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In brushless dc motor modern electrical commutation is done, unlike conventional dc motor where it is employed with classic mechanical commutation. The construction of brushless dc motor is very much likely to permanent magnet synchronous motor. Here, the rotor consist of permanent magnet, whereas the polyphase winding is placed on the stator. Combination of drive, its electronic drive and rotor position sensor, the word " brushless dc motor is used ". The electronic drive circuit in the brushless DC motor is an inverter consists of the transistor, and it feeds stator winding, where the transistors are controlled by pulses generated by rotor position sensors ensuring that the rotor revolves at an angular speed. This angular speed of the rotor is nearly equal to the average speed of the field produced by the stator. The drive circuit is fed by DC supply like other conventional DC motor where the fields of stator and rotor remain stationary with respect to each other at all the speeds. The speed to the brushless DC motor is controlled by the DC supply voltage. Due to all these similarity with conventional DC motor, this type of motor is known of brushless DC motor.
In unipolar brushless dc motor, optical sensors are embedded on the rotor to excite these sensors. Three phototransistors P1, P2, P3 are mounted on the end plate of the motor and seperated by an angle of 120 o. The armature of a brushless dc motor consist of a three phase winding ( polyphase ) mounted on the stator, and the field consists of a two-pole permanent magnet. The driving circuit consists of three transistors Q1, Q2, Q3, which are used to excite the stator winding.
When phototransistors get exposed to light due to the revolution of the shutter. The phototransistors generate the pulse PI1, PI2, PI3 for each revolution, . Each transistor will conduction for 120 o. The phototransistor ( P1 ) generate a pulse when it receives light, and it generates pulse PI1. Hence the transistor ( Q1 ) gets turned ON. Therefore, current flows through the phase winding Ph1 producing the north pole face of Ph1. Suddenly, the rotor revolves in anticlockwise direction and the light stop falling on P1 due to the revolution of the rotor. Now, the phototransistor ( P2 ) generate a pulse when it receives light, and it generates pulse PI2. Hence the transistor ( Q2 ) gets turned ON. Therefore, current flows through the phase winding Ph2 producing the north pole face of Ph2. Rotor further rotates in the anticlockwise direction and it reaches the axis of the pole face Ph2 and light stops falling on P2. Now, the phototransistor ( P3 ) generate a pulse when it receives light, and it generates pulse PI3. Hence the transistor ( Q3 ) gets turned ON. Therefore, current flows through the phase winding Ph3 producing the north pole face of Ph3. Suddenly, the rotor revolves in anticlockwise direction and the light stop falling on P3 due to the revolution of the rotor. This switching sequence is repeated. As a result, the continuous rotation of rotor is obtained.
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