A Proposed Modified Constant Frequency Variable Duty PWM Based Speed Control Technique for DC Motor Drives: A New Method to Achieve More Precise Control over Speed

In this paper, the authors have proposed a new method for speed control of DC motor drives via modified constant frequency-variable duty (CFVD) pulse width modulation (PWM) technique. The armature of the DC motor rotating in very low speed has to accept abrupt mechanical shocks (kicks) due to the application of large amplitude narrow pulse trains (very low duty cycle) while the speed of the DC motor is controlled by using the conventional CFVD PWM method. These consecutive abrupt mechanical shocks lead to reduction in life of the DC motor. In the proposed method, an extra control signal (having modified sinusoidal form) is subtracted from the conventional fixed amplitude CFVD PWM signal to obtain the modified speed control signal, which provides gradual periodic increase and decrease in the amplitude of consecutive pulses for all duty cycles. Thus for very low duty cycles (for achieving very low speed) the abrupt mechanical shocks on the armature can be avoided. Moreover, the peak amplitude of the subtracted extra control signal provides an additional control over the speed of DC motor along with the duty cycle of the conventional PWM signal. A circuit for generating the modified control signal has been designed and also fabricated for testing. Finally the experiments are carried out to study the speed control characteristics of a 9 V DC motor drive. The proposed method is found to be better than the conventional CFVD PWM speed control method as regards flexibility in speed control, transient response and abrupt electrical breaking.