BLDC MOTOR

A Dc Motor is a device that converts Electrical energy into mechanical energy.

It works on the principle that when a current carrying conductor is placed in a magnetic field it experiences a force that causes it to move. In DC motor, the current is passed through the armature windings which are wound on the rotor. And the magnetic field is produced by the electro magnetized field windings placed on the stator. Thus the total force of each conductor on the armature winding causes the rotor to rotate. The steady rotating force is called as torque. As the rotor rotates, the armature winding moves through the plane perpendicular to the magnetic field resulting in the reversal of torque. This makes the rotor to reciprocate from one direction to the other, but to ensure that the torque remains unidirectional, the current in the armature windings is to be reversed every time the winding moves through the plane perpendicular to the magnetic field. 

A device called as Commutator is employed to keep the torque unidirectional. A Commutator is a split ring device that causes the current to reverse every time the winding moves through the plane perpendicular to the magnetic field. The current from the external circuit is passed to the split rings through brushes. The brushes are to be placed in a proper position, failing which the current in the armature windings will not reverse properly and may cause sparking at the brush contatcts. This causes more wear and tear on the brushes and reduces the operating life. The operating  conditions the noise levels are high, restricting its usage where silent operation is preferred. Also the brushes need regular maintenance, replaced if necessary so as to avoid the deposit of dust and moisture which may cause a conducting path resulting in the short circuit of the commutator and damaging the motor ultimately. The tendency of the sparks at the brush contacts restricts its usage in inflammable environment. The figure below shows a Motor with commutator and brushes depicted in it:

Where,

A- Commutator

B- Brush

C- Armature Windings (Rotor)

D- Field Windings (Stator)

E-Brush Guides.

In order to overcome these problems associated with the brushes, Brush Less DC (BLDC) Motors are developed. As the name itself states, these are the motors without brushes and commutator. Here the commutation action is carried out by Electronic devices like multiple feedback sensors. BLDC motors often incorporate internal or external position sensors to sense the actual position of the rotor. The most commonly used sensors are hall sensors that work on the principle of Hall Effect. Depending on the sensor output, the polarity reversal is performed. Power transistors are used for polarity reversal. The power transistors are switched in synchronization with the rotor position and cause the polarity reversal. The figure below shows the BLDC motor along with the Hall Sensors: 

BLDC motors offer high efficiency, a maintenance free and safe operation with reduced noise levels. They also provide high power density and reliability and hence employed in computers, industrial automation, aerospace, military, traction motors for Electric Vehicles and household products. The main disadvantage of BLDC Motors arises due to the situation that may arise due to the failure of the sensor. In order to overcome this disadvantage, sensor less BLDC motors are being developed. 

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Minnie Florence V

Asst.Professor

EEE Department 

Sphoorthy Engineering College

Sphoorthy Engineering College