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Which of the following thing must keep in mind during Plugging?
Change phase sequence - When motor reached near zero speed
Disconnect Supply - When the motor reached a synchronous speed
Change phase sequence - When the motor reached a synchronous speed
Disconnect Supply - When motor reached near zero speed
Disconnect Supply - When motor reached near zero speed
Plugging (or reverse current braking) is an electric braking method where the phase sequence of a three-phase induction motor is reversed by swapping any two stator leads. This reverses the direction of the rotating magnetic field, producing a strong braking torque in opposition to the rotor's rotation. To prevent the motor from starting and accelerating in the reverse direction, the supply must be disconnected via a zero-speed switch as soon as the motor speed drops near zero.
Plugging (or reverse current braking) is an electric braking method where the phase sequence of a three-phase induction motor is reversed by swapping any two stator leads. This reverses the direction of the rotating magnetic field, producing a strong braking torque in opposition to the rotor's rotation. To prevent the motor from starting and accelerating in the reverse direction, the supply must be disconnected via a zero-speed switch as soon as the motor speed drops near zero.
spluggingтАЛ=2тИТsmotoringтАЛ тАФ Slip during plugging phase, which ranges between 1 and 2
TbтАЛтЙИTstartтАЛ тАФ Initial braking torque during plugging is higher than normal operating torque
When two stator phases are interchanged, the synchronous speed changes from +NsтАЛ to тИТNsтАЛ, while the rotor continues to spin in the forward direction at +N due to inertia. Consequently, the slip becomes s=тИТNsтАЛтИТNsтАЛтИТNтАЛ=1+NsтАЛNтАЛ, which is greater than 1. This produces a heavy braking torque that rapidly decelerates the motor to a standstill.
Plugging provides quick stopping (rapid braking) compared to mechanical or dynamic braking.
A Zero-Speed Switch (Plugging Switch) is mechanically coupled to the motor shaft to trip the supply contractor when speed approaches zero.
Enormous rotor I┬▓R losses occur during plugging as electrical power from the supply plus mechanical energy from inertia are dissipated as heat.
Provides extremely fast motor deceleration and stopping.
Simple circuit configuration requiring only contactor switching.
Inefficient as kinetic energy and supply energy are completely wasted as heat.
High current peaks during braking subject windings to thermal stress.
Rolling mills and machine tools requiring rapid stopping.
Elevators, cranes, and printing presses for emergency braking.
Option A is incorrect because changing the phase sequence is the initiation step of plugging, not the final requirement near zero speed.
Option B and C are incorrect because plugging is used to stop the motor from running conditions, not when it reaches synchronous speed.
D is correct тАФ Disconnecting the supply when the motor reaches near zero speed prevents the motor from accelerating in the reverse direction.
Always remember for exam questions: Slip during motoring is 0<s<1, during regenerative braking s<0, and during plugging 1<s<2.