Question

A 3-phase, 15 Hp, 460 V, four pole, 60 Hz, 1728 rpm induction motor delivers full output power to a load connected to its shaft. The windage and friction loss of the motor is 750 W. The rotor copper loss is______W. Assume (1 hp = 746 W)

• A. 497.5

Answer 1

The correct option is A 497.5
$\text{Full load shaft power}=15\times 746=11190W$

$\text{Mechanical power developed}=11190+750=11940W$

$\text{Air gap power}=\frac{\text{Mechanical power developed}}{1-s}$

$\text{Slip, s}=\frac{N_s-N_r}{N_s}$

$N_s=\frac{120f}{P}=\frac{120\times 60}{4}=1800rpm$

$s=\frac{1800-1728}{1800}=0.04$

$P_{ag}=\frac{11940}{1-0.04}=12437.5W$

$\text{Rotor copper loss}$

$=s{P}_{ag}=0.04\times 12437.5$

$=497.5W$