Brushless DC Motor (BLDC Motor)

BRUSHLESS DC MOTOR (BLDC Motor)

The term BLDC motor is applied to a specific class of AC synchronous motors in which semiconductor control is used to control stator currents so that a rotating torque is available to rotate the rotor without the use of brush and commutator. Solid state switching devices like thyristors, IGBT or transistors triggered by rotor position sensors can replace the brush and commutator arrangement as in conventional DC motors by fully electronic processes.

Construction

The stator of the BLDC motor is similar to that of a conventional reduction motor. The stator has a three‒phase winding which is star connected. The stator core is made up of silicon steel stompings which are stacked together and fixed on the stator frame. The stator windings are supplied through a variable voltage current source inverter (CSI) insure a sequence that they produce a rotating magnetic field around the airgap. The rotor in a BLDC motor is of salient pole permanent magnet type.

Position sensors paced on the rotor shaft provide signals to the control and gate drive circuit which cause the SCRS in the inverted circuit to be triggered in proper sequence so as to be in synchronism with the rotor's mechanical position. The stator and rotor fields thus get locked to each other and remain in synchronism at any and every rotor speed. The schematic diagram of a BLDC motor along with its drive system is shown in Fig. 4.26

Commonly used rotor position sensors are Hall‒effect sensors and optical sensor consisting of LEDs and photo transistors.

Operation

When supply is switched on, current flows through the armature winding whose distribution depends on the rotor position and switching ON the devices at various instants. Due to the interaction between current and magnetic field, rotor experiences torque, and if this torque is greater than the load torque, rotor starts moving, BLDC motor is thus a self‒starting motor. As the rotor moves, a relative velocity exists between the rotor and the stator. This results in the emf induction in armature winding whose direction is opposite to the applied voltage. Due to this back emf, a momentary change in torque and speed will occur. The armature draws more current and increases the torque and hence the speed returns to the some value. These changes are very fast and the performance is not affects.