A transformer is a static electrical device that transfers energy by inductive coupling between its winding circuits.
The transformer is one of the simplest of electrical devices. Its basic design, materials, and principles have changed little over the last one hundred years, yet transformer designs and materials continue to be improved. Transformers are essential in high voltage power transmission providing an economical means of transmitting power over large distances. The simplicity, reliability, and economy of conversion of voltages by transformers was the principal factor in the selection of alternating current power transmission in the "War of Currents" in the late 1880's. In electronic circuitry, new methods of circuit design have replaced some of the applications of transformers, but electronic technology has also developed new transformer designs and applications.
Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to gigawatt units used to interconnect large portions of national power grids, all operating with the same basic principles and with many similarities in their parts.
Transformers alone cannot do the following:
- Convert DC to AC or vice versa
- Change the voltage or current of DC
- Change the AC supply frequency.
A simple transformer consists of two electrical conductors called the primary winding and the secondary winding. If a time-varying voltage
is applied to the primary winding of 
turns, a current will flow in it producing a magnetomotive force (MMF). Just as an electromotive force
(EMF) drives current around an electric circuit, so MMF drives magnetic
flux through a magnetic circuit. The primary MMF produces a varying magnetic flux 
in the core (shaded grey), and induces a back electromotive force (EMF) in opposition to . In accordance with Faraday's Law, the voltage induced across the primary winding is proportional to the rate of change of flux :
and 
. It thus follows that:
The EMF in the secondary winding, if connected to an electrical circuit, will cause current to flow in the secondary circuit. The MMF produced by current in the secondary opposes the MMF of the primary and so tends to cancel the flux in the core. Since the reduced flux reduces the EMF induced in the primary winding, increased current flows in the primary circuit. The resulting increase in MMF due to the primary current offsets the effect of the opposing secondary MMF. In this way, the electrical energy fed into the primary winding is delivered to the secondary winding.
Neglecting losses, for a given level of power transferred through a transformer, current in the secondary circuit is inversely proportional to the ratio of secondary voltage to primary voltage. For example, suppose a power of 50 watts is supplied to a resistive load from a transformer with a turns ratio of 25:2.
- P = E·I (power = electromotive force · current)
- Now with transformer change:
In a practical transformer, the higher-voltage winding will have more turns,of smaller conductor cross-section, than the lower-voltage windings.
Since a DC voltage source would not give a time-varying flux in the core, no back EMF would be generated and so current flow into the transformer would be unlimited. In practice, the series resistance of the winding limits the amount of current that can flow, until the transformer either reaches thermal equilibrium or is destroyed.
Comments
Post a Comment