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?

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  1. Back EMF increases considerably while loading the motor.
  2. At no load, the back EMF is zero.
  3. Back EMF is not necessary for the electromechanical energy conversion in a motor.
  4. The back EMF decreases considerably while loading the motor.

Answer (Detailed Solution Below)

Option 4 : The back EMF decreases considerably while loading the motor.
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Detailed Solution

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Equivalent Circuit of DC motor

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\(E_b=V-I_aR_a\)

where, Eb = Back EMF

Ia = Armature Current

Ra = Armature Resistance

Back EMF (Electromotive Force) is the voltage generated in the armature of a DC motor that opposes the applied voltage. It is caused by the motor’s rotation and is proportional to the speed of the motor.

Explanation

At no load (Ia = 0), the motor runs at its highest speed, and hence, the back EMF is at its maximum value, nearly equal to the supply voltage, because there is very little load to oppose the rotation.

\(E_b=V-(0)R_a=V\)

When the motor is loaded, the speed of the motor decreases due to the increased torque requirement. As the speed decreases, the back EMF also decreases. This allows more current to flow through the armature, which in turn provides more torque to handle the load.

Therefore, when the motor is loaded, the back EMF decreases as the motor slows down to supply more current and torque.

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