DC Machines: Principle of operation

DC Machines: Principle of operation

1. What a DC Machine Is (Motor vs Generator — Same Construction, Opposite Operation)

DC machine = electromechanical energy converter with commutator, can be generator (mechanical→electrical) or motor (electrical→mechanical). Structure same, operation opposite (reversible).

Principle:

• Generator: Faraday + motion: conductor moving in magnetic field $B$ with velocity $v$ generates emf

(or $e = -N dφ/dt$). Fleming's right-hand gives direction.

• Motor: Lorentz force: current $I$ in conductor length $l$ in $B$ experiences force

, producing torque

. Fleming's left-hand gives direction.

Commutator (split ring) rectifies AC induced in armature to DC at brushes (for generator) and reverses current to maintain unidirectional torque (for motor).

Generator: prime mover rotates armature in B → e = Blv → via commutator DC at brushes → load.

Motor: DC supply via brushes/commutator → I in armature in B → F = BIl → torque → rotation.

Back emf: motor also generates Eb = k·φ·N (proportional to speed, opposes supply) ⇒ Eb = V - I_a Ra

4. Exam Points

• State generator $e=Blv$ (Faraday) and motor $F=BIl$ (Lorentz) — 5 marks, very frequent (almost certain to ask principle).
• Explain reversibility and commutator role — 5 marks, very frequent.
• Give Fleming's right (generator) vs left (motor) — 3-5 marks, frequent.
• Mention back emf $E_b = V - I_a R_a$ for motor — 3 marks, frequent.