Page 84 - Build Your Own Combat Robot

Chapter 4:
Motor Selection and Performance
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The output power is always less than the input power. The difference between
the two is the amount of heat that will be generated due to electrical and frictional
losses. It is best to design and operate your robot in the highest efficiency range to
minimize the motor heating. If the motor is able to handle the heat build-up, it
might be best to design the robot (or weapon) to be operated at a higher percent-
age of the motor’s maximum power (to keep the motor as light as possible). For
example, a motor that is used to recharge a spring-type weapon might be fine if
operated at near-stall load for just a few seconds at a time. The maximum amount
of heat is generated when the motor is stalled. A motor can tolerate this kind of
heat for short periods of time only, and it will become permanently damaged if it’s
stalled for too long a period of time. This heat is generated in the armature wind-
ings and the brushes, components that are hard to cool by conduction.
Figure 4-1 shows a typical motor performance chart. These charts are usually
obtained from the motor manufacturer, or a similar chart can be created if you
know the motor constants. The motor shown in Figure 4-1 is an 18-volt Johnson
Electric motor model HC785LP-C07/8, which can be found in some cordless
drills. The constants for this motor are shown in Table 4-1. This motor is dis-
cussed here as an example motor to describe how all of the motor constants relate
to each other and how they affect the motor performance.
Figure 4-1 graphically displays how the motor speed decreases as the motor
torque increases and how the motor current increases as the applied torque on the
motor increases. For this particular motor, maximum efficiency is approximately

FIGURE 4-1
Typical motor
performance
curves.