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Q)

# An electric motor operation on a $50\;v$ d.c supply draws a current of $12\;A$ . If the efficiency of the motor is $30\%$ , estimate the resistance of the motor .

$\begin{array}{1 1} 30 \;\Omega \\ 0.3\;\Omega \\ 3\;\Omega \\ 0.03\; \Omega \end{array}$

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A)
Solution :
given $V= 50\;V$
$I= 12\;A$
n$= 30\%$
Input power = VI
$\qquad= 50 \times 12 =600 walt$
Since $n=30\%$ , the energy dissipated as heat
$P= \large\frac{70}{100}$
$\quad= \large\frac{70}{100}$$\times 600$
$\qquad= 420\;w$
$P= I^2 R$
$R= \large\frac{P}{I^2}$
$\quad= \large\frac{420}{12 \times 12}$
$\quad= 2.9 \Omega$
or $3\;\Omega$ (approx)