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Which of the following is/are the must condition during Self-Excited Dynamic Braking of three-phase induction motor?
A) Disconnect All phases from Line
B) Three capacitors are permanently connected to the motor
C) The capacitor must enough to supply reactive power
D) All of these
All of these
Self-excited dynamic braking (or capacitive braking) of a three-phase induction motor requires disconnecting the motor from the AC supply line and connecting a capacitor bank across its stator terminals. The kinetic energy of the rotating rotor provides mechanical input while the connected capacitors supply the necessary reactive power (Q) for self-excitation, establishing a rotating magnetic field to produce braking torque.
Self-excited dynamic braking (or capacitive braking) of a three-phase induction motor requires disconnecting the motor from the AC supply line and connecting a capacitor bank across its stator terminals. The kinetic energy of the rotating rotor provides mechanical input while the connected capacitors supply the necessary reactive power (Q) for self-excitation, establishing a rotating magnetic field to produce braking torque.
XCтАЛ=2╧АfC1тАЛтЙдXmтАЛ тАФ Condition for self-excitation (Capacitive reactance must be less than or equal to magnetizing reactance)
QCтАЛ=3V2╧ЙC тАФ Reactive power supplied by delta-connected capacitors to stator
When the stator is isolated from the 3-phase line and connected to capacitors, residual magnetism in the rotor core induces a small voltage in the stator windings. This voltage drives a capacitive current through the capacitor bank, which leads the voltage by 90┬░ and supplies magnetizing reactive power. This reinforces the air-gap magnetic flux, building up stator voltage through a process analogous to self-excitation in a DC shunt generator. As the motor rotates, kinetic energy is dissipated as electrical energy in the stator/rotor resistances, bringing the machine to a smooth stop.
The stator must be disconnected from the mains supply to prevent short-circuiting or power feed-back.
Capacitors must be permanently or selectively switched across stator terminals during the braking cycle.
Presence of residual magnetism in the rotor core is mandatory to initiate voltage buildup.
The minimum capacitance value must be sufficient to supply the required magnetizing reactive VARs.
No external DC auxiliary supply required (unlike conventional DC dynamic braking).
Energy dissipated is purely proportional to speed, producing smooth deceleration without mechanical shock.
Braking torque drops rapidly as motor speed decreases, collapsing entirely at lower speeds (around 20-30% of rated speed).
Requires large and expensive AC capacitors for effective braking.
Emergency stopping of industrial drives where external power failure may co-occur.
Textile mill drives and machine tools requiring controlled deceleration without external DC sources.
Option A is correct because isolating the motor from the AC line is essential before applying self-excitation.
Option B is correct because capacitors connected across the terminals provide the leading current required for excitation.
Option C is correct because if capacitance C is too small, the magnetizing curve will not intersect the capacitor IтИТV line, preventing self-excitation.
D is correct тАФ All listed conditions (isolating the mains, connecting capacitors, and ensuring adequate capacitive reactive power capacity) are mandatory for self-excited dynamic braking.
For competitive exams (GATE/ESE/SSC JE), remember that self-excitation fails if the motor lacks residual magnetism or if the connected capacitance is less than the critical capacitance (C<CcritтАЛ).