can be eliminated. While the differential compound configuration is of questionable value—its curve shows a tendency to runaway speeds at high armature current values—the cumulative compound motor appears to offer benefits to EV operation. You can tailor a cumulative compound motor to your EV needs by picking one whose series winding delivers good starting torque and whose shunt winding delivers lower current draw and regenerative braking capabilities once up to speed. When you look, you might find these characteristics already exist in an off-the-shelf model. • Reversing—A short-shunt compound motor resembles a series motor, so reversing its applied voltage polarity normally does not reverse the motor direction. A long-shunt compound motor resembles a shunt motor, so reversing the voltage supply leads normally reverses the motor. As with speed and torque, compound motors can be tailored to do whatever you want to do in the reversing department.
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