An electric motor operating on a 50 V dc supply draws a current of 12 A. If the efficiency of the motor is 30%, the estimated resistance of the motor would be:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 2.9 Ω
The input power, $P_{in}$ is -
$P_{in}=V×I$
$=50×12=600W$
Efficiency ($η$) = 30%
$∴$P_{out}=η×P_{in}$
$=0.30×600=180W$
Hence, Power loss, $P_{loss}=P_{in}-P_{out}$
$=600-180=420W$
The power loss as heat in motor is -
$P_{loss}=I^2×R$
$R=\frac{P_{loss}}{I^2}=\frac{420}{144}=2.9 Ω$