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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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DC Dynamic braking of induction motor is done by
A) disconnecting the one phase from the source
B) disconnecting the All phases
C) disconnecting the All phases from the line and connected it with DC Supply
D) reversing the phase sequence of the motor
disconnecting the All phases
DC dynamic braking of an induction motor is achieved by disconnecting all three stator phases from the AC power line and connecting a DC supply across two or three of the stator terminals. This converts the stationary stator into a DC electromagnet that establishes a stationary magnetic field.
DC dynamic braking of an induction motor is achieved by disconnecting all three stator phases from the AC power line and connecting a DC supply across two or three of the stator terminals. This converts the stationary stator into a DC electromagnet that establishes a stationary magnetic field.
Tb∝Idc2⋅Rr — Braking torque proportional to square of DC current and rotor resistance
Ploss=Ir2Rr — Kinetic energy dissipated as heat in the rotor circuit
When the stator is energized with DC, it creates a stationary magnetic field in the air gap. As the rotor continues to rotate due to inertia, its conductors cut this stationary magnetic flux, inducing currents in the rotor winding. The interaction between the stationary magnetic field and the induced rotor currents produces a counter-torque (braking torque) that rapidly brings the motor to a stop. The kinetic energy of the rotating mass is dissipated as electrical heat in the rotor resistance.
Braking torque drops to zero as speed approaches zero, so a mechanical hold brake is required to hold the load at rest.
Braking magnitude can be smoothly controlled by varying the magnitude of the applied DC voltage/current.
DC supply voltage required is low because it only needs to overcome the DC resistance of the stator windings.
Smooth and controlled deceleration without harsh mechanical mechanical wear
No risk of motor reversing direction at zero speed (unlike plugging)
Requires a separate auxiliary DC power source
Braking torque diminishes as speed drops, becoming zero at standstill
Machine tools and lathes requiring controlled stopping
Cranes and hoists during deceleration phase
| Feature | DC Dynamic Braking | Plugging (Reverse Current) |
|---|---|---|
Supply Requirement | Auxiliary DC Source required | 3-Phase AC Source (reversed) |
Energy Efficiency | Moderate energy loss as heat in rotor | High energy loss (line + kinetic energy) |
Behavior at Zero Speed | Comes to rest, torque drops to zero | Will reverse direction unless disconnected |
Option A (disconnecting one phase) results in single-phasing, which causes unbalanced operation and reduced torque rather than proper braking.
Option B (disconnecting all phases) allows the motor to coast to a stop naturally under mechanical friction, which is not braking.
Option D (reversing phase sequence) defines Plugging, another electric braking method where reverse torque is produced.
C is correct — DC dynamic braking requires disconnecting the 3-phase AC line supply from the stator and energizing it with a DC supply.
For competitive exams, remember: Plugging causes maximum energy loss and reverse rotation risk; DC Dynamic Braking requires a DC supply and cannot reverse; Regenerative Braking feeds power back to the AC grid when speed exceeds synchronous speed (N>Ns).