Pulse Width Modulation also called Pwm, is a great recipe of controlling the amount of power delivered to a load without dissipating any wasted power because the average power delivered to the load is proportional to the modulation duty cycle.
Many population endeavor to control the speed of a D.C. Motor using a large variable resistor (called a rheostat) in series with the motor to control the amount of current and therefore the speed of the motor. While this may work it generates a lot of heat and wasted power in the resistance. One simple way to control the speed of a motor is to regulate the amount of voltage across its terminals. This is achieved using "Pulse Width Modulation" or Pwm by driving the motor with a series of "On-Off" pulses rather than a continuously varying (analog) voltage. By increasing or decreasing the pulse widths, the Pwm circuit regulates power flow to the motor 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 voltage applied to the motors terminals. By changing or modulating the timing of these pulses the speed of the motor can be controlled, i.e. 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. The difference in the ratio between the "On" pulse and the "Off" pulse is called the duty cycle. Duty cycle is expressed as a division relating to the fraction of time that the production voltage is "On" and may be discrete from 0 to 100 percent. So for a duty cycle of 0 percent the motor is fully-off, 50 percent the motor is operation at half speed, and 100 percent the motor is fully-on.
The use of pulse width modulation to control a small motor or fan has the benefit in that the power loss in the switching device is small because it is either fully "On" or fully "Off" therefore the switching device has a much reduced power dissipation giving a linear type of control resulting in best speed stability. Also the amplitude and frequency of the motor voltage remains constant so the motor is all the time at full strength.
One supplementary benefit of pulse width modulation is that the pulses reach the full supply voltage and will yield more torque in a motor by being able to overcome the internal motor resistances more easily. The result is that the motor can be run slowly without stalling. Also, in a pulse width modulation circuit, small ordinarily available potentiometers can be used to control a wide range of motor loads whereas large and high-priced high power variable resistors called Rheostats are needed for resistive controllers.
Pulse width modulation can also be used to control the brightness of light sources such as Dc lamps or Light Emitting Diodes. Light emitting diodes (Leds), turn "On" and "Off" extremely fast and would appear to flicker if supplied with a low frequency voltage. The result of this flicker can be reduced by increasing the pulse width modulation frequency. If the "On/Off" switching frequency is sufficiently rapid, the human eye can not see this switch performance and only sees the brightness of the lamp without flicker.
Pulse Width Modulation is a great recipe for controlling the amount of average power delivered to a load without dissipating any wasted power. Pwm circuits can be used in many dissimilar applications to control the speed of motors and fans or to dim the brightness of Dc lamps or Led's. If you need to control it, use Pulse Width Modulation to do it.
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