High Voltage Reduces Power Losses
In order for the electrical power distribution network to function, voltages must be stepped up before power is transmitted great distances over power lines. One major problem is that power is lost between the power plant and the consumers because currents use some of the power to heat the transmission lines. The power transmitted along the line is equal to the voltage times the current. The higher the voltage the lower the current that must flow within the transmission lines to deliver the same power. Lower currents produce much less heating and much less power loss. Of course, the high voltages (needed to drive the low currents) must be stepped back down before power is supplied to our homes. Transformers are the critical elements that step up and down the voltages at each end of the line.
What is a Transformer?
A transformer is just a piece of iron with a pair of wires coiled around it - one with many more turns in the coil than the other. The coils of wire are not physically connected. The iron core is immersed in an insulating oil bath which does not conduct electricity well.
How Does It Work?
The basic physical process underlying the operation of transformers is electromagnetic induction. If a conductor, such as a copper wire, is sitting in a magnetic field that is changing, a current will flow in the conductor. This current will not be steady but will also be changing. Alternatively, if a changing current is present, it will produce a changing magnetic field. The typical current flowing in our homes and in power distribution networks changes direction 60 times every second. Around every wire, through which this current flows, a magnetic field is produced.
A transformer works only with AC (alternating current) circuits. The changing AC current enters the primary coil of wire in the transformer. A magnetic field is produced that is concentrated in the iron core of the transformer. A secondary coil of wires (also conductors) is wrapped around the iron core, not physically touching the first set of wires. The changing magnetic field produced by the first coil is experienced by the second coil and current begins to flow in these wires as well. The second coil has many more turns of wire and offers a higher resistance to the current flow than the first coil. The greater resistance means that a larger voltage drop (than is present across the first coil of wire) is produced from one end of the coil to the other. Therefore a low voltage enters the transformer and a high voltage exits, or vice versa.
How Power Moves Through the Transformer
But the transformer is a passive device -- it cannot add power. Power is equal to the product of the voltage and the current. If the voltage increases the current drops. A high voltage and low current exits the transformer carrying almost the same amount of power along the transmission lines that the initial low voltage and high current did. Most transformers operate at high efficiency, under normal conditions, transmitting about 99% of the power that enters them. (About 1% of the power is lost in heating the transformer.)