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The history of power regulation

REOVAR variable transformer
In 1837 Thomas Davenport a Vermont blacksmith built the first DC electrical rotating machine. Some time after, when Thomas Edison created the light-bulb in 1879 and subsequently illuminated 50 blocks in lower Manhattan using six 500kW steam powered generators. These were exciting days indeed.

It was only a matter of time before public buildings, such as theatres, began installing the new electrical lighting and so created a need for methods of control, primarily for dimming. The early systems comprised long lengths of resistance wire, formed into a coil with a wiper controlled by a manual lever. They were crude, large and difficult to adjust smoothly.

In Berlin REO (then known as Hase & Von Wolff G.M.B.H.) were busy producing resistors for dimming large public buildings and ballasts for arc lamps.

In 1933 General Radio announced the launch of a new product, an adjustable autotransformer called the "Vari`c" (short for variable AC). Eduard Karplus, a graduate of the Institute of Technology, Vienna was its creator. The first unit, the type 200-C had a current rating of 5 amperes and was used to provide an adjustable voltage from a mains supply of 115V or 230V. The output voltage range was from zero to something above line voltage and was virtually stepless. In the 1960's the Superior Electric Company produced large numbers of variable transformers under the trade names of "Luxtrol" or "Powerstat" for theatre and television applications throughout the USA and Canada.

In the meantime further uses had been found for the variable transformer and another popular, consumer, application was for speed control of model train sets, especially in the 1950's and 1960's. These transformers were essentially step-down units, the highest voltage obtainable from the secondary winding being substantially less than the primary voltage of 110 to 120 volts AC. The variable-sweep contact provided a simple means of voltage control with little wasted power; much more efficient than control using a variable resistor. There were industrial applications too and during this era variable transformers were used to control larger DC motors used in large printing and textile lines.

The invention of the transistor in 1948 by Bardeen, Brattain and Schokley brought us to the threshold of the solid-state electronics age. Then in 1956 Bell Telephond Laboratory invented the thyristor (PNPN transistor), which was later commercialized by General Electric Co. By using these devices it was possible to make much smaller and lighter power systems and so there was a natural tendency for engineers to use thyristor techniques in place of iron and copper or resistance wire.

Nowadays MosFets and IGBT's are the vogue, operating at much higher switching frequencies. The advantage being that size reduces considerably as frequency increases. Switch mode power supplies are now taking the place of thyristor controllers but there are associated problems to contend with such as harmonics and interference. ← Back to all Technical Resources