DC Motor Burnout on Startup – Rankers Physics
Topic: Electromagnetic Induction
Subtopic: Motional EMF

DC Motor Burnout on Startup

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

Solution:

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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