Category
BLDC Motor Supplier
info@bldcmotor.org
Home » BLDC Motor Wiki » 6-Step Commutation of Sensorless BLDC Motor
6-Step Commutation of Sensorless BLDC Motor
Every time the BLDC motor swifts its direction, there is a winding connected to the positive electrode of the power source (the current enters the winding); the second winding is connected with the negative electrode (the current flows out therefrom); and the third winding is at a power-losing state. The torque is generated between the magnetic field and the permanent magnet. Under the ideal state, the torque peak appears at the orthogonality of two magnetic fields, and the torque is the weakest when the two magnetic fields are the weakest. In order to maintain rotation of the motor, the magnetic field generated by the stator's winding should keep on changing the position, because the rotor will rotate in the direction parallel to the stator's magnetic field. The "six-step commutation" defines the sequence of the winding electrification.
Comutation sequence: To turn by every 60°electric angle, one of the Hall sensors will change its state. Therefore, it takes six steps to finish the electricity cycle. Under the simultaneous model, the phase current is switched once when rotating by every 60°of electricity angle. However, one electric cycle might not be corresponding to a complete rotor's mechanical rotation cycle. The number of electric cycles to be repeated by accomplishing one cycle of mechanical rotation is decided by the number of the rotor magnetic poles. Generally, to a six-step inversion method, the n magnetic poles divide the electric motor's rotating angle into 360/ (6*n magnetic poles)°. Every pair of rotor magnetic poles calls for accomplishment of one electric cycle. Therefore, the number of electric cycles divided by the number of rotations is equal to the number of rotor magnetic poles. Take the Hall sensor signals with 60° of phase shift between the magnetic poles for example. The phase shift between Hall sensors can be 60°or 120°. In choosing the controller for the specific BLDC motor, we should adhere to the sequence defined by the electric motor manufacturer.
If the signal labelled with PWMx switches between "on" and "off" according to this sequence, the electric motor will move at a rated rotating speed. Assume that the AC bus voltage is equal to the electric motor's rated voltage plus the voltage loss on two ends of the switch. To change the rotating speed, these signals must be higher than the electric motor's frequency for PWM. As an experiential rule, the PWM frequency should be at least ten fold as high as the electric motor's highest frequency. When PWM duty ratio changes in one inversion process, the average voltage provided by the PWM duty ratio reduces, which will then decrease the rotating speed. Another advantage of PWM is that, when the DC busbar voltage is much higher than the electric motor's rated voltage, the percentage of the PWM duty to the electric motor's rated voltage can be limited to control the electric motor. This will increase the flexibility, enabling the controller to work with the electric motor of different rated voltages. By controlling the PWM duty, the average output voltage of the controller can be matched with the electric motor's rated voltage.
Comutation sequence: To turn by every 60°electric angle, one of the Hall sensors will change its state. Therefore, it takes six steps to finish the electricity cycle. Under the simultaneous model, the phase current is switched once when rotating by every 60°of electricity angle. However, one electric cycle might not be corresponding to a complete rotor's mechanical rotation cycle. The number of electric cycles to be repeated by accomplishing one cycle of mechanical rotation is decided by the number of the rotor magnetic poles. Generally, to a six-step inversion method, the n magnetic poles divide the electric motor's rotating angle into 360/ (6*n magnetic poles)°. Every pair of rotor magnetic poles calls for accomplishment of one electric cycle. Therefore, the number of electric cycles divided by the number of rotations is equal to the number of rotor magnetic poles. Take the Hall sensor signals with 60° of phase shift between the magnetic poles for example. The phase shift between Hall sensors can be 60°or 120°. In choosing the controller for the specific BLDC motor, we should adhere to the sequence defined by the electric motor manufacturer.
If the signal labelled with PWMx switches between "on" and "off" according to this sequence, the electric motor will move at a rated rotating speed. Assume that the AC bus voltage is equal to the electric motor's rated voltage plus the voltage loss on two ends of the switch. To change the rotating speed, these signals must be higher than the electric motor's frequency for PWM. As an experiential rule, the PWM frequency should be at least ten fold as high as the electric motor's highest frequency. When PWM duty ratio changes in one inversion process, the average voltage provided by the PWM duty ratio reduces, which will then decrease the rotating speed. Another advantage of PWM is that, when the DC busbar voltage is much higher than the electric motor's rated voltage, the percentage of the PWM duty to the electric motor's rated voltage can be limited to control the electric motor. This will increase the flexibility, enabling the controller to work with the electric motor of different rated voltages. By controlling the PWM duty, the average output voltage of the controller can be matched with the electric motor's rated voltage.
Post a Comment:
You may also like:
