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In AC Dynamic braking of induction motor, negative (opposite) torque produced due to
A) negative slip
B) Induction Generator Mode
C) Unbalancing
D) DC generator Mode
Unbalancing
In AC dynamic braking (also known as single-phase braking) of a three-phase induction motor, the stator winding is disconnected from the three-phase AC supply and connected to a single-phase AC supply. This single-phase connection creates an unbalanced magnetic field (or two counter-rotating fields), which produces a net negative (opposing) braking torque that slows down the motor.
In AC dynamic braking (also known as single-phase braking) of a three-phase induction motor, the stator winding is disconnected from the three-phase AC supply and connected to a single-phase AC supply. This single-phase connection creates an unbalanced magnetic field (or two counter-rotating fields), which produces a net negative (opposing) braking torque that slows down the motor.
TnetтАЛ=TfтАЛтИТTbтАЛ тАФ Net torque due to forward (TfтАЛ) and backward (TbтАЛ) rotating fields
sbтАЛ=2тИТs тАФ Slip corresponding to the backward rotating field
According to Double Field Revolving Theory, a single-phase AC magnetic field can be resolved into two equal and opposite rotating magnetic fields: a forward rotating field (s) and a backward rotating field (2тИТs). Because the supply is unbalanced, the backward rotating field produces a strong counter-torque (opposing torque) against the rotor's direction of rotation, thereby bringing the motor dynamically to a stop.
AC dynamic braking uses a single-phase AC supply across stator terminals instead of a three-phase supply.
The unbalanced magnetic field breaks down into forward and backward components; the backward component provides negative braking torque.
External resistance is often inserted into the rotor circuit to maximize braking torque and limit heating.
Does not require a separate DC source unlike standard DC dynamic braking.
Smooth deceleration without sudden mechanical shocks.
High rotor and stator heating due to unbalanced single-phase operation.
Braking torque reduces to zero as motor speed approaches zero, requiring mechanical brakes for holding.
Used in induction motor drives where continuous DC power source is unavailable for dynamic braking.
Cranes, hoists, and industrial machinery needing controlled deceleration.
Option A (Negative slip): Negative slip corresponds to Induction Generator Mode operating above synchronous speed (s<0), which occurs during regenerative braking.
Option B (Induction Generator Mode): Occurs when rotor speed exceeds synchronous speed (N>NsтАЛ), producing regenerative braking rather than dynamic braking.
Option D (DC generator Mode): Applies to DC dynamic braking where a DC current is injected into the stator, turning the machine into a synchronous/DC generator dissipated via rotor resistors.
C is correct тАФ AC dynamic braking produces negative torque due to the unbalancing created by connecting single-phase AC to the induction motor stator.
Remember: Regenerative braking occurs when slip s<0 (N>NsтАЛ), DC Dynamic Braking requires DC injection, and AC Dynamic Braking relies on unbalancing single-phase AC fields.