Back EMF is a significant quantity during operation of a DC motor. Which of the following statements regarding

Question

Back EMF is a significant quantity during operation of a DC motor. Which of the following statements regarding the concept of Back EMF is correct?

Examoogle · Accepted Answer

The correct answer is Option A: The back EMF decreases considerably while loading the motor.. Back EMF is the EMF induced in the armature conductors of a DC motor as they rotate in the magnetic field, opposing the applied supply voltage. When a motor is loaded, its speed decreases slightly, which directly reduces the Back EMF, allowing more armature current to flow to meet the increased load demand.

Step-by-Step Solution & Concepts:
Back EMF is the EMF induced in the armature conductors of a DC motor as they rotate in the magnetic field, opposing the applied supply voltage. When a motor is loaded, its speed decreases slightly, which directly reduces the Back EMF, allowing more armature current to flow to meet the increased load demand.

Working Principle: The principle is governed by Lenz's Law, where the induced EMF opposes the cause (the applied voltage). As the load increases, the electromagnetic torque required to drive the load increases, causing the motor speed to drop. Since $E_b = \frac{P\Phi Z N}{60A}$, a reduction in speed ($N$) results in a proportional decrease in Back EMF ($E_b$). Consequently, the net voltage ($V - E_b$) increases, which draws more current ($I_a = \frac{V - E_b}{R_a}$) to develop the required torque.

Key Formulas: $E_b = \frac{P\Phi Z N}{60A}$ — Back EMF equation of a DC motor; $I_a = \frac{V - E_b}{R_a}$ — Armature current equation based on Back EMF

Key Points: Back EMF is essentially a self-regulating mechanism in a DC motor.; It acts as a governor, adjusting the armature current automatically based on load requirements.; Without Back EMF, the armature current would be dangerously high, potentially damaging the windings.

Advantages: Provides self-regulation of current based on load demand.; Prevents the motor from drawing excessive current under steady-state conditions.

Disadvantages: Limits the maximum speed of the motor.; Necessitates the use of a starter for DC motors to prevent high inrush current at standstill.

Applications: DC shunt motors; DC series motors; DC compound motors

Additional Info: At starting (speed N=0), Back EMF is zero, which is why a starter is used to limit the starting current.. Option B is incorrect because at no load, Back EMF is at its maximum (slightly less than supply voltage).. Option C is incorrect because Back EMF is essential; it differentiates a motor from a simple resistor.. Option D is incorrect because loading causes a speed drop, leading to a decrease, not an increase, in Back EMF.

Answer: A is correct — The Back EMF decreases as the motor speed drops due to the increased mechanical load, allowing higher current to sustain the required torque.

Exam Tip: Always remember the relation $I_a = \frac{V - E_b}{R_a}$ when solving problems related to DC motor speed regulation; if load increases, $N$ decreases, $E_b$ decreases, and $I_a$ must increase to balance torque.

Examoogle · Answer

Option B: At no load, the back EMF is zero.

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Option C: Back EMF is not necessary for the electromechanical energy conversion in a motor.

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Option D: Back EMF increases considerably while loading the motor.

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