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AC Dynamic braking in induction motor is done by
A) disconnecting the one phase from the source
B) disconnecting the All phases
C) by disconnecting the All phases from the line and connected it with DC Supply
D) reversing the phase sequence of the motor
disconnecting the one phase from the source
AC dynamic braking (also known as single-phase braking) in a 3-phase induction motor is achieved by disconnecting one of the stator phases from the AC supply while keeping the remaining two phases connected across the AC line. This single-phase supply produces a pulsating magnetic field rather than a revolving magnetic field, generating a net retarding torque that brings the motor to a stop.
AC dynamic braking (also known as single-phase braking) in a 3-phase induction motor is achieved by disconnecting one of the stator phases from the AC supply while keeping the remaining two phases connected across the AC line. This single-phase supply produces a pulsating magnetic field rather than a revolving magnetic field, generating a net retarding torque that brings the motor to a stop.
TeтАЛ=TfтАЛтИТTbтАЛ тАФ Net torque developed during single-phasing, where TfтАЛ is forward torque and TbтАЛ is backward torque
sbтАЛ=2тИТs тАФ Slip with respect to the backward rotating magnetic field
When one phase is disconnected, the single-phase AC current flows through the remaining two stator windings, creating a pulsating stationary magnetic field. According to double revolving field theory, this pulsating field resolves into two counter-rotating fields (forward and backward) with equal magnitude. Since the motor is initially running at a speed near synchronism, the backward rotating field produces a strong braking torque, while the net torque becomes negative (retarding) to slow down the rotor quickly.
AC dynamic braking is also referred to as single-phase braking.
It requires no additional external power supply (unlike DC dynamic braking).
The torque developed at zero speed is zero, meaning the motor cannot reverse direction.
Does not require an auxiliary DC power source.
Eliminates the risk of motor reversal near zero speed.
Produces lower braking torque compared to DC dynamic braking and plugging.
Unbalanced currents flow in stator windings during the braking period.
Small induction motor drives where moderate braking is sufficient.
Cranes and hoists as a preliminary slowing-down method before mechanical mechanical braking is applied.
| Feature | AC Dynamic Braking | DC Dynamic Braking |
|---|---|---|
Supply used for braking | Single-phase AC supply | Fixed DC supply |
External DC Source | Not required | Required |
Braking Torque | Comparatively lower | Higher and uniform |
Option B (Disconnecting all phases) simply cuts off power, causing the motor to coast to a stop due to inertia, which is simple coasting, not braking.
Option C refers to DC Dynamic Braking, where all AC phases are disconnected and a DC source is connected across the stator windings.
Option D refers to Plugging (or Phase Reversal braking), where two supply lines are interchanged to reverse the phase sequence.
A is correct тАФ AC Dynamic braking in an induction motor is performed by disconnecting one phase from the AC source while keeping the remaining two connected.
For competitive exams, remember: Plugging reverses phase sequence (s>1), DC Dynamic Braking uses DC supply to stator, and AC Dynamic Braking disconnects one phase from the source.