Wow !
1 horsepower = 746 watts
2.1 horsepower = (2.1 x 746) = 1,566.6 watts
Since the motor is 71% efficient, 1,566.6 watts = 71% of the input power.
1,566.6 = 0.71 P
Divide each side by 0.71 : input Power = (1,566.6 / 0.71) watts
(Let's leave it like that for right now. The final answer will be more accurate
if we can avoid rounding until the very end.)
Input power =(voltage) x (current)
(1,566.6/0.71 watts) = (171 volts) x (Current)
Divide each side by 171 volts: Current = (1,566.6) / (0.71 x 171) = 12.903 Amps.
Current = 12.9 Amperes.
Check:
(volts x Amps) x (efficiency) = output power
(171 x 12.9)(watts) x (71%) x (1 HP / 746 watts) =
(171 x 12.9 x 0.71) / (746) HP = 2.0994 HP
Close enough to 2.1 for Engineers !
yay !