15-12-2012, 05:08 PM
SPEED CONTROL OF DC MOTOR USING PULSE-WIDTH MODULATION
SPEED CONTROL.pdf (Size: 78.97 KB / Downloads: 56)
Pulse-width modulation (PWM) or duty-cycle variation methods are commonly used in speed control of DC motors. The duty cycle is defined as the percentage of digital ‘high’ to digital ‘low’ plus digital ‘high’
pulse-width during a PWM period. Fig. 1 shows the 5V pulses with 0%
through 50% duty cycle. The average DC voltage value for
0% duty cycle is zero; with 25% duty cycle the average value is 1.25V (25% of 5V). With 50% duty cycle the average value is 2.5V, and if the duty cycle is 75%, the average voltage is 3.75V and so on. The maximum duty cycle can be 100%, which is equivalent to a DC waveform. Thus by varying the pulse-width, we can vary the average
voltage across a DC motor and hence its speed. The circuit of a simple speed controller for a mini DC motor, such as that used in tape recorders and toys, is shown in Fig. 2. Here N1 inverting Schmitt trigger is configured as an astable multivibrator with constant period but variable duty cycle. Although the total in-circuit resistance of VR1 during a complete cycle is 100 kilo-ohms, the part used during positive and negative periods of each cycle can be varied by changing the position
of its wiper contact to obtain variable pulse-width. Schmitt gate N2 simply acts as a buffer/driver to drive transistor T1 during positive incursions at its base. Thus the average amplitude of DC drive pulses or the speed of motor M is proportional to the setting of the wiper position of VR1 potmeter.