17-09-2012, 03:45 PM
Transformer
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A transformer is a device that transfers electrical energy from one circuit to another throughinductively coupled conductors—the transformer's coils. A varying current in the first orprimary winding creates a varying magnetic flux in the transformer's core and thus a varyingmagnetic field through the secondary winding. This varying magnetic field induces a varyingelectromotive force (EMF), or "voltage", in the secondary winding. This effect is calledinductive coupling.
If a load is connected to the secondary, current will flow in the secondary winding, and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer
Electric Power Distribution
Before the transformer was developed, long-distance direct-current electric power distribution required the use of spinning motor-generator systems to step a high transmission voltage down to utility voltage for lighting. [5] G = power plant generator, M/D = motor-generator at customer's location, L = lights
Before the development of the transformer, electric power distribution primarily used direct current. It was difficult for a DC utility-power generation station to be more than a few kilometers from the user, because up until about 1897, light bulbs could only be effectively designed to operate at up to 110 volts maximum,[6], and up to 220 volts by 1917.[7]. It is expensive to send energy long distances at utility voltage (100-250 volts) due to the very high amperage of many customers, and the need for very thick transmission wires capable of handling the current.
It was understood that high voltages allowed long distance transmission with low amperage (250 volts at 5000 amps = 25000 volts at 50 amps) so the transmission wires can be smaller and less expensive, but it still needed to be stepped back down to utility voltage at the customer's location. At the time the only way to efficiently convert DC from one voltage to another was with a spinning motor-generator device, and this would be needed at each customer site. Each motor-generator has brushes constantly rubbing on a commutator, and axle bearings that need lubrication. The brushes wear out and need to be periodically replaced, and the commutator wears down and needs to be resurfaced, then the whole machine is rebuilt when the commutator wears too thin.
Detailed operation
The simplified description above neglects several practical factors, in particular, the primary current required to establish a magnetic field in the core, and the contribution to the field due to current in the secondary circuit.
Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero resistance.[39] When a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core.:[39] The current required to create the flux is termed the magnetizing current. Since the ideal core has been assumed to have near-zero reluctance, the magnetizing current is negligible, although still required, to create the magnetic field.
The changing magnetic field induces an electromotive force (EMF) across each winding.[40] Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP and VS measured at the terminals of the transformer, are equal to the corresponding EMFs. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF".[41] This is in accordance with Lenz's law, which states that induction of EMF always opposes development of any such change in magnetic field.