Did you ever turn a bicycle upside-down and spin its wheels? Most kids spin the wheel by slapping it. The more slaps per unit of time, the faster it turns. Also, the longer the hand is in contact with the tire, the faster it turns. This is how something called Pulse Width Modulation (PWM) works. Pulsing for example an electric DC motor with the full voltage and current produces the same magnetic force in the armature, but for a shorter time. Thus, the force provided by the motor is about the same. Hit the tire or pulse the motor only half the time however, and the average voltage is cut in half. The motor will turn at half its full-voltage speed but still supply the same force. The signal to the motor would look something like this for a 50% duty cycle, which is to say it's on half of the time (figure 1.1).
Figure 1.1: Signal with a 50% duty cycle.
The motor will run at half its normal speed, providing the pulses are fast enough to take advantage of the motor's moving inertia. If you try to slow the shaft down with your fingers however, you will find that the motor has about the same force (torque) as it does at full speed.
Here's another one. If the duty cycle is 30% the motor will run at about 30% of its full speed (figure 1.2).
Figure 1.2: Signal with a 30% duty cycle.
The simplest analogue form of generating fixed frequency PWM is by comparison with a linear slope waveform such as a sawtooth. As seen in figure. 1.3, the output signal goes high when the sine wave is higher than the sawtooth. This is implemented using a comparator whose output voltage goes to logical value HIGH when the non-inverting input (+) is greater than the other (-).
Figure 1.3: Sine-sawtooth PWM.
Other signals with straight edges can be used for modulation. A rising ramp carrier will generate PWM with Trailing Edge Modulation.
Figure 1.4: Trailing Edge Modulation.