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PLUGGING

                                        PLUGGING :


PLUGGING :
                       Plugging means phase reversal. It is a simple method of electric braking. During the braking period reverse torgue produce on the motor. A special device is required to cut of the supply as soon as the motor comes to rest. This methed of braking can be applied to both A.C and D.C motors, 
Plugging applied to D.C motor :
                         In D.C motor the direction of torque depends on the direction of rotation of the armature. Therefore the torque developed in D.C machine is reversed either by inter changing the field terminals or the armature terminals. But usually the connections of the armature are interchanged. 


Plugging applied to D.C shunt motor 

           
                              (a) NORMAL OPERATION
                                (b) BRAKING OPERATION
                                               fig 1.1

                         Fig 1.1 (a) shows the normal working of the shunt motor. In this case back emf (E) opposes the applied voltage (V). The armature current flows from A to B. When plugging is applied the armature connections are interchanged as shown in fig 1.1 (b). When the armature connections are reversed the applied voltage V and back emf E direction, Hence at the instant of reversal of armature are in same connections a voltage equal to (V + E) is available acorss the armature terminals, (namely twice the supply voltage), which cause a heavy current to flow around the circuit. Hence in order to limit the heavy current to the safey valve during the braking period, additional resistance R is inserted in the armature circuit as shown in fig 1.1 (b). 

Plugging applied to D.C series motor :
           
                        (a) NORMAL OPERATION 
                        (b) BRAKING OPERATION 
                                           FIG.1.2
 In D.C series motor while plugging the field current is also reversed. This results the production of the torque the same direction as it was working normally. Therefore in order to develop torque in the reverse direction, the direction of field current should remain unchanged. Fig. 1.2 (a) shows the connection of the D.C series motor before plugging is applied. 


As shown in fig 1.2 (b) the direction of field current remains unchanged whereas direction of armature current gets reversed. Now a total voltage (V + E) is available across the armature terminals i.e nearly twice of the supply voitage. It causes heavy current to flow around the circuit, Hence the additional resistance R is connected in series to limit the current. This method is simple and efficient but power is wasted in the resistance. 

Plugging applied to Induction motor :
             
                             (a) NORMAL OPERATION 
                             (b) BRAKING OPERATION 
                                               FIG. 1.3
 Plugging is achieved in induction motor by reversing any two of the three phases interchanged as shown in fig.1.3(b). When reversing any two phases the rotation of magnetic field is reversed. Therefore the torque produced in the rotor also will be reversed. At the instant of plugging the relative speed between the rotor conductors and the magnetic field will be approximately twice the synchronous speed. So voltage induced in the rotor will be twice that of normal induced voltage, which causes to flow abnormal current in the stator and rotor. Therefore additional insulation is provided for the windings in order to withstand this high voltage. High resistance may have to be inserted in the rotor or stator circuit for the purpose of protection. 

Plugging applied to synchronous motor :
            When three phase voltage is supplied to the stator of synchronous motor a rotating magnetic field is produced in the stator. D.C voltage is applied to the rotor tield windings producing permanent magnetic field. the direction of rotating magnetic field is reversed by interchanging the two phases of the stator then the braking torque is not produced. Therefore in the case of plain synchronous motor plugging is not possible. But in the case of self started synchronous motor the braking effect will be produced due to eddy current induced damper windings provided for starting purpose.


               
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