In order to make the operation more reliable,
more efficient and less noisy,
the recent trend has been to use brushless dc motors
They are also lighter compared to brushed motors
with the same power output
The brushes in conventional dc motors wear out over time
and may cause sparking
Thus the brushed dc motor
should never be used for operations that demand long life and reliability.
Let’s see how a brushless DC motor works.
The rotor of a BLDC motor
is a permanent magnet.
The stator has a coil arrangement as shown.
By applying DC power to the coil,
the coil energize and become an electromagnet.
The operation of a BLDC is based
on the simple force interaction between the
permanent magnet and the electromagnet.
In this condition
when the coil A is energized,
the opposite poles on the rotor and stator are attracted to each other.
As the rotor nears coil A, coil B is energized.
As the rotor near coil B, coil C is energized.
After that coil A energized with the opposite polarity.
This process is repeated and the rotor continues to rotate.
A humorous analogy to help remember it is
to think about BLDC operation like the story
of the donkey and the Carrot.
Where the donkey tries hard to
reach the carrot but the Carrot keeps moving out of reach.
Even though this motor works it has one drawback.
You can notice that at any instant
only one coil is energized.
The two dead coils greatly reduce
the power output of the motor
here is the trick to overcome this problem.
When the rotor is in this position
along with the first coil which pulls the rotor
you can energize the coil behind it
in such a way that
it will push the rotor.
For this instant
a same polarity current ispassed through the second coil
the combined effect produces more torque
and power output from the motor
the combined force also makes sure
that a BLDC has a beautiful constant torque nature
with this configuration two coils need to be energized separately
but by making a small modification to the stator coil
we can simplify this process
just connect one free end of the coils together
when the power is applied between coils A and B
let’s note the current flow through the coil
it’s just like the separately energized state
that’s how a BLDC works but you
might have some intriguing doubts in your mind
how do I know which stator coils to energize
how do I know when energizer
so that I will get a continuous rotation from the rotor
in a BLDC
we use an electronic controller for this purpose.
A sensor determines the position of the rotor
and based on this information
the controller decides which coils to energize
most often a Hall effect sensor is used for this purpose.
the BLDC design we have discussed so far
is known as the outrunner type.
Inrunner BLDC design is also available in the market.
We hope you had a nice introduction on the working of BLDC motors.