A física dos transformadores elétricos e suas aplicações indispensáveis

The war of currents was a series of events surrounding the introduction of electrical power transmission systems in the late 1880s. On one side was Thomas Edison who advocated the large-scale use of direct current and on the other Nikola Tesla, who on the contrary, he advocated the use of high voltage alternating current. The outcome of this epic dispute, as we know, was favorable to Tesla and the key factor in his triumph was a useful magnetic device for converting voltages: the indispensable electrical transformer.

That's who this video will be about. Hello everyone, I'm Eudes Fileti. I'm a physicist and I work at the Federal University of São Paulo.

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TRANSFORMERS One of the most important applications of high induction and mutual induction, which we saw in the previous video, occurs in a transformer. A transformer is a device that is used to lower or raise an alternating voltage. In electrical applications it is often useful to be able to change the voltage from one value to another.

For example, high voltage lines can operate at voltages as high as 750,000 volts. But before this electrical power can be used at home, it must be lowered to 127 volts. Likewise, the 127 volts from a wall socket can be lowered back to 9 or 12 volts to power a cell phone battery.

On the other hand, some equipment, such as old tube TVs, required the voltage to be raised from 110 to 15 thousand volts, necessary for the device to work. Furthermore, to make an appliance compatible with your electrical network, it is often convenient to change the output voltage from 220 volts to 127 or vice versa. For all these voltage conversions, it is the transformer that does the work.

HOW THE TRANSFORMER WORKS The transformer consists of two coils, two windings electrically isolated from each other, but wound on the same core, a primary coil and a secondary coil, each with a certain number of turns on the wire. Here, the primary coil is the one that is connected to the generator. The coil core is typically made of a material such as iron, with a very high relative permeability.

This keeps the magnetic field lines almost completely within the core, which becomes strongly magnetized leading to a drastic increase in the mutual inductance of the two coils. The alternating current in the primary coil establishes a variable magnetic field in the iron core that greatly increases the magnetic field and drives the field lines to the secondary coil. If the core is well designed, almost all the magnetic flux that passes through the primary coil will also pass through the secondary coil.

As the flux through the coils is variable, then the electromotive force will be induced in both the coils according to Faraday's law. Although the electromotive force induced in both coils is a function of the same flux, they are proportional to the number of turns in each. So if we divide one equation by the other, the flux cancels and we obtain that the ratio between the induced electromotive forces is equal to the direct ratio between the numbers of turns in each coil.

This is the transformer equation. Thus, a turns ratio of say eight to one implies that the secondary coil has eight times as many turns as the primary coil. On the other hand, a turns ratio of one to 8 implies the opposite, that the secondary has one-eighth the number of turns of the primary.

In a high quality transformer the resistance of the coils is negligible so that the electromotive force in each one is essentially the same as its voltage. Therefore, the transformer equation can be written in terms of terminal voltages. So, if the number of turns in the secondary coil was less than the number of turns in the primary coil, the voltage will be reduced.

In this case, we are dealing with a voltage step-down transformer. On the other hand, if the number of turns in the secondary coil is greater, the voltage is increased and hence we are dealing with a voltage step-up transformer. ENERGY CONSERVATION Before you start thinking that transformers release energy from what they consume, which would violate the principle of conservation of energy, it is good to know the full story: As energy must always be conserved, the average power in the primary circuit, which is given by the product between current and voltage, must be the same as the power average in the secondary circuit.

This implies that the ratio between the currents in the primary and secondary coils will be inversely proportional to the ratio between the corresponding voltages. Therefore, if the voltage increases, the current decreases. For example, if the number of turns in the secondary coil is twice the number of turns in the primary coil, then the transformer will double the voltage of the secondary circuit and at the same time reduce the current by half.

This is consistent with the principle of Conservation of energy. POWER TRANSMISSION. TESLA x EDSON The use of Tesla's high voltage system allowed the alternating current system to transmit energy from the generating station, for longer distances, much more efficiently, with much less power loss than the direct current system of Edson.

As the resistance of a wire is directly proportional to its length, the longer the wire, the greater the energy dissipation. Thus, when electrical energy is transmitted over a long distance, the finite resistivity of the wires, which carry the current, becomes very significant. Think: If the current-carrying wire dissipates power as residual heat, which depends on the square of the electric current, then an obvious way to reduce this energy loss during transmission is to reduce the current.

In fact! For long-distance power transmission it is desirable to use the lowest possible current associated with the highest possible voltage. This reduces losses in transmission lines, allowing electrical transmission with maximum efficiency, saving many resources for both energy companies and consumers.

And this is where the transformer shows its value. In a typical situation, a plant produces a voltage of 12 thousand volts. This voltage is then increased 20 times, to 240 thousand volts, by a step-up transformer, 20 to one.

High voltage power is sent via long-distance transmission line. When it reaches the city, the voltage is reduced about 30 times to about 8 thousand volts at a substation by a step-down transformer, from one to 30. Before any domestic use, the voltage is reduced further to 220 or possibly 127 volts through another step-down transformer, which is usually mounted on a pole.

The energy is then distributed to the homes that consume it. Note that even the relatively low voltage supplied by a household wall socket is too high for many electronic devices, so an additional step-down transformer is required. This is the role of adapters used to recharge a cell phone or a laptop from the voltage line.

These adapters contain a step-down transformer that converts the line voltage to an even lower value typically 13 to 12 volts. Finally you might wonder why Edson didn't also use transformers in his system since they were so useful for Tesla. To see that this is not possible, remember that the operation of a transformer depends on a varying magnetic flux to create an induced electromotive force in the secondary coil.

If the current is constant in direction and magnitude, as is the case with direct current, then there would simply be no electromotive force. This video ends here. If you liked it, give it a like and subscribe to the channel.

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