This devices are mainly used for governing the speed in the machinery. Two main causes of maintaining the rate of machinery by using variable speed drives are to regulate the process also to conserve energy. Though these drives are produced for process control, energy conservation is becoming its prime objective.
Many industrial processes need different speed to operate different products. We can save energy by controlling the speed of those processes. Since we are able to find the speed from the pre-set ranges, it's called the "adjustable" speed. This can be referred as variable speed because its output could be changed without undergoing any steps. These are purely mechanical variations.
Some advantages of using VSD are, they give smoother operation and acceleration control and enable you to choose different speed for each process, decrease operation for setup purposes, adjust rate of production, control torque, and compensate variables in changing process.
Although we can find many applications of AC in industry, we still find that there are many industrial processes that require DC for their operation.
Some of these applications include battery charging, emergency lighting, welding, electroplating, metal refining, metal reduction, communications and many others.
Unlike AC generators, the speed of DC generators are not so critical. Therefore, small DC generators can be belt-driven, gear-driven or direct driven by prime movers. Large DC generators are always direct driven. Some of the prime movers used are AC motors, petrol or diesel engines, water or steam turbines.