AMT General – Fundamentals Of Electricity Practice Exam

How much current does a 30-volt, 1/2 horsepower motor that is 85-percent efficient draw from the bus?

14.6 amperes 12.4 amperes 14.1 amperes 15.2 amperes

Correct Answer:

14.6 amperes

Explanation:

The current drawn from the bus depends on the input power, which equals the output power divided by the efficiency. Start by converting the motor’s output power to watts: 0.5 horsepower is 0.5 × 746 ≈ 373 watts. With 85% efficiency, the input power is P_in = 373 W / 0.85 ≈ 439 W. The bus voltage is 30 V, so the current is I = P_in / V ≈ 439 W / 30 V ≈ 14.6 A. So the motor draws about 14.6 amperes from the 30-volt bus. If you ignored efficiency, you’d get about 12.4 A, which would be incorrect for this case because not all input power becomes useful output due to losses.

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