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We encounter DC motors in all sorts of technology today
(computer fans, electric toothbrushes, toys, powered wheelchairs,
hybrid/electric cars, elevators, cranes, belts, and mixers just to name a few).
Before the development of the AC inverter, these devices were the best answer
for applications requiring variable speed operation while fully loaded.
And while we could rage about the epic battle between AC and
DC motors, I'd prefer to just let them duke it out on their own… (Image Sources: Grainger, Inc. |
cognitivedistortion.com)
That said, even though the AC (induction) motor has stolen
some of the DC motor's individuality, it still boasts a smaller footprint and simpler
speed control. And whether you're a frequent consumer or a new hobbyist, you
probably have found there is a lot worth considering when sourcing one of your
own.
Considerations
First off, you have a large list of types based on
construction:
- Shunt wound motors
are used for precise control of speed and torque.
- Series wound
motors exhibit high starting torques for moving heavy loads.
- Compound wound
motors are for constant speed and high starting torque.
- Permanent magnet
motors are more efficient than wound types but have less speed regulation.
- Disc armature
motors are best for quick startup and shutdown under constant load.
- Coreless/slotless
motors boast high acceleration, speed control, and little vibration, best used
as servomotors.
Then comes the tricky part: speccing (if I may use that
term) the motor. The most important specifications for understanding a DC
motor's performance are speed and torque.
- Shaft speed is
the speed of the motor's rotating shaft. Specs generally include the no-load
speed, which is the maximum speed the motor can reach when no torque (load) is
applied.
- Output torque is
the load the motor can generate or handle. Specs generally include the
stall-torque (fully loaded at zero speed) and continuous torque (maximum torque
at normal running conditions).
Of course, these can be explained further by analyzing the
torque/speed graph of a motor.
The graph above (Credit:
Baldor Electric Company) is a torque/speed trend graph for a shunt wound DC
motor. It indicates that the motor will provide consistent torque over a wide
range of speeds.
In addition, we can look at motor commutation. Brushed DC
motors, which use contact brushes, are less costly to construct and provide
simpler control mechanisms, but require frequent maintenance to replace
brushes. Brushless motors, which use Hall Effect sensors, require no
maintenance and are more efficient, but require more expensive controllers to
operate.
Much More to Know
This only scratches the surface of the theory behind DC
motor selection. From here, we can look at voltage, output power, power dissipation,
sizing, and a number of application-specific features. But I'll leave that
elaboration to GlobalSpec's How
to Select DC Motors page.
What sorts of projects have you used DC motors for? Have
your sourcing quests provided you with any words of wisdom to those new to the
field?
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