SPEED CONTROL OF BLDC MOTOR
Speed control of BLDC motor is essential for making the motor work at desired rate. Speed of a brushless dc motor can be controlled by controlling the input dc voltage / current. The higher the voltage more is the speed.
Many different control algorithms have been used to provide control of BLDC motors. The motor voltage is controlled using a power transistor operating as a linear voltage regulator. This is not practical when driving higher-power motors. High-power motors must use PWM control and require a microcontroller to provide starting and control functions.
The control algorithm must provide three things:
- PWM voltage to control the motor speed
- Mechanism to commutate the motor
- Method to estimate the rotor position using the back-EMF or Hall Sensors
Pulse-width modulation is used to apply a variable voltage to the motor windings. The effective voltage is proportional to the PWM duty cycle. When properly commutated, the torque-speed characteristics of the BLDC motor are identical to a dc motor. The variable voltage can be used to control the speed of the motor and the available torque.
The commutation of the power transistors energizes the appropriate windings in the stator to provide optimum torque generation depending on the rotor position. In a BLDC motor, the MCU must know the position of the rotor and commutate at the appropriate time.