Question

An electric motor operating on $50V$ D.C. supply draws a current of $10A$. If the efficiency of the motor is $40\%$, then the resistance of the winding of the motor is:

• A. $1.5\Omega$
• B. $3\Omega$
• C. $4.5\Omega$
• D. $6\Omega$

Answer 1

The correct option is B $3\Omega$

The efficiency of motor is $40\%$,the remaining $60\%$ of power is dissipated in heat form.
Total power delivered is $P=VI=50\times 10W$

Dissipated power is $60\%$ of total power supplied.
$50\times 10\times \frac{60}{100}=I^{2}R$
$50\times 10\times \frac{60}{100}=100R$
$R=3\Omega$