Build Your Own Combat Robot

Chapter 4: Motor Selection and Performance 73
Another method that can be used to help control the heat buildup in the motors is
to use an electronic speed controller (ESC). The ESC is a device that meters the flow
of current to your motor. It does this by rapidly switching the current on and off,
several hundred to several thousand times per second. One way in which controllers
from different companies differ is in the frequency at which they chop the current
to the motor. The motor takes a time average of the amount of time the current is
on versus the time between each cycle. As a result, the motor will see a lower “average”
current and voltage than it would if it were on continuously. Hence, the
motors will see less heating.
As stated before, nothing happens for free in the world of physics. Electronic
controllers get hot and require heat sinking. They also can generate radio frequency
interference, which might cause problems in a radio-controlled robot.
Chapter 7 will provide a more detailed discussion on electronic speed controllers.
High-Performance Motors
If you are still not satisfied with the performance of your motor (and money is no
object), you might want to purchase a high-performance motor. High-performance
motors have one major difference (and several minor ones) from regular
motors—in a word, efficiency. We have been discussing motors with 50- to
75-percent efficiencies. That is the range for fair to very good ferrite magnet motors.
When we step up to rare-earth magnets, we get into a whole new realm of
performance. The efficiency figures for small rare-earth magnet motors range
from about 80 to 90 percent.
Rare-earth magnets are made from either cobalt or neodymium alloys. The
magnetic fields are so powerful that they are actually dangerous to handle. A moment’s
inattention may result in a nasty crush as your finger is caught between
them and a stray piece of metal. The added bonus with cobalt alloy magnets is that
they are resistant to demagnetization, no matter how much voltage you pump into
it or how hot it gets. Motors with rare-earth magnets run much cooler than ferrite
motors. While running under ideal operating conditions, a ferrite motor turns
about 33 percent of the power it consumes into heat, whereas the rare-earth motor
wastes only about 10 to 20 percent of the electricity you feed it.
Another class of high-performance motor is the brushless PMDC motor.
The brushes in an ordinary motor can be the source of several problems: they
spark and cause radio interference, they are a source of friction, and they wear
out. The brushless motors have sensors that detect the position of the rotor relative
to the windings. This information is sent thousands of times a second to a
special controller that energizes the windings at the optimum moment on each
revolution of the motor. In a brushless motor, the windings are stationary and
the magnets spin—exactly the opposite of a conventional motor. This configuration
is capable of much higher speeds. You can get motors that spin at
50,000 RPM or more. The major drawback to the high-performance motors is
that they are significantly more expensive then regular motors.