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A 220-V d.c. machine has an armature resistance of 1.0 Ω. If the full-load armature current is 10 A, find the induced e.m.f. when the machine acts as (a) generator (b) motor. (Assume d.c. machine to be shunt connected and in each case it is considered negligible)

A 220-V d.c. machine has an armature resistance of 1.0 Ω. If the full-load armature current is 10 A, find the induced e.m.f. when the machine acts as (a) generator (b) motor. (Assume d.c. machine to be shunt connected and in each case it is considered negligible) Correct Answer 230 V, 210 V

Concept:

E.M.F equation when DC shunt machine acts as:

Case 1: Generator

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E.M.F. in case of DC generator is given by:

Eb = V + IaRa

Case 2: Motor

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E.M.F. in case of DC motor is given by:

Eb = V - IaRa

Calculation:

Case 1: When the DC machine is acting as a generator

VT = 220V,  Ra = 1Ω,  Ia = 10A

Eg = 220 + (10× 1)

Eg = 230 V

Case 2: When the DC machine is acting as a motor

Eg = 220 - (10× 1)

Eg = 210 V

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