Variable frequency operation has been around a long time, at least since the invention of the AC (alternating current) generator and the induction motor.
By changing the rotational speed of a generator, you can alter its output frequency. In fact, before the discovery of high speed transistors, this was one of the only ways you could vary motor speed. But frequency changes were limited because generator speed reduction reduced the output frequency but didn’t effect the voltage.
Controlling Rotational Speed
One of the ways to use a multi-pole motor is to wind it in such a way that allowed a single switch or multiple switches to vary the number of active stator poles at any given moment. Rotational speed could then be changed manually or by using a sensor connected to one or more of these switches.
Some types of equipment — such as hot and chilled water circulators, pool pumps, and cooling tower fans and pumps — still use this method. Other domestic booster pumps use fluid-driven of variable belt-driven systems to vary pump sped based on feedback provided by a pressure diaphragm valve.
A Simpler Way
Frequency converters simplify the process by changing pump speed any time you want, either manually or automatically.
Most frequency converters allow you to set motor speed manually via a multi-position switch or keypad. Others use sensors — such as those that monitor pressure, flow, temperature, levels and so on — to automate the process.
The Solid State Transistor
The use of frequency converters has simplified the process of controlling rotational speed capitalizing on the efficiency of the solid state device known as the transistor. Thanks to the modern frequency converter, the variable speed of pumps and other types of equipment essentially has been revolutionized.
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