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Unformatted text preview: (B) Calculate the efficiency of your system at maximum motor current. SOLUTION When R P = R M = 10 Ω , the two parallel currents are equal and their sum equals the total current from the 12V source. The effective resistance of R P and R M in parallel is 5 Ω and the total current from the source is 2*f*200mA. The source current is I = 12V/(R + 5 Ω ) = 2*f*200mA Therefore, R = 12V /(2* f*200mA )  5 Ω R 1 0  V OUT MOTOR R M = 10 Ω Max = f*200mA R = ??? Ω R P = 010 Ω I MOTOR The efficiency is the motor power over the total source power supplied (P S = I*12V) Therefore, Eff. = [(f*0.200A) 2 R M ] / [12V * (2 * f*0.200A)] Eff. = [(f*0.200A) R M ] / (24V) = [I M *R M ] / (24V) EXAMPLE: Say f = 0.650. Then I M = 130 mA = 0.130 A R = 12/(0.4f)  5 = 41.15 Ω . Eff = (0.130A*10 Ω )/24V = 0.054 = 5.4% Higher efficiency would be obtained with a higher motor current limit....
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 Spring '10
 Shankar
 Voltage divider, Resistor, Electric motor, rp, variable current divider

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