CHAPTER 17 ■ DC MOTORSLimitations of Brushless Motors
Because of the voltage requirements of the chips and components on a brushless motor, it accepts a narrow
range (around ±15%) of voltages for motor operation. On the other hand, the brush motor consists of wires
and magnets that are usually perfectly content to accept the wide power ranges available from ordinary
consumer batteries.
Aside from the more rigorous voltage requirements, other downsides of robot movement by brushless
motors are that brushless motors tend to be less available, more expensive, and provide lower pushing
power (torque). For these reasons, the robots in this book use brush motors.
Looking Inside a Coreless Permanent-Magnet DC Brush Motor
The first motor type examined in this chapter was a brush motor containing an iron core. A coreless
variation (also called ironless) exists that is very similar in all other aspects. Note that this is a brush motor,
not brushless.
A coreless permanent-magnet DC brush motor (see Figure 17-10) consists of roughly the same sections,
materials, and parts as the classic iron-core motor. The permanent magnets are still attached to the stator,
although they’re mounted towards the center rather than near the outside walls.
The armature shows the biggest change. Instead of individual shoes and windings, the armature
consists of overlapping windings. There is an empty space between the shaft and the windings so that the
permanent magnets on the stator can slide in between.
Comparing Coreless vs. Iron Core
Without the heavy iron core, the coreless rotor is much more nimble. Motor acceleration and deceleration is
improved. Without the shoes, the rotation is very smooth: no cogging.
Then again, without the massive iron core, the motor can’t dissipate heat as well. A high enough
temperature destroys the motor by melting the plastic that holds the windings in shape. (Overheating should
never occur during normal operation, as long as the robot provides adequate ventilation and power usage
within manufacturer’s specifications.)
The shape of the windings in a coreless motor provides greater efficiency. In an iron-core motor, the
ends of the loops are not in the proper orientation to provide magnetic forces that contribute to the rotation.
The thin edges of the coreless windings waste very little mass or resistance in non-productive directions.
Figure 17-10. Guts of a coreless DC brush motor: (left to right) stator with permanent magnets mounted near
the center; rotor with shaft, armature, windings, and commutator; and cap with brushes