In the given example, the motor being described has a nameplate current rating of 23 amperes. What is the motor’s horsepower in that example?

In the given example, the motor being described has a nameplate current rating of 23 amperes. What is the motor’s horsepower in that example?

• A. 7.5 hp
• B. 5 hp
• C. 10 hp
• D. 12 hp

Answer: (A)

The test is showing how to relate the electrical nameplate current to the motor’s mechanical horsepower. Horsepower is just the mechanical output power, and you estimate it from the electrical input power (and the motor’s efficiency). For a single‑phase motor, input power in watts is roughly voltage times current times the power factor: P_in ≈ V × I × PF. Convert that to horsepower by dividing by 746: HP ≈ (V × I × PF × Efficiency) / 746. In the example, the current is 23 A and the voltage on the nameplate is typically 230 V. If you assume PF × Efficiency roughly equals 1 for a quick estimate used in the problem, then P_in ≈ 230 × 23 ≈ 5290 W, and HP ≈ 5290 / 746 ≈ 7.1 HP. The problem’s rounding conventions then place it at about 7.5 HP, which matches the chosen answer. If the motor were three-phase, you’d use P_in ≈ √3 × V_LL × I × PF and then convert to HP the same way.

The test is showing how to relate the electrical nameplate current to the motor’s mechanical horsepower. Horsepower is just the mechanical output power, and you estimate it from the electrical input power (and the motor’s efficiency). For a single‑phase motor, input power in watts is roughly voltage times current times the power factor: P_in ≈ V × I × PF. Convert that to horsepower by dividing by 746: HP ≈ (V × I × PF × Efficiency) / 746.

In the example, the current is 23 A and the voltage on the nameplate is typically 230 V. If you assume PF × Efficiency roughly equals 1 for a quick estimate used in the problem, then P_in ≈ 230 × 23 ≈ 5290 W, and HP ≈ 5290 / 746 ≈ 7.1 HP. The problem’s rounding conventions then place it at about 7.5 HP, which matches the chosen answer.

If the motor were three-phase, you’d use P_in ≈ √3 × V_LL × I × PF and then convert to HP the same way.