In which of the following braking, we can recover kinetic energy stored in the rotor.

Question

Examoogle · Accepted Answer

Option C: Regenerative Braking

Explanation & Concepts:
Regenerative braking occurs when the motor operates as a generator, converting the stored kinetic energy of the rotor and driven load back into electrical energy and returning it to the supply mains. This occurs when the rotor speed ($N$) exceeds the synchronous speed ($N_s$), making the slip negative ($s

Working Principle: When the rotor speed increases above the synchronous speed due to an overhauling load or a reduction in supply frequency, the relative speed between the rotor conductors and the rotating magnetic field reverses. This changes the direction of the induced rotor electromotive force (EMF) and current, reversing the electromagnetic torque to act as a braking torque while feeding power back to the AC grid.

Key Formulas: $s = \frac{N_s - N}{N_s}$ — Slip equation (negative during regenerative braking when $N > N_s$); $P_m = (1-s) P_g$ — Mechanical power (negative value indicates mechanical energy conversion to electrical power); $T_e = \frac{3}{2\pi N_s} \cdot \frac{s E_2^2 R_2}{R_2^2 + (s X_2)^2}$ — Electromagnetic torque (reverses direction when $s

Key Points: Regenerative braking is the most energy-efficient braking method as it recovers kinetic energy instead of wasting it as heat.; It can only occur naturally when the speed of the motor exceeds its synchronous speed ($N > N_s$).; It is widely used in electric traction, elevators, and cranes during overhauling loads or downhill movement.

Advantages: High energy efficiency as kinetic energy is fed back into the supply grid.; Reduces overall operating costs and minimizes mechanical wear on conventional friction brakes.; No external braking resistors or power dissipation devices are needed during operation.

Disadvantages: Cannot stop the motor completely or hold it at zero speed ($N = 0$).; Cannot be applied if the power grid is incapable of absorbing the regenerated power.

Applications: Electric traction systems (trains, trams, and electric vehicles).; Elevators, hoists, and cranes during lowering of heavy loads.; Variable Frequency Drive (VFD) controlled induction motor applications.

Additional Info: Plugging (Reverse Voltage Braking) involves reversing two supply phases, dissipating energy as heat through resistors and causing high electrical stress.. Dynamic Braking converts kinetic energy into heat by disconnecting the AC supply and connecting a braking resistor across the stator windings DC excitation.. Only Regenerative Braking successfully converts and feeds kinetic energy back to the source.

Answer: C is correct — Regenerative braking converts the stored kinetic energy of the rotor into electrical energy and feeds it back to the supply system when the motor speed exceeds synchronous speed.

Exam Tip: In competitive exams, remember that regenerative braking requires $N > N_s$ ($s

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Option A: Plugging or Reverse Voltage Braking

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Option B: Dynamic Braking

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Option D: All of these

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