The so-called pulse width modulation refers to a method to control the motor speed through change the duty cycle of motor armature voltage witched on and off, that known as pulse width modulation (PWM).
For DC motor speed control system, it is extremely convenient to control speed with FPGA. The method is changing time ratio (ie, duty cycle) of the motor armature voltage turn-on and power-on to control the motor speed.
In the pulse action, when the motor is energized, the speed increases; when the motor is power off, speed decreases. Just according to certain rules, changing the time of power off or power on, you can control the motor speed. When the motor is always power on, its maximum speed is Vmax, set duty cycle is D = t1 / T, the average speed of the motor
Vd = Vmax . D.
Vd: the average speed of the motor,
Vmax: motor full time(maximum),
D=t1/T: duty cycle
Vd and the duty ratio D is not entirely linear relationship, when the system is allowed, it can be approximated seen as a linear relationship. Thus it can be seen as the duty cycle D is proportional to the motor armature voltage Ua. Change the size of duty cycle to control the motor speed.
Seen from the above description: the speed of the motor is proportional to armature voltage of the motor, and the duty cycle that controls waveform is proportional to the motor armature voltage, so the speed of the motor is proportional to the duty cycle, the greater the duty cycle, the faster motor revolution, when the duty cycle α = 1, the motor speed reach to his maximum value.