A three-phase induction motor delivers 50 hp at full load. At this load its efficiency is 91% and its power factor is 0.86 lagging. The motor is supplied at 480 V (line-to-line). The full-load line current is most nearly:

57.3 A (correct answer)
99.3 A
52.2 A
49.3 A

Worked solution

P_{out}=50\times746=37.3

kW;

P_{in}=P_{out}/\eta=37.3/0.91=40.99

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Study handbook

47 concepts — worked theory, key equations, and common pitfalls for every topic.

Sample concept

Three-Phase Circuits and Power

Wye and delta relationships, balanced three-phase power, and the √3 conventions that underpin nearly every exam calculation.

Three-phase is the native language of power engineering: three sinusoidal sources of equal magnitude displaced

120^\circ

apart deliver constant (non-pulsating) total power, use conductors more efficiently than single-phase, and create the rotating magnetic field that motors depend on. A large shar…

V_{LL} = \sqrt{3}\,V_{LN}\angle{+30^\circ}, \quad I_L = I_\phi

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PE

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Reference

NCEES PE Power Reference Handbook + NEC 2020 & NESC 2017

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Practice questions

kW;

S=P_{in}/pf=47.66

kVA;

I=S/(\sqrt3\,V_{LL})=47.66\times10^3/(\sqrt3\times480)=57.3

A. Distractors forget the efficiency (use shaft output as electrical input), forget the power factor (treat

P_{in}

as kVA), and drop the

\sqrt3

.

PE Electrical and Computer: Power

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