Page 89 - Build Your Own Combat Robot

Build Your Own Combat Robot
70
FIGURE 4-3
Motor speed and
torque changes
by doubling the
input voltage.

and the maximum stall torque is doubled when the input voltage is doubled. Re-
call from equation 4 that the motor’s speed is proportional to the applied voltage.
In Figure 4-3, you will notice that the current draw line from the 18-volt and
36-volt cases are on top of one another. Remember that the current draw is only a
function of the applied torque on the motors, and it is not related to the voltage. So
for a fixed torque on the motor, the current draw will be the same regardless of the
speed of the motor.
Figure 4-4 shows how the output power from the motor is affected by doubling
the applied voltage. You can see that increasing the voltage can significantly in-
crease the output power of the motor. The maximum power at 36 volts is approxi-
mately four times greater than the maximum power at 18 volts. The maximum
power of this 18-volt motor is 448 watts, or 0.6 horsepower. By doubling the voltage,
this motor has become a 2.5-horsepower brute! Not only does the power increase,
so does the motor’s efficiency. The maximum efficiency of the motor at 18 volts is
74.5 percent, and at 36 volts the maximum efficiency is 81.6 percent—a 7 percent
increase in efficiency just by doubling the voltage!
A big factor in choosing a motor is the conditions under which it will operate.
Will the motor run continuously, or will it have a short duty cycle? A motor can be
pushed much harder if it is used for a short time and then allowed to cool. In fact,
heat is probably the biggest enemy of the PMDC motor.
By doubling the motor’s voltage, you can double the top speed of the robot, and
you can even double the stall torque of the motor. But be forewarned: These im-
provements do not come without a cost. Figure 4-5 shows the heat generated in
the motors as the applied torque increases. Doubling the voltage, and therefore