Which of the following phenomena will happen during the breaking period?

Question

Examoogle · Accepted Answer

Option D: All of these

Explanation & Concepts:
Electric braking in electric motors is a method used to bring the motor and its connected load to a quick stop or slow down rapidly. During the braking period, the motor acts as a generator, producing an opposing (negative) electromagnetic torque that opposes rotation. Consequently, the motor slows down to a standstill while converting the stored kinetic energy into heat (in dynamic braking) or feeding it back into the electrical supply (in regenerative braking).

Working Principle: When braking is initiated, the direction of electromagnetic torque is reversed relative to the direction of rotation, resulting in negative torque ($T_{e}

Key Formulas: $T_{b} = -T_{e}$ — braking torque acts in opposition to the direction of motion; $E_{k} = \frac{1}{2}J\omega^2$ — stored rotational kinetic energy in the motor and load; $\frac{d\omega}{dt} = \frac{T_{L} - T_{b}}{J}$ — angular deceleration equation during braking

Key Points: Negative torque is produced because the electromagnetic torque opposes the direction of motor rotation.; The kinetic energy stored in the rotor and connected mechanical load is either dissipated as heat ($I^2R$ losses) or regenerated back into the AC/DC source.; Electric braking ensures quicker stopping, reduced mechanical wear, and precise position control.

Advantages: Rapid stopping time compared to natural coasting to rest.; Eliminates mechanical wear and tear associated with physical brake shoes or pads.; Regenerative braking option allows recovery of energy, improving overall drive efficiency.

Disadvantages: Plugging and dynamic braking generate significant heat in the motor windings or external resistors.; Requires additional control equipment, contactors, or power electronics converters.

Applications: Electric traction systems (trains, trams, and electric vehicles).; Cranes, hoists, and elevators requiring quick and controlled stopping.; Industrial rolling mills, machine tools, and centrifuges.

Additional Info: Three main types of electric braking exist: Regenerative Braking (energy fed to power grid), Dynamic/Rheostatic Braking (energy dissipated across external resistors), and Plugging/Reverse Current Braking (phase reversal creating heavy retarding torque).. Option A, Option B, and Option C all describe accurate physical aspects of the electrical motor braking process, making Option D the correct answer.

Answer: D is correct — During braking, negative torque opposes rotation, stored kinetic energy converts into heat or electrical energy, and the motor eventually reaches a standstill.

Exam Tip: In competitive exams, remember that Plugging offers the fastest braking action but wastes maximum energy as heat, whereas Regenerative braking is the most energy-efficient mode but requires the motor speed to exceed synchronous speed ($N > N_s$) or rely on an active inverter.

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Option A: The motor came at standstill

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Option B: negative torque will be produced in the motor

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Option C: Kinetic energy stored by the motor is converted into Heat or Electrical Energy

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