A 440V shunt motor takes a total current of 95A and runs at 1200 rev/min. If the iron, friction and windage losses amount to 1.8kW, the shunt field resistance is 50Ω and the armature resistance is 0.3Ω, determine the overall efficiency of the motor.
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11
Given :
For DC shunt motor
Line current = armature current + shunt current
= 20 A = ...........1
∵ Shunt current = = \dfrac{supply voltage}{shunt resistance}
shuntresistance
supplyvoltage
i.e Shunt current = = 1 Amp( a )
= 20 A - 1 A
= 19 A
So, Armature current = I_aI a = 19 amp
( b )For Motor
∵ EMF = V - I_aI a × R_aR a
= 250 - 19 × 0.3
= 250 - 5.7
= 244.3 volt
i.e EMF = 244.3 volt
Hence,
( a ) Armature current is 19 amp
( b ) Back EMF is 244.3 volt
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