Control Systems

Electronics is being used more and more to control indust­rial processes, particularly where the speed of a motor has to be accurately controlled. One big advantage is that valves, unlike relays, have no moving parts, no contacts to shatter or burn, and practically no inertia; and their speed of operation is very much greater. Two special types of valves, known as "thyrotrons" and "ignitrons" are generally used for motor control.

A thyrotron is a gas-filled triode, whose gas ionizes when the grid voltage rises above a certain value. "Ionization" is a scientific term for "knocking spots off atoms". An atom con­sists roughly of a positive nucleus at the centre with a number of negative electrons around it. It is electrically neutral, because the positive nucleus balances the negative electrons. But if an atom loses an electron, this balance no longer holds, and it becomes positively charged. It is then known as a "positive ion."

In the thyrotron, an electron emitted from the hot cathode collides with a gas molecule and we have two free electrons and a positive ion; all tend to collide with other gas molecules. These chain effects soon fill the tube with positive ions which make nice conductors, and a current can flow between cathode and anode. Neon lamps, mercury and sodium vapour street lamps and fluorescent tubes are examples of ionization.

Unlike hard valves, a thyrotron once conducting cannot be cut off by reducing the grid voltage alone, it is necessary to drop or remove the anode supply voltage in order to allow the valve to deionize. It has one great advantage, however, in that it can handle very much larger currents, than vacuum valves. This is why it is used for motor control.

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