CHAPTER 17 ■ DC MOTORSEfficiency Characteristic of DC Motors
Regardless of how efficiently a robot utilizes a motor, the motor itself has an efficiency associated with its
ability to convert electrical energy into mechanical energy. Well-designed motors can be as much as 90%
efficient. That means almost all of the electricity is turned into useful motion, with only 10% wasted.
Wasted electricity can take many forms, such as noise, vibration, sparks (light, sound, heat, physical
damage), and magnetic fields. However, the motor expends most wasted electricity as heat. Therefore, a
more efficient motor not only permits the robot’s batteries to run longer, but also permits the motor itself to
run cooler.
Because motors usually guzzle the largest portion of the robot’s power supply, more efficient motors
may be worth the extra cost. This extends the robot’s run time before needing a recharge.
Toy motors hover at around 30% efficiency or less. Precision motors, like escap or Maxon, are around
75% efficient or better. The motor’s datasheet provides a specific number along with a curve representing the
efficiency under various speeds and loads. Motors are usually most efficient when carrying a load about^1 / 7
their maximum torque.
Audible Noise Characteristic of DC Motors
A squeaky motor can be annoying. Perhaps it just needs some lubrication. Maybe the motor has a defect. Or,
perhaps the motor is wearing out.
In any case, the unpleasant grinding or squealing of motors can be a detractor from your robot. Before
ordering a large quantity of a particular motor, it is worth sampling a couple to make sure they don’t have
any unadvertised negatives such as noise.
Oiling the Rotor Shaft
Motors that squeal can often be quieted (with a bonus of increased performance) by applying a tiny dab of
light oil to the places where the shaft rests on the motor body. This is normally at the front and back of the
motor body, just where the shaft sticks out.
Don’t apply oil or other lubrication if the motor is working correctly, as the existing lubrication may be a
special formulation or it may be incompatible with your choice of lubrication.
Electrical Noise Characteristic of DC Motors
Electrical noise is not the same as audible squealing and squeaking. Electrical noise appears as rapid, spiked
fluctuations in voltage in attached circuitry or nearby components. The most common human experience of
electrical noise is static on a television when someone is vacuuming.
Robots can have big problems with electrical noise. Noise can scramble information as it is being
communicated throughout a circuit board. The chips can reset as though power had been turned off and on.
The affected board doesn’t even need to be in close proximity to a noisy motor. Electronics and motors
sharing the same power source (battery) can transmit the electrical noise throughout all attached circuit
boards.
More efficient motors tend to produce less electrical noise. (Likewise with motors that have built-in
capacitors). Figure 17-22 shows lots of spikes generated by the brushes connecting and disconnecting from a
toy motor’s rotor commutator segments.