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Metropolitan Waterworks Authority has their policy to improve and develop production and transmission of water with high standard, stability and preciously to step into “The Best -in- Class of Metropolitan Waterworks in Asian’’ .According to the maintenance records of the high voltage motors in transmission pump for the last 2 years, it was found that there were 22 maintenance records which caused 886 shut down days and the Polarization Index of the motors were under standard limit. In consequence, it is necessary to improve efficiency of the high voltage motors in transmission pump station-1 that has been operated over 38 years. The causes of maintenance were analyzed and the 4 alternatives were suggested which were 1) changing only the winding of the old motor, 2) changing the old motor to synchronous motor, 3) changing the old motor to induction motor and 4)changing to induction motor with variable speed drive. The Analytic Hierarchy Process was conducted and it was found that the changing of new induction motor was the best alternative because of better tolerance, easy to operate and less maintenance requirement. And due to high investment cost, economic analysis was also taken into consideration, the incremental investment analysis was applied for selection, it was found that changing winding of old motor was the economical alternative, due to the lowest investment cost. The sensitivity analysis of the 3 variables which were machine and installation costs, electricity cost, and labor cost at the range of +25 % the selected alternative was still the economical selection. It was concluded that if the available budget was at 12.45 million baht, the alternative of changing only winding was recommended but if the budget was at 29.55 million baht, the technically feasible alternative of changing to induction motor was the preferable alternative.
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