27-03-2014, 12:34 PM
Transformers
Function:
Transformers are extremely versatile devices that can be used to either step up and step down AC voltages or to step up and step down AC current. They can also allow AC to pass and block DC.
The most common type of transformer is the voltage transformer, which consists of two windings, the primary winding and the secondary winding. The windings are not electrically connected together, which is a safety feature in AC electrical circuits, but are wound on the same laminated soft iron core.
If an AC voltage is applied to the primary winding, the resultant changing flux links with the secondary winding. The changing flux is concentrated by the iron core and causes an EMF to be induced in the secondary winding. The magnitude of the EMF is proportional to the ratio of the number of turns between the primary and secondary windings.
Auto transformers (continue)
Auto transformers are a special type, since they have no electrical isolation between the primary and secondary windings. A single continuous winding is wound on a laminated iron core, where part of the winding is used as the primary, whilst the other part is used as the secondary, as shown below.
Current transformers
Current transformers differ from the voltage transformer, because the primary circuit consists of a supply feeder cable rather than a winding connected across a supply, as shown below.
In this arrangement, the alternating magnetic field associated with the load current is linked to the current transformer secondary winding via a laminated soft iron core, through which the feeder (primary) passes. The secondary current is used to feed a meter and typically registers the current flowing from an AC generator to the busbar or load. The secondary current can additionally be used to supply power meters and to monitor the load-sharing in an electrical circuit.