When the armature of a DC motor rotates, emf induced is

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back emf

Explanation: When a DC motor armature rotates within a magnetic field, the conductors cut the magnetic flux, inducing an EMF according to Faraday's Law of Electromagnetic Induction. This induced EMF acts in opposition to the applied supply voltage, hence it is termed 'Back EMF' (Eb).

Working Principle: As the motor rotates, the induced EMF ($E_b$) opposes the terminal voltage ($V$) applied to the armature. This relationship is defined by the Kirchhoff's Voltage Law equation $V = E_b + I_aR_a$, where $I_a$ is the armature current and $R_a$ is the armature resistance. The back EMF acts as a self-regulating mechanism, controlling the amount of current drawn by the motor based on its load and speed.

Key Formulas: $E_b = \frac{P \Phi Z N}{60 A}$ — Induced Back EMF in terms of machine constants; $V = E_b + I_a R_a$ — Voltage balance equation for DC motor armature

Key Points: The back EMF is always less than the applied terminal voltage ($V > E_b$).; At starting, the speed $N = 0$, therefore $E_b = 0$, leading to high starting current.; Lenz's Law explains the 'opposing' nature of the Back EMF.; Back EMF provides the inherent self-regulating property of a DC motor.

Advantages: Automatic control of armature current based on load; Essential for stable operation of DC machines

Disadvantages: Requires high starting current due to zero back EMF at start; Potential for commutator sparking if the machine is overloaded

Applications: DC series motors for traction; DC shunt motors for industrial drives

Additional Info: The magnitude of back EMF is proportional to speed ($E_b \propto N$).. Option A (self-induced emf) refers to inductance effects in a coil, whereas option B refers to transformer action.

Answer: C is correct — The emf induced in the armature of a DC motor is called back emf because it opposes the applied supply voltage.

Exam Tip: Remember that in a DC Generator, the induced EMF is the output voltage (generated EMF), while in a DC Motor, it is the Back EMF (opposing voltage).

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