The DC motors are used in many applications like process control and automation in industry, Robotics, printers, and scanners also. Speed control is an important factor in these applications. Speed of DC motor depends upon the average DC voltage applied across its armature. If the polarity of the power supply is changed, then the motor runs in reverse direction.
Dc Motor inner section
Pulse Width Modulation: PWM In modern days the speed of the DC motor is controlled using pulse width modulation (PWM) method. By using PWM average voltage applied to the DC motor is varied, thereby the speed of the DC motor is controlled. That is, by changing (modulation) the width of the pulse applied to the DC motor, we can increase or decrease the amount of power provided to the motor. There by increasing or decreasing the motor speed.
Dc Motor Armature:-
WAVE FORM: F-1. ON period is less and OFF time period is more Therefore ne average output is less.
WAVE FORM: F-2. ON period is more and OFF period is less therefore net average output is more.
In PWM the pulse period is kept constant; pulse on time and off time are changed; put together, the total time is maintained constant.
The basic circuit arrangement for speed control using a transistor SL100 as a switch. When control signal applied to its base is at high level (During T on period) the motor circuit is closed.
When the control signal applied to the base of SL100 is zero (During T off period) the motor circuit is open circuited.
Related Link: Stepper Motor details here.