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Brushless Electric Motors
How does a brushless
electric motor work?
BY MARSHALL BRAIN
In a typical DC
motor, there are permanent magnets on
the outside and a spinning armature on
the inside.
The
permanent magnets are stationary, so they are called the stator. The armature rotates, so it is
called the rotor.
The
armature contains an electromagnet.
When
you run electricity into this electromagnet, it creates a magnetic field in the
armature that attracts and repels the magnets in the stator.
So
the armature spins through 180 degrees. To keep it spinning, you have to change
the poles of the electromagnet.
The
brushes handle this change in polarity. They make contact with two spinning
electrodes attached to the armature and flip the magnetic polarity of the
electromagnet as it spins.
This
setup works and is simple and cheap to manufacture, but it has a lot of
problems:
· The brushes eventually wear out.
· Because the brushes are making/breaking
connections, you get sparking and electrical noise.
· The brushes limit the maximum speed of the motor.
· Having the electromagnet in the center of the
motor makes it harder to cool.
· The use of brushes puts a limit on how many
poles the armature can have.
With
the advent of cheap computers and power transistors, it became possible to
"turn the motor inside out" and eliminate the brushes.
In
a brushless DC motor (BLDC),
you put the permanent magnets on the rotor and you move the electromagnets to
the stator.
Then
you use a computer (connected to high-power transistors) to charge up the
electromagnets as the shaft turns.
This
system has all sorts of advantages:
· Because a computer controls the motor instead of
mechanical brushes, it's more precise. The computer can also factor the speed
of the motor into the equation. This makes brushless motors more efficient.
· There is no sparking and much less electrical noise.
· There are no brushes to wear out.
· With the electromagnets on the stator, they are very
easy to cool.
· You can have a lot of electromagnets on the stator for
more precise control.
The only disadvantage of a brushless motor is its higher initial
cost, but you can often recover that cost through the greater efficiency over
the life of the motor.
Marshall Brain, Founder
Marshall Brain is the founder of HowStuffWorks. He holds a bachelor's degree in electrical engineering from Rensselaer Polytechnic Institute and a master's degree in computer science from North Carolina State University. Before founding HowStuffWorks, Marshall taught in the computer science department at NCSU and ran a software training and consulting company. Learn more at his site.
Marshall Brain is the founder of HowStuffWorks. He holds a bachelor's degree in electrical engineering from Rensselaer Polytechnic Institute and a master's degree in computer science from North Carolina State University. Before founding HowStuffWorks, Marshall taught in the computer science department at NCSU and ran a software training and consulting company. Learn more at his site.
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