Question

A 220V, 10 kW, 900 rpm separately excited DC motor has an armature resistance $R_a=0.02\Omega$. When the motor operates at rated speed and with rated terminal voltage, the electromagnetic torque developed by motor is 70 N - m. Neglecting the rotational losses of the machine, the current drawn by the motor from 220 V supply is

• A. 22 A
• B. 34.2 A
• C. 6.95 A
• D. 30 A

Answer 1

The correct option is D 30 A

By Neglecting rotor losses,

$E_b{I}_a=10kW$

$(V-I_a{R}_a)I_a=10kW$

$(220-0.02I_a)I_a=10000$

$\Rightarrow 0.02I_a^2-220I_a+10000=0$

On solving $I_a=45.64A$

$T=\frac{60}{2\pi N}{E}_b{I}_a=\frac{60}{2\pi \left(900\right)}\times 10000$

$=106.10N-m$

Separately excited motor,

$\therefore T\propto I_a$

$\therefore$ For Torque 70 N - m

$I_a=\frac{70\times 45.64}{106.10}=30.11A$