Machine Parts

Assume that two generators and a load are connected parallely. Let us assume that generator 1 is operating and supplying current to the load. The polarity of the voltage at terminals of the generator is indicated in the diagram. The load of generator 1 now increases to the point where the rated current is exceeded. The second generator must be placed in operation to share the load. The prime mover of the second generator started and voltage builds up in generator 2. Before anything else is done after the voltage of generator 2 is build up. The polarity of generator 2 should be compared with the running generator 1. Since both of   these generators are to be connected in parallel. The positive terminal of each generator is to be connected together and the negative terminals of two generators are connected together. If the generators are not properly connected as just indicated, the improper connection will place both generators in series, with only the internal resistance of

The shunt generator is not satisfactory for supplying power to remotely located points because not only does the terminal voltage of the generator drop as load increases but there is also a volt drop in the line. The greater the load, the greater the volt drop in line. The combined effect of the reduction in generator voltage plus the line drop makes the shunt generator unsatisfactory for transmission of power over long distances. To overcome the reduction in generator voltage we can use cumulative compound generator. The cumulative compound generator is the same as a shunt generator with the addition of a series field winding which produces flux to aid the flux of the shunt field. We can use cumulative compound generator for the following characteristics. ·          The drop in the terminal voltage is small. ·          Maintaining the constant terminal voltage.           The terminal voltage rises as load as is applied to generator

The generator is failed to build up for the following reason:- ·          No residual magnetism:   if there is little or no residual magnetism, because of inactivity or jarring in shipment, no voltage will be generated that can produce field current. ·          Field connection reversal: the voltage generated due to residual magnetism is applied to the field and current flow in such a direction as to produce lines of flux in the same direction as the residual flux. If the field connections are reversed, the lines of the flux produced by the current flow will oppose the residual flux so that the generated voltage will decrease rather than increase. ·          Field- circuit resistance too high:   if the field current resistance is greater than the critical voltage. Then this prevents voltage buildup. ·          Dirty commutator:   a dirty commutator does not permit good contact between brushes and the commutator. This poor contact shows up as a high resistance to

Commutation:                                         Commutation is the process a coil undergoes after leaving the magnitude field of one pair of poles and just before entering the magnetic field of the next pair poles. Converting ac voltages on a machines’ armature to dc voltages by commutator :                            This function is performed by a commutator. The ends of the coil are connected through the commutator to the brushes which lead to the external circuit.   An explanation of fig 1 will clarify this process while the coil moves from position 1 to position 2 the brushes remain in contact with the commutator segment and the current direction remain as indicated although the magnitude decreases.    At position 2 of fig 1 the voltage induced in the coil is zero, and the current in the external circuit is also zero. At this instant segment A leaves brush 1and makes contact with brush 1. As the coil moves from position 2 to position 3, segment

Armature reaction:                                              When current flows in the armature conductors a flux surrounds these conductors. The direction of this armature flux is such that it reduces the flux from the field poles, resulting reduction in both generated voltage and terminal voltage. So we can say that the interaction between armature flux and field flux is called armature reaction. Effects on a loaded generator:                                                  When a load is connected to a generator, of course current flow exists. When current flows through a conductor a magnetic field is set up around the conductor. When several conductors are placed together, the flux from each conductor is combined to produce a resultant flux. The flux from the conductor on the left side of the armature and the flux from the conductor on the right side of the armature cause a resultant flux in the centre of the armature that is downward in direction. There

The important affecting variables are given below: Radius on punch: A general rule to prevent excessive thinning is to design the punch with a radius of from 4 to 10 times the metal thickness. Otherwise a sharp radius will require higher forces when the metal is folded around the punch nose & may result in excessive thinning on tearing at the bottom of the cup. Draw radius on die: Theoretically the radius on the draw die should be as large as possible to permit fall freedom of metal flow as it passes over the radius. If the draw radius is too large, the metal will be released by the blank holder too soon & wrinkling will result. To sharp a radius will hinder the normal flow of metal & cause uneven thinning. Friction: The force of static friction between the work piece blank & draw die surface must be overcome in a drawing operation. Since the blank holder pressure causes higher frictional forces, it should be only high enough to prevent wr