Correct AnswerC. It will oppose the supply voltage
Quick Explanation
The induced armature emf in a running DC motor is the back emf. By Lenz's law it opposes the applied supply voltage, limiting armature current as the motor gains speed.
Formula / Key Relation
V = Eb + Ia Ra
Eb ∝ ΦN
At start: N = 0 ⇒ Eb = 0
Detailed Explanation
Concept & ReasoningWhen the armature of a DC motor rotates in the magnetic field, an emf is induced in its conductors. The direction of that induced emf is governed by Lenz's law: it opposes the cause producing it. Since the supply causes the armature to rotate, the induced emf acts opposite to the supply and is therefore called back emf. The armature circuit is commonly written V = Eb + IaRa (neglecting brush drop). At starting Eb = 0, which explains the high starting current; as speed rises, Eb rises and current falls.How to Solve It in the ExamMotor induced…
Concept & Reasoning
When the armature of a DC motor rotates in the magnetic field, an emf is induced in its conductors. The direction of that induced emf is governed by Lenz's law: it opposes the cause producing it. Since the supply causes the armature to rotate, the induced emf acts opposite to the supply and is therefore called back emf. The armature circuit is commonly written V = Eb + IaRa (neglecting brush drop). At starting Eb = 0, which explains the high starting current; as speed rises, Eb rises and current falls.
How to Solve It in the Exam
Motor induced emf = back emf = opposes supply.
Important Exam Point
Use the armature-voltage equation to connect back emf with starting-current behavior.
Related Revision Path
This question sits in the revision path Electrical Machines → DC Machines → Back EMF .
Quick Trick
Motor induced emf = back emf = opposes supply.
Common Mistake
Do not confuse motor back emf with generator emf polarity; both follow electromagnetic induction but circuit roles differ.
Exam Tip
Use the armature-voltage equation to connect back emf with starting-current behavior.
SEO + Knowledge GraphRelated PYQs & Exam ContextExam path, knowledge path and deeper practice links.
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