What is PWM Motor Control?

In this tutorial, we are going to learn about PWM Motor Control.

The magnetic field produced by the sensor’s permanent magnets is fixed and therefore cannot be changed but if we change the strength of the armatures electromagnetic field by controlling the current flowing through the winding more or less magnetic flux will be produced resulting in a stronger or weaker interaction and therefore a faster or slower speed. Then the rotational speed of a DC motor (N) is proportional to the back emf (Vb) of the motor divided by the magnetic flux (which for a permanent magnet is a constant) times an electrochemical constant depending upon the nature of the armatures winding (KC) giving us the equation of : N ∞ V/ KCɸ.

As per below circuit for control the flow of current through the motor. Generally we control the speed of a DC motor using a large variable resistor (Rheostat) is series with motor as per image.

It generate a lot of heat and wasted power in the resistance, One simple and easy way to control the speed of a motor is to regulate the amount of voltage across its terminals and this can be achieved using “Pulse Width Modulation”  or PWM.

As per name, pulse width modulation speed control works by driving the motor with a series of ‘ON – OFF’ pulses and varying the duty cycle, the fraction of time that the output voltage is ‘ON’ compared to when it is ‘OFF’, of the pulses while keeping the frequency constant.

The power applied to the motor can be controlled by varying the width of these applied pulses and thereby varying the average DC voltage applied to the motors terminals. By changing or modulating the timing of these pulses the speed of the motor can be controlled, like that the longer the pulse is ‘ON’, the faster the motor will rotate and likewise, the shorter the pulse is ‘ON’ the slower the motor will rotate.

In other word, the wider the pulse width, the more average voltage applied to the motor terminals, the stronger the magnetic flux inside the armature winding and the faster the motor will rotate .

Leave a Comment