Question

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

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

Answer 1

The correct option is C $2.9\Omega$

$v=50v,I=12A,\eta =30\%,$

Input power $=vI=50\times 12=600w$

Since $\eta =30\%⟹P=\frac{70}{100}\times 600=420w$

$P=I^{2}R⟹R=\frac{P}{I^2}=\frac{420}{12\times 12}=2.9\Omega$