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Speed of DC shunt motor is directly proportional to___________(a) Flux(b) Back emf(c) Terminal voltage(d) Armature resistance dropI have been asked this question in examination.My doubt is from Operating Characteristics of DC Shunt Motor topic in chapter Characteristics of DC Motors of DC Machines

Answer»

Right answer is (b) Back emf

The best explanation: The armature CIRCUIT equation for DC shunt motor is given by EA = Vt -IARA. But back emf is also equal to kφn. Thus, kφn= Vt -IaRa, which further gives n = (Vt -IaRa)/ kφ. HENCE, speed is DIRECTLY proportional to back emf.



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