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REGENERATIVE BRAKING


                          REGENERATIVE BRAKING :


REGENERATIVE BRAKING :
                      In this type of braking the motor is not disconnected from the supply but remains connected to it . The machine is made to work in such a way that it generates energy and fed it back to the supply . This type of braking is better than the above two types because no energy is wasted . 


REGENERATIVE BRAKING APPLIED TO D . C SHUNT MOTOR:                        
         
                           (a) NORMAL OPERATION 

                           (b) BRAKING OPERATION 
                                          FIG 1.1
    If the emf induced in the motor is greater than the supply voltage power will be fed back into the supply . The emt induced in the motor depends upon the speed and excitation of the motor . If the field is disconnected from the supply and the field current is increased by exciting it by another source as shown in fig. 1.1( b ) , the induced emf will exceed the applied voltage and the motor will feed energy into the supply . The motor slows down to the speed corresponding to this new value of the field current since.


REGENERATIVE BRAKING APPLIED TO
 D.C SERIES MOTOR:                    
               
                          (a) NORMAL OPERATION 
                         (b) BRAKING OPERATION 
                                         FIG 1.2

Regenerative braking is not applied to ordinary in D.C series motor. in regenerative braking, at the instant of reversal of armature current, the emf induced in the motor will be small. Therefore the current flow through the field is in wrong direction. This will reverse the field and causes the motor emf to help the supply voltage. This will result in short circuit of the supply. Due to these complications, this method Is not used for common industrial purposes. This method of braking with series motor is used for traction purposes either by modification of winding or supplying the machines with separate excitation. Fig 1.2(a) and (b) shows the connections of a D.C series motor for regenerative braking using French method. It is provided with a main series winding and auxiliary field windings connected in parallel with the main series winding in a single D.C series motor. During braking period, the auxiliary windings are put in series with each other and switched across the supply as shown in fig 1.2 (b). The motor acts as a differentially compound generator.

REGENERATIVE BRAKING APPLIED TO 
INDUCTION MOTOR :
           When an induction motor runs above synchronous speed the emf induced in the rotor is greater than the supply voltage. Hence the motor works as an induction generator and feeds power back to the supply line. No extra auxiliaries are needed for this purpose. Its application is very useful to lift and hoist for a descending load at a speed slightly above synchronous speed. It is also advantageous on mountain railways

 ADVANTAGES OF REGENERATIVE BRAKING :
1. A part of energy is returned to the supply system.
2. The wear of brake shoe is small. This will increase the life of brake shoe. 
3. Higher value of braking retardation is obtained. Thus the braking period for retarding the train is reduced. 
4. Small amount of brake dust is produced when the mechanical brakes are applied. 
5. It is possible to run the train at high speeds on the gradient because of high braking retardation. 
6. It enhances the running safety. 

DISADVANTAGES OF REGENERATIVE BRAKING :
1. Because additional equipments are required with this type of braking system, the initial cost of the syste will be more. 2. In case of D.C traction, the D.C. motor required is of higher size.


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