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BRAKING

                                          BRAKING :

BRAKING :
                  If the load is removed from an electric motor and supplied to it be disconnected it will continue to run for sometime due to inertia. To avoid danger to the worker or damage to the products manufactured quick stopping of motor Is required. It is done by braking. The braking system should be reliable and quick in action. The braking torque must be controllable. 

There are two types of braking.

                  1. MECHANICAL BRAKING. 
                  2. ELECTRICAL BRAKING. 

MECHANICAL BRAKING :
                         In this method, the stored energy is dissipated as heat by a brake shoe or a brake lining which rubs against a brake drum. 

ELECTRIC BRAKING :
                          The stored energy of rotating parts is converted into electrical energy and dissipated in the resistance in the form of heat or returned to the supply in case of electric braking. The motors used should have suitable braking characteristics. In electric braking the driving motor operates as a generator during the period of braking and comes to standstill. It is possible to bring an equipment to rest by electric braking but in some applications it may not be possible to hold the equipment at rest. In such cases mechanical brakes are also provided for holding. 

FEATURES OF GOOD BRAKING SYSTEM :

The main features of a braking system are, 

(1) The braking should be quick and reliable in action. 

(2) The braking torque must be controllable. 

(3) Some suitable means must be provided for dissipation of kinetic energy of the rotating parts of the motor and its driving machines.

ADVANTAGES OF ELECTRIC BRAKING:

1. Electric braking is smooth
2. Electric braking is fast and cheap 
3. Heat produced in this method does not affect the system 4. Capacity of the system can be increased by the way of heavy loads and high speeds. 
5. In some cases, a part of output energy is returned to the supply during braking. Hence running cost is considerably saved. 
6. The system should have easy speed control. 
7. Maintenance is cheap. 
8. Wear and tear will not take place. 

DISADVANTAGES :

1. High initial cost 
2. It can not be appled to all types of motors 
3. Electric braking can stop the motor but it cannot hold it stationary. To hold it stationary mechanical braking is required. 

TYPES OF ELECTRIC BRAKING :
             
             1. Plugging 
             2. Dynamic braking (or) Rheostatic braking 
             3. Regenerative braking

PLUGGING :
                 Plugging means phase reversal. It is a simple method of electric braking. During the braking period reverse torque produce on the motor. A special device is required to cut ff the supply as soon as the motor comes to rest. This method of braking can be applied to both A.C and D.C motors. 



DYNAMIC BRAKING (OR) RHEOSTATIC BRAKING :
                          Dynamic braking is also called rheostatic braking. In this method of braking, a motor is disconnected from the supply and operated as a generator driven by the kinetic energy of the rotor. The kinetic energy of rotation is converted into electric energy, which is dissipated in the external resistance connected across the motor at the braking instant. The advantage of this type of braking is that no energy is required from the supply to brake the motor. This method of braking can be applied to brake the D.C motors, synchronous motors and induction motors. Dynamic. 


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.


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 fype of braking system, the initial cost of the system will be more. 
2. In case of D.C traction, the D.C. motor required is of higher size.




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