Evans Water Engineers


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EVANS Electronic load governing

Electronic Load Governing came from a system that my father developed back in 1949 for controlling the waterwheel at Trecarrell Mill. At the time there was no satisfactory way of governing a waterwheel, since any reduction in the electrical load caused an over-speed and over voltage because of the time taken to open or close the sluice gate. The electric or eddy current brake had been used for water turbine governing since the nineteen thirties, but it was cumbersome and wasted all the braking energy. The "differential load governor" as my father called it, required two generators; one for the main load and the other for the ballast or heating load. The output of the main load generator passed through an auxiliary "reverse wound field winding" in the second generator. The effect of an increase in main load was to reduce the field and hence the load on the second control generator.

The original installation operated continuously for about 10 years, by which time the National Grid had reached the back woods of Cornwall and a connection was made for the princely sum of £12. The plant continued to be used for heating until the highly subsidized price of mains electricity began to rise. My own interest began when we started to look at using Thyristors for controlling the output of an alternator that we had installed as a replacement to the original DC system. Having made a few prototypes and contacted several electronic engineering firms I met up with Gerry Pope, who was at the time manufacturing audio equipment. Although this was a major departure from his normal line of work he quickly produced a working system that was the worlds first "Electronic Load Governor". From 1974 for the next ten years we improved and promoted the system on a very small budget and quite a lot of derision from other manufacturers who said that it would never catch on.

The system uses only one standard generator and a ballast load connected in parallel with the main output. The ballast load, which must not be disconnected, has a slightly larger capacity than the maximum output of the turbine and is controlled by Thyristors or similar solid-state switching device. If the speed (and frequency) rises because the load on the generator is not great enough, the controller begins to increase the voltage in the ballast circuit until it once again balances the output from the turbine. In other words, if the turbine is producing 10 kW but only 4 kW are required by the main load, the other 6 kW will be diverted into the ballast load. Another version switches discrete ballast loads in combination and permutations, or in conjunction with a variable control device.

Having patented the system I went public on the system and persuade the Intermediate (ITDG) now called ‘Practical Action’ to promote microhydro and the control system for use in developing countries. With Peter Fraenkel and later with Ray Holland the message slowly spread around the world, and is now an industry standard. In 1986 Gerry Pope and myself jointly received an "International Inventors Award" for our efforts. The story is by no means over since many similar systems now exist around the World for both synchronous and asynchronous generators. My own systems continue to develop continuing a family involvement in governing going back over 100 years to when Joseph Evans manufactured the famous ‘Pickering Governor’ for steam engines.