EDUCATION

                              DYNAMIC BRAKING   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 BRAKING TO INDUCTION MOTOR :                                                                  fig 1.1                In 3 phase squirrel cage induction motor, the stator is disconnected from the A.C supply mains and D.C supply is given to the stator winding as shown in fig. 1.1(b). A st

                          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 i

                                         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              

                                          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

   SELECTION OF MOTORS FOR DIFFERENT DUTY CYCLES : 1. Continuous duty cycle :                          It denotes the motor operation at a constant load torque to reach steady state temperature. The load time and. temperature time graph are shown in fig 1.1 EXAMPLES :               Paper mill drives compressors, conveyers, centrifugal pumps and fans                                              fig. 1.1 2. Short time duty :                   It denotes the operation of motor at constant load for short period followed by rest to cool down to the original starting temperature. Short time duty timings are generally 10, 30, 60 and 90 minutes. The load time and the temperature time graph are shown in fig 1.2  EXAMPLES :               Crane drivers, drives for household appliances, sluice gate drives, valve drives and machine tool drives                                              fig 1.2 4. Intermittent period duty with starting :

                               FACTORS GOVERNING                                              THE                              SELECTION OF MOTORS :                 The following factors are taken into consideration while selecting a motor for industrial applications.  1. ELECTRICAL CHARACTERISTICS  a) Running.characteristics b) Starting characteristics c) Speed control  d) Braking  a) RUNNING CHARACTERISTICS :                                     The running characteristics of a motor include the speed-torque or the speed-current characteristics, losses, magnetising current, efficiency and power factor at various loads. The magnetising current and power factor are to be considered in case of A.C motors only.  b) STARTING CHARACTERISTICS :  The starting torque developed by a motor should be sufficient to start and, accelerate the motor at its load to the rated speed in a reasonable time. Some motors may have to start against full load torque. e.g-mot