الدينامو والمولدات - الطاقة الكهربائية .. الفيزياء

Dynamo and Generators - Electrical Energy .. Physics

Dynamo and Generators

When an electric conductor moves within a magnetic field, an electric current is generated in it. The current generated in this way has been the source of most of the electricity allocated for domestic and industrial uses. The dynamo, or electric generator, is nothing but a device in which a conductor rotates within a magnetic field. In its simplest form, it consists of a permanent magnet that supplies it with the necessary field, and a single coil that rotates within the field, induced in it as a result of an electric current that is picked up by two carbon brushes that press on the slip rings. When the coil cuts the largest number of lines of force, that is, when its direction is horizontal (positions 2 and 4), the generated current reaches its maximum on the sides of the coil. When its direction is vertical (positions 1 and 3), in this case it does not cut any of the lines of force, and the current is zero in it. As it continues to rotate, each side of it cuts all the lines of force in opposite directions, so the resulting current is reversed after half a cycle. In general, the direction of the current is related to the field and rotation directions at the same time. We can see the total effect of one cycle in the drawing on the opposite page.

This type of current is called alternating, and it is known as a sine wave because of the equation that expresses it, which is Y = Y Q cos (Z), where Y represents the value of the current when the value of the angle between the coil and the horizontal line is Z, and Y Q represents the maximum value of the current. The maximum value of the current varies depending on the field strength and the speed of rotation of the coil.

It is worth noting that the power of the electric current is related to the square of its intensity (Y2m) and since the square of any negative quantity is positive, the power resulting from the negative half-wave is positive (blue line on the drawing). It is a mistake to rely on the maximum current value in calculating the current coming out of the generator because it is practically less than that if we take the entire cycle into account, and the effective current is defined as equal to the quantity YQ/square root 2, i.e. approximately three-quarters of the maximum current value.

(Above) The use of fuel in generating electricity.

(To the left) The angles (to the right) The current between the windings of a three-phase generator and the power in a three-phase device.

To obtain a direct current from the generator, the two slip rings must be replaced by a split ring called the commutator, whose task is to reverse the negative part of the sine wave, thus giving a smooth outgoing current whose smoothness increases once other coils are added to the generator and the commutator is divided into several parts.

Electricity is still difficult to store—the batteries and accumulators are usually small—but it is cheaper to transport from one place to another than to store it. For this reason, electricity has been the main source of energy that has sustained world civilization throughout this century. Electricity produced on the principles set out on pages 48 and 49 produces all the electricity for domestic and industrial consumption. More than 99% of generators are built in such a way that they are turned by coupling them to steam turbines. The rest are turned by water turbines (in hydroelectric projects), diesel engines, or steam turbines. The reason for distributing electrical energy in this way is that most good generators produce alternating currents that can be easily changed by transformers. The best method used to transmit alternating current over long distances is to use high voltage, low current, and three-phase groups. The use of high voltage and low current is necessary to reduce the amount of heat loss (related to the square of the current value, Y2m), and thus allows the use of thin wires. The use of three-phase devices is due to the fact that generators made of three separate coils are more efficient than generators made of a single coil.