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Question

# A 20 kW, 300 V dc shunt motor is driving a constant torque load with line current of 100 A at a speed of 1600 rpm. Motor has armature and field winding resistance as 0.02 Ω and 100 Ω respectively. If 50 Ω external resistance is added in field circuit, then motor is running at (Assume linear magnetzation curve)

A
1812.9 rpm
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B
2100 rpm
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C
2392.18 rpm
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D
1594.8 rpm
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Solution

## The correct option is C 2392.18 rpmCase - I: Armature current, Ia1=100−3=97A back emf, Ea1=300−97×0.02 Ea1=298.06 V Case - II: When 50Ω external resistance added in field circuit, Load torque is constant T∝ϕIa ϕ2Ia2=ϕ1Ia1 ϕ∝If Ia2=Ia1ϕ1ϕ2=97×32=145.5A Ea2=300−145.5×0.02 Ea2=297.09V We know that, Ea∝ϕ ωm Ea2Ea1=ϕ2ϕ1×N2N1 297.09298.06=23×N21600 N2=2392.18 rpm

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