History
In the past, when only partial power was needed (such as for a sewing machine motor), a rheostat (located in the sewing machine's foot pedal) connected in series with the motor adjusted the amount of current flowing through the motor, but also wasted power as heat in the resistor element. It was an inefficient scheme, but tolerable because the total power was low. This was one of several methods of controlling power. There were others—some still in use—such as variable autotransformers, including the trademarked 'Autrastat' for theatrical lighting; and the Variac, for general AC power adjustment. These were quite efficient, but also relatively costly.
For about a century, some variable-speed electric motors have had decent efficiency, but they were somewhat more complex than constant-speed motors, and sometimes required bulky external electrical apparatus, such as a bank of variable power resistors or rotating converter such as Ward Leonard drive.
However, in addition to motor drives for fans, pumps and robotic servos, there was a great need for compact and low cost means for applying adjustable power for many devices, such as electric stoves and lamp dimmers.
One early application of PWM was in the Sinclair X10, a 10 W audio amplifier available in kit form in the 1960s. At around the same time PWM started to be used in AC motor control
Read more about this topic: Pulse-width Modulation
Famous quotes containing the word history:
“History is the present. That’s why every generation writes it anew. But what most people think of as history is its end product, myth.”
—E.L. (Edgar Lawrence)
“We don’t know when our name came into being or how some distant ancestor acquired it. We don’t understand our name at all, we don’t know its history and yet we bear it with exalted fidelity, we merge with it, we like it, we are ridiculously proud of it as if we had thought it up ourselves in a moment of brilliant inspiration.”
—Milan Kundera (b. 1929)
“The best history is but like the art of Rembrandt; it casts a vivid light on certain selected causes, on those which were best and greatest; it leaves all the rest in shadow and unseen.”
—Walter Bagehot (1826–1877)