Question

An electric motor operates on a $50\, V$ supply and a current of $12\, A$. If the efficiency of the motor is $30\%$, what is the resistance of the winding of the motor?

• A. $6\,\Omega$
• B. $4\,\Omega$
• C. $2.9\,\Omega$
• D. $3.1\,\Omega$

Answer 1

Answer: (C)

$2.9\,\Omega$

Answer 2

$I=\frac{E-e}{R}$ ...(i) and $\eta \times 100=\left(\frac{e}{E}\right) \times 100$ ...(ii) $\therefore 30=\frac{e}{50} \times 100$ [from Eqs.(ii)] $\Rightarrow R=15\, V$ Thus, $R=\frac{50-15}{12}=2.9\, \Omega$