Question

A 220 V DC shunt motor having a field flux of 0.8 Wb runs at a speed of 900 rpm. Find the speed of the rotor if the field flux is reduced to 0.6 Wb by field resistance control method.

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Answer 1

Explanation:

A 220 V DC shunt motor having a field flux of 0.8 Wb runs at a speed of 900 rpm. Find the speed of the rotor if the field flux is reduced to 0.6 Wb by field resistance control method.

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Answer 2

Answer:

Shunt motor supplied at V1 = 250 V

Speed N1 = 900 rpm

Armature current

Reduced speed = 600 rpm

Reduced current

Now, the motor equation is given by,

V = Eb + IaRa

Case 1:

= 238 V

Now, Eb ∝ N⋅ϕ

For shunt DC motor flux is constant

Eb ∝ N

Case 2:

250 V = 158.667 + 20 ×

Now, (armature resistance)

Resistance in series with the armature,

⇒ Rext

⇒ Rext = 4.17 Ω.

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