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

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

• A. 14.6 amperes
• B. 12.4 amperes
• C. 14.1 amperes
• D. 15.2 amperes

Answer: (A)

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.

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.