EGR 115 Measurement Uncertainty Homework An AC electric motor operates continuou

Question

EGR 115 Measurement Uncertainty Homework An AC electric motor operates continuously at a speed of 3,600 rpm. The measured input voltage and current are 480 VRMS and 300 ARMS, respectively. The power factor is 1. The load is dissipated to a dynamometer that indicates 186 horsepower at the shaft. The motor is cooled entirely by water that flows throughout the housing. The measured water flow rate is 2 gallons per minute (gpm). Measured inlet and exit water temperatures are 70 F and 85 F, respectively. Your instrumentation has the following ranges and uncertainties:

Explanation / Answer

1) power input = Irms*Vrms cos theta = 480 *300 = 144 KW

power input to dynamo= 139.5 KW

2) Heat loss = m*c*del T= 8.33* 16.6* 4.2/1000 = 0.58 KW

3) efficiency = 143.42/144 *100 =99.5 %

4) shaf output power = 139.5 KW

efficiency= 96.8 %

5) there is a decrease in efficiency from 3 to 4

due to the extra loss by friction

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