A 220 V dc shunt motor is running at 500 rpm when armature current is 50 A. The value of armature resistance is 0.2 ohm. The speed of motor at the double torque will be

Concept:

DC shunt motor is a motor in which shunt field winding is directly connected across supply and since the voltage is constant therefore it is also called a constant flux device.

Performance eqⁿ of DC shunt motors are

E_b = V_t - I_aR_a

or, E_b = K_aϕω       _ _ _ _ _ (1)

and τ = K_aϕI_a       _ _ _ _ _ (2)

E_b1 = 220 - 50 × 0.2

= 220 - 10 = 210 V

Since, Torque is double i.e., armature current is doubled

i.e., I_a2 = 2 × I_a1 = 2 × 50 = 100 A

∴ E_b2 = 220 - 100 × 0.2

= 200 V

Therefore, the correct option is (4).