When a transformer primary is energized from a voltage source and the
secondary is opened circuited, a small current will flow in the primary. This
current is usually less than 5 percent of the rated full load value of the
transformer. Since no current flows in the secondary winding . The primary may
be considered as a coil with a large reactance due to the iron core. Thus
causes a small current .
But when load is applied to the secondary terminals. In
accordance with Lenz’s law, the current that flows through the secondary
winding must act in such a direction as to oppose the flux set up by the
primary current. When the flux is momentarily reduced, the induced emf in the
primary winding is also reduced and therefore more current flows, in the
primary winding. When more load current flows in the secondary the primary
current will again increase.flux
How can the problem be remedied : No residual magnetism, reversed field connection and high resistance of field circuit are responsible for the failure of build up process of a dc generator. The factors are explained below: No residual magnetism: For the starting build up process requires some residual magnetism in the magnetic circuit then no voltage will be generated that can produce current flow through field circuit. Reversed field connection: The voltage induced inside the coil of field circuit due to residual magnetism that can permit the flow of current. For the following direction of this flux will be created and the direction of this flux will be same direction as the residual flux. If the field connections are reversed the lines of flux produced by current flow will oppose the residual flux, as a result generate voltage will decrease rather than increase. So voltage build up process failed. Field circuit resistance too high: A field
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