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:

2.9 Ω

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 -

$P_{loss}=I^2×R$

$R=\frac{P_{loss}}{I^2}=\frac{420}{144}=2.9 Ω$