FLOMATCHER/MARATHON
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Marathon Electronics Wattmiser slip energy recovery controls were chosen to replace 5- 1500 HP wound rotor resistor type secondary controls at a major metropolitan sewage pumping station based on energy savings, maintenance costs, and human comfort. Adjustable speed pumping in this sewage lift station is required to match the wide variations in flow from the incoming sewers. A multiple number of pumps are programmed on and off to maintain a constant level in the wet well. When this pumping facility was first installed in the1960's, wound rotor motor drives with secondary resistor speed control were a common and economically effective method for adjustable speed pumping. thousands of such controls were installed throughout the country. WASTE HEAT CALCULATION With the escalating cost of energy, however, the wasted heat energy dissipated in these controls has been a serious concern. This wasted energy can be calculated as: HP Wasted = (T x RPM)/5250 Where T = Load torque in lbs. ft.; RPM = Slip below full load speed. For example, assume a 1500 HP 10-pole wound rotor motor drives a centrifugal pump requiring 1500 HP at full load motor speed of 356 RPM, the torque load varies as the square of the speed (which is typical of a centrifugal pump), and the average operating speed of the pump is 80% of full load speed. The HP wasted in the resistors can be calculated as: HP=((>80)² x 22121 lb ft. x 71.2 RPM)/5250 = 192 HP If the pump is operated 12 hours per day for a year at an energy cost of 6 cents per kilowatt hour, the cost of wasted energy would be: $ Cost =m 192 HP x .746 x 12 hrs. x 365 days x $.06/KWH = $37,642 per year Costs for different operating profiles of speed and time and different energy costs can be easily calculated. WATTMISER SIMPLE AND EFFECTIVE The Marathon Electronics Wattmiser offered a simple and effective way to recover the slip energy of the wound rotor motors and return this energy to the power line as savings. The single line diagram of the Wattmiser circuit (Fig. 2) shows how the rotor current is rectified to DC in the diode bridge, smoothed by an inductor, and returned to the line by a simple DC SCR bridge operating as a line commutated inverter. The simplicity of the DC SCR bridge is very important since this is well proven solid state circuitry and easily maintained hardware. Trouble shooting is accomplished by simple replacement of modular circuit boards. SOLVES EXCESSIVE HEAT PROBLEMS This pumping station also had other serious problems due to the wasted heat - particularly during the summer months. Comfort levels for plant operators could not be maintained even with ventilation systems full on. In addition, maintenance on the older controls had become a problem, and resistor burnouts had occurred. This excessive heat build-up is a common complaint - particularly on large horsepower drives or where building dimensions are small in relation to horsepower. The Wattmiser control is being specified widely by consulting engineers and industrial users for adjustable speed pumping applications on both new installations and retrofits. It is being chosen over variable frequency drives for its simplicity and over eddy current and hydraulic clutch drives for its far superior efficiency. USE WITH MEDIUM VOLTAGE MOTORS The Wattmiser can easily be applied to motors with primary voltage ratings of 230, 460, 2300 or 4160 volts. This is an important consideration where it is desirable to serve large motors at a medium distribution system voltage. Marathon Electronics has manufactured the Wattmiser since 1972 and offers the most in-depth experience in design and manufacture of slip recovery systems. WATTMISER FEATURES:
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