Motor Speed
A brushless DC motor consists of a rotor in form of a permanent magnet and stator in form of polyphase armature windings. It differs from conventional dc
motor in such that it doesn’t contains brushes and the commutation is done using electrically, using a electronic drive to feed the stator windings.
Basically a BLDC motor can be constructed in two ways- by placing the rotor outside the core and the windings in the core and another by placing the windings outside the core.
In the former arrangement, the rotor magnets act as a insulator and reduce the rate of heat dissipation from the motor and operates at low current. It is typically used in fans. In the latter arrangement, the motor dissipates more heat, thus causing an increase in its torque. It is used in hard disk drives.
Motor Speed Control
In a 4 pole, 2 phase brushless dc
motor, a single hall sensor is used, which is embedded on the stator. As the rotor rotates, the hall sensor senses the position and develops a high or low signal, depending on the pole of the magnet (North or South). The hall sensor is connected via a resistor to the transistors. When a high voltage signal occurs at the output of the sensor, the transistor connected to coil A starts conducting, providing the path for the current to flow and thus energizing coil A.
The capacitor starts charging to the full supply voltage. When the hall sensor detects a change in polarity of the rotor, it develops a low voltage signal at its output and since the transistor 1 doesn’t get any supply, it is in cutoff condition. The voltage developed around the capacitor is Vcc, which is the supply voltage to the 2nd transistor and coil B is now energized, as current passes through it.