First slide

Magnetic flux and induced emf

Question

An electric motor operating on a 60 V dc supply draws a current of 10 A. If the efficiency of the motor is 50%, the resistance of its winding is

Moderate

A

$3 Ω$
B

$6 Ω$
C

$15 Ω$
D

$30 Ω$

Solution

$Efficiency η = 50 % So e = E / 2$
$and i = \frac{E - e}{R} \Rightarrow i = \frac{E - E / 2}{R} = \frac{E}{2 R}$
$\Rightarrow R = \frac{E}{2 i} = \frac{60}{2 \times 10} = 3 Ω$

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