Page 88 - Build Your Own Combat Robot

Chapter 4:
Motor Selection and Performance
which will increase the power requirements. Do you have an efficient drive train, 69
or are you using power-robbing worm gears? How fast do you want to go?
An internal combustion engine produces its peak horsepower at about 90 per-
cent of its maximum RPM, and peak torque is produced at about 50 percent of
maximum RPM. The higher the RPM, the more energy it consumes. Compare this
to the PMDC motor, which consumes the most energy and develops its peak
torque at zero RPM. It consumes little energy at maximum RPM, and it produces
its peak horsepower at 50 percent of its unloaded speed.
At 50 percent of maximum speed, the PMDC motor will draw half of its maxi-
mum stalled current, as seen earlier in Figure 4-1. Unfortunately, much of the cur-
rent going into the motor at this high power level is turned into heat. Figure 4-2
shows how much heat is generated in the example motor used to create the statis-
tics in Figure 4-1.
It is obvious to see that the minimum amount of heating occurs when running
the motor near its maximum speed and efficiency. It can also be seen in Figure 4-2
that as the motor torque increases, a near exponential increase in motor heat re-
sults. Motors can tolerate this amount of heat only for short periods of time. Con-
tinuously running a motor above the maximum power output level will seriously
damage or destroy it, depending on how conservatively the manufacturer rated
the motor.
Many motors are rated to operate continuously at a certain voltage. You can
increase the power of your motor by increasing the voltage. Figure 4-3 shows how
a motor’s speed, torque, and current draw are affected by increasing the input
voltage to the motor. In Figure 4-3, you can see that the motor speed is doubled

FIGURE 4-2
Heat generated in
an electric motor.