To determine the current of a three-phase motor from horsepower, voltage, efficiency, and power factor, which quantities must be known?

To determine the current of a three-phase motor from horsepower, voltage, efficiency, and power factor, which quantities must be known?

• A. Horsepower, voltage, efficiency, and power factor.
• B. Horsepower and voltage only.
• C. Voltage and efficiency only.
• D. Horsepower, voltage, and efficiency.

Answer: (A)

Current in a three-phase motor comes from how real input power, voltage, and power factor relate to current. The real electrical input power is P_in = √3 × V × I × PF. The motor’s mechanical output is given in horsepower, so P_out = HP × 746 W, and this relates to input power through efficiency: P_out = η × P_in, or P_in = (HP × 746) / η. Combine these: I = P_in / (√3 × V × PF) = (HP × 746) / (η × √3 × V × PF). So you must know horsepower, line voltage, efficiency, and power factor to determine the current. If any of these is missing, you can’t compute I with a single known voltage and horsepower.

Current in a three-phase motor comes from how real input power, voltage, and power factor relate to current. The real electrical input power is P_in = √3 × V × I × PF. The motor’s mechanical output is given in horsepower, so P_out = HP × 746 W, and this relates to input power through efficiency: P_out = η × P_in, or P_in = (HP × 746) / η. Combine these: I = P_in / (√3 × V × PF) = (HP × 746) / (η × √3 × V × PF). So you must know horsepower, line voltage, efficiency, and power factor to determine the current. If any of these is missing, you can’t compute I with a single known voltage and horsepower.