Acts as feedback
Sensor – Senses position of rotor
It converts the information of rotor position
into a suitable electrical signal.
This signal is used to switch ON and OFF of
various semiconductor devices of electronic
switching circuitry of BLPM motor
Rotor position can be determined by a Hall
Effect device (or devices), embedded in the
stator, which provide an electrical signal
representing the magnetic field strength. The
amplitude of this signal changes as the magnetic
rotor poles pass over the sensor.
Other sensing methods are shaft encoders and
also sensing the zero crossing points of currents
generated in the unenergised phase windings.
The latter method is known as "sensorless"
Optical position sensor
Hall effect position sensor
Electronic position sensors
Mainly used in Unipolar BLDC machines
Rotor consists of optical sensors.
The optical sensor consists of
◦ A light source
◦ three phototransistors P1, P2 and P3 mounted on
the end plate of the motor, separated by 120o
from each other
◦ A revolving shutter coupled to the shaft of the
When the shutter revolves, the
phototransistors get exposed to the light in
the sequence of their numbers. In each
revolution, the phototransistors generate the
pulses PI1, PI1 and PI1which have duration and
phase displacement of 120o.
When light falls on the phototransistors P1, it generates a pulse
and transistor Q1 gets turned on.
Current starts flowing through stator winding Ph1. This produces
north pole at pole face of Ph1.
South pole gets attracted towards it and reaches the axis of pole
face of Ph1. Hence rotor revolves in anticlockwise direction.
During the mean time, the light stops falling on P1 and starts
falling on P2. Hence pulse PI1 is generated which turns on the
Current now starts flowing through the winding Ph2, producing a
north pole. Hence rotor further rotates in anticlockwise direction
so that rotor reaches the axis of the pole face of .
In the meantime, starts falling on P3. This causes transistors Q3
to turn on which produces north pole at the pole face of Ph3.
This rotates the rotor further in anticlockwise direction.
Switching sequence repeats and continuous rotation of the rotor
The Hall Effect uses three hall sensors within the
Brushless DC Motor to help detect the position of
The magnetic field changes in response to the
transducer that varies its output voltage.
Feedback is created by directly returning a
voltage, because the sensor operates as an
The distance between the Hall plate and a known
magnetic field can be determined with a group of
sensors, and the relative position of the magnet
can be deduced.
In a brushless DC motor (BLDC), the
rotor has permanent magnets and the
stator has an electronically-controlled
rotating field, using sensors (rotary
encoders or back-EMF) to detect rotor
position. As such they have no
commutator, and tend to be more
efficient and more powerful than
commutated motors. They do require a
more complicated motor controller.
High resolution sensing of absolute rotational
position and speed without shaft encoders or
High reliability - eliminates mechanical and
Sensing circuits are integrated with motor
drivers, automatically driving commutation and
providing precise speed and position control
Uses the magnetic characteristics of the motor to
sense rotor position, eliminating the need for
mechanical adjustments or alignments
Works at any speed, even while motor is
Eases packaging - no requirement to place
components near motor
No injection of extraneous sense signals into
Works with standard brushless motors
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