A 3 horsepower, 100 V DC motor is 85% efficient when developing its rated output. What is the current?

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Multiple Choice

A 3 horsepower, 100 V DC motor is 85% efficient when developing its rated output. What is the current?

Power in to the motor equals its electrical input power, and that input power is the mechanical output divided by efficiency. Convert the mechanical output to watts, then find the electrical input and current. The motor develops 3 hp. Using 1 hp ≈ 746 W gives P_out ≈ 3 × 746 = 2238 W. With 85% efficiency, P_in = 2238 / 0.85 ≈ 2633 W. At 100 V, the current is I = P_in / V ≈ 2633 / 100 ≈ 26.33 A, which rounds to about 26.300 A.

Power in to the motor equals its electrical input power, and that input power is the mechanical output divided by efficiency. Convert the mechanical output to watts, then find the electrical input and current.

The motor develops 3 hp. Using 1 hp ≈ 746 W gives P_out ≈ 3 × 746 = 2238 W. With 85% efficiency, P_in = 2238 / 0.85 ≈ 2633 W. At 100 V, the current is I = P_in / V ≈ 2633 / 100 ≈ 26.33 A, which rounds to about 26.300 A.

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