DC Motor Burnout on Startup – Rankers Physics

Motional EMF: Practice Problem & Solution

Assertion (A): The probability of burn out of a dc motor is maximum, when the motor is just switched on. Reason (R): No back emf is developed in the armature of dc motor, when it is just switched on.  
Both (A) & (R) are true and the (R) is the correct explanation of the (A)
Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
(A) is true but (R) is false
Both (A) and (R) are false

Short Solution (Best for NEET):

To solve this problem, we apply the core principles of Motional EMF. Understanding the underlying formula is key to arriving at the correct answer below:

Assertion (A) is true: When a DC motor starts, its speed is zero, thus the back EMF (\(epsilon_b\)) is zero. This leads to the maximum current (\(I = \frac{V - \epsilon_b}{R_a}\)) drawn from the supply, which can cause burnout. Reason (R) is true: Back EMF is proportional to the motor's angular speed (\(epsilon_b = k\Phi\omega\)), so it is zero at startup (\(omega = 0\)). Reason (R) correctly explains Assertion (A).

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