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WHEELS OF STEEL TURN FROM GOLD TO PLATINUM
A PID controller is inserted within the control loop so that the loop can be tuned to ensure stable and accurate control with good response times. Because the PID controller is built into the ABB drive, the benefits of closed loop control can be realized without the additional expense of external control equipment. A Proportional-Integral-Derivative controller is a universal type of controller that helps eliminate any error and improve steady state performance, as well as improve dynamic performance. Magnetto-Topy use four drives, rated from 37 to 55 kW, on water pumps within four cooling towers that supply water to the wheel making process. Before installing variable speed drives, the company used star-delta switching which turned the pumps on or off. Thus, the pumps were left running at full speed on 50Hz. With drives fitted, the company can reduce the running speed on a 55 kW pump to 35Hz, resulting in a 53% energy saving. A further assessment showed that by applying closed loop control, the energy consumption could be reduced by an additional 50%. But Magnetto-Topy was cautious about further slowing the pumps, because of the risk presented by legionella bacteria, which can spawn in cooling towers. To prevent this, a continuous water flow is essential. Along with water flow, the water temperature needs to be maintained at a maximum of 20º C to prevent the growth of legionella bacteria. Changing the water flow rate can also affect the water temperature. So the system was tuned for these worse case conditions, which normally only occur during the summer, when the water temperature reaches its 20º C maximum. But during the winter months when the water temperature is naturally cooler, there is an opportunity to slow the pumps down even further. This is where PID control comes in. A transducer is placed into the water tank and this feeds the temperature measurement back to the drive. So at 20º C, the drive is running at 40 Hz. As the temperature drops to 13º degrees C and below, the drive runs at 25 Hz. As the temperature of the water rises, the pump speeds up and allows more water to cascade over the cooling tower. "The result is that with PID control, the energy bill has been halved yet again," says Phil Smith, Maintenance Service Administrator. "PID control is a compensatory control method. Without it we have to cater for the worse case which means running pumps at 40Hz, which is the summer setting."
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