There are many industrial applications for which control of power input and/or output is required. Examples of such applications are variable speed drives, illumination controllers, and temperature regulators. The initial developments in power-control schemes were based on variable tap-changing transformers and series and shunt regulators (consisting of resistors or reactors) which produce a change in the applied voltage, and thereby vary the power. These schemes either were found to be inefficient or involved costly equipment. Further, the power control was usually in steps. Such drawbacks have since been partially overcome by saturable-core reactors. The advent of magnetic amplifiers paved the way for complete static control of power without moving parts. During the period before World War II, tremendous improvements were made in the performance and design of magnetic power controllers, and the evolution of square-loop materials improved their range of amplification. Magnetic control of power is still adopted for many military and industrial applications where reliability and sturdiness are of prime importance. The major limitation of this method is the bulkiness of the controller which prohibits its use in certain applications. Also, the core-magnetising current and iron losses result in low input power factor and low efficiency.
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