Introduction to BLDC motor. Construction and Working.

BY    AFZAL YOUSAF 

Brushless DC Motor (BLDC) is mostly used in Electric vehicles like E-bikes, E-rickshaws, small electric loading vehicles etc and is more popular among EV enthusiasts and designers. Popularity of BLDC motor is because of its exciting properties that makes it suitable and significant contributor of modern drive technology.

In this article you can get an overview of BLDC Motor, construction, electronic position sensors , BLDC working and Advantages.

Rapid gain in popularity has seen in BLDC motor due to its properties like High efficiency, Better speed vs torque characteristics, less weight, low cost, simple construction, reliable and easily availability. All these properties are due to modern power electronics and advancement in Microprocessor technology that helped to thrive BLDC Motor.

BLDC Motor as name suggests they are brushless and communicated via electronically. In conventional DC Motors, brushes are used to transmit the power to the rotor whey they get excited by the magnetic field. BLDC motor replaces this mechanically torned brushes by electronic communication (Usually Hall sensors) and thus more efficient.

BLDC Motor with Hall Sensor

Most familiar type of BLDC Motor with Hall sensor for electronic communication is shown in above picture. Wheel hub BLDC Motor is also popular and widely used in electric two wheelers. Electronic communication process in BLDC is shown below.

Construction 

Construction of BLDC Motor is very simple and main difference between brushed and brushless DC Motor is the replacement of mechanical commutator with an electric switch circuit. BLDC Motor is a synchronous motor in which magnetic fields are generated in the stator and rotor revolve at the same frequency. BLDC is available in three configurations like Single phase, Two phase and Three phase but Three phase is the most common one.

BLDC Cross-section

Important parts in their construction are:-

• Stator
• Rotor
• Position or Hall sensors.

Stator is usually made of stacked steel laminations with axially cut slots for winding. Even though structure of BLDC  stator is similar to that of Induction motor, the winding is slightly different. Most BLDC Motor consist of three stator winding usually connection in star type without neutral. And based on interconnections, stator windings are further divided into Trapezoidal and Sinusoidal based on the shape of Drive current and Back emf.

Stator

Rotor is the part made of permanent magnets. Number of poles in the magnet can vary from 2 to 8 based on the applications. North and south  poles are placed alternatively and arrangement of poles are in three different manner.

• Magnets placed on outer periphery of rotor.
• Magnets embedded on rotor.
• Magnets inserted in the core of the rotor.

Simple Rotor

Position or Hall sensor is used for the electronic communication in BLDC. Position or Hall sensor works on Hall effect principle. For the motor to rotate, stator windings must be energised in a sequence and position of rotor must be known to precisely energize the stator winding. Hall sensor detects the position of rotor and converts it into an electric signal. 

Output of hall sensor will be HIGH or LOW depending on whether North or South pole of rotor passes near it. Usually BLDC uses three hall sensors in the stator to sense rotor position and combines the output of all to find exact sequence of energizing.

Working 

Consider the stator with windings A, B and C and let rotor be a single magnet for easy understanding. We know when a current is applied through a coil, a magnetic field is generated and the orientation of the field lines i.e. the poles of the generated magnet will depend on the direction of the current flowing through the coil.

So if we apply current to current to coil A, it will generate a magnetic field and attract the rotor magnet. The position of the rotor magnet will shift slightly clockwise and will align with A. If we now pass current through coils B and C one after the other (in that order), the rotor magnet will rotate in clock wise direction. This is how motor rotates. And when we reverse the direction of current, motor rotating direction also changes.

To increase efficiency, we can wind the opposite coils using a single coil so that we get double attraction. Further increasing the efficiency, we can energize two coils at the same time so that one coil will attract the magnet and the other coil will repel it. During this time, the third will be idle. Mostly 6 possible combination of coils are available in 360 degree rotation of motor. 

Advantages of BLDC

• No wear and tear as electronically communicated.
• High efficiency.
• Better Speed vs Torque characteristics.
• Longer life.
• Low cost and Easily available.
• Less weight.
• Noiseless operation.
• Significantly Higher RPM.

Comment your thoughts and queries. Keep Reading.