The control system investigator (CSI) at American Control & Engineering Service Inc., Rose Hill, Kan., faced a mystery in “The Case of the Overeager Pump.” Their case broke down this way:
The Mystery. Operators in the freshwater division of a municipal water works complained that one of three pumps at a lift station ran constantly, causing the hydrostatic chamber to drain empty. This, in turn, caused the other pumps to come on, which eventually put so much pressure on the system that the pipes burst. The system was rated only for 93 psi, but with the other pumps running, the pressure shot up to 130 psi.
The Clues. The customer had hired an electrician who thought the issue was with the starter, which he replaced. Afterward, the pump would work correctly for a short time but then return to its hyperactive pumping. If the customer shut down the pump manually, the relay eventually would become unstuck and work correctly again, but this was not a permanent solution. The customer asked American Control & Engineering’s CSI to solve the case.
The Perp. Called to the scene, the investigator observed that the relays for pumps No. 2 and No. 3 were 12 V DC relays rated to 10 A, and pump No. 1 was rated to just 0.6 A. However, the initial amps coming from the starter were well over 0.6 A, which caused the relay’s contacts to stick. Once stuck, the pump continued to run.
The Solution. The CSI removed the circuit board to which the relay was attached, and then removed the relay, as well as the terminal strips on either side. The investigator installed a standard 12 V DC base and relay. To power the board’s components, he soldered half of one of the terminal strips to the board, then soldered two wires to the board to deliver power to the relay.
Case closed. Pump No. 1 now switches on and off when it should.
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