Dynamo: the principle of operation of a DC generator, features of the device, the role of an armature and a collector

Dynamos are similar to alternating current devices in that they convert mechanical energy the electrical requires the same components: the stator (stationary part) and the generator armature (rotating element). The same structural elements are also applicable to the creation of a DC motor. Therefore, due to their complete reversibility without any changes, such generators are called DC machines.

History and evolution

Dynamos were the first electrical machines capable of producing energy for industry, and the foundation on which many other rotary devices for converting mechanical and electrical power were designed, including electric motor. The theory of operation of electromagnetic generators was created by Michael Faraday in 1832.

He also built the first device known as the Faraday disk. This device created a low DC voltage, for the production of which was used the rotation of a copper disk between the poles of a horseshoe magnet. The unipolar generator created by Faraday for demonstration purposes was completely unsuitable for practical use,

since it had two serious drawbacks:

1. The current induced directly in the area of ​​the magnet's action was self-closed in the remaining parts of the disk, in connection with which the generated electricity mainly did the work of heating the rotating copper canvases.
2. The voltage generated by the device was extremely low due to the singleness of the conductor passing through the magnetic flux.

These problems could be solved by increasing the number of magnets around the perimeter and using coils with windings instead of a disk. A similar scheme became typical for all subsequent designs of dynamo machines. From the further history of the development of generators the following dates can be distinguished:

• 1832 g. - French instrument manufacturer Pixie built the first dynamo based on Faraday principles;
• 1860 g. - the Italian professor of physics Pacinotti created a generator of an almost modern type;
• 1866-1867 - Independently of each other, Whitson, Siemens and Varley obtained patents for self-excited dynamos;
• 1871 g. - Belgian Gramm, based on Pacinotti's design, created the first commercial generator for industry.

The simplest electrical machine

According to Faraday's law of electromagnetic induction, a change in magnetic flux on a turn of a wire will produce a magnetic force that causes electrons to move in the conductor. This creates an electric current in the coil. This phenomenon is called electromagnetic induction, it serves as the foundation for the design of electrical machines. Briefly the principles of the DC generator are as follows:

1. The magnetic force applied to the electrons creates an electromotive force, which sets the electrons in motion in the circuit.
2. The strength and direction of this EMF is determined by the strength and direction of the magnetic field, as well as the speed of the moving part, which can be both a conductor and a magnet.

Essentially, all electrical generators operate on the same principle, regardless of whether they produce AC or DC current. In the armature windings of dynamos, in fact, an alternating current is induced, which is converted into direct current with the help of a collector and a brush unit.

It is convenient to consider the operation of this class of devices using the example of a simple generator, additionally equipped with a switch for current rectification. A good illustrative model for understanding the processes taking place in a dynamo can be a rotating coil of a rectangular conductor placed between two opposite poles of a magnet.

With a full revolution of such a frame, an electric current will be induced in it, circulating through the loop. Its direction can be determined using Fleming's right-hand rule, which says that if you position your hand so that the palm entered the magnetic flux, and direct the bent thumb in the direction of the movement of the conductor, then the index finger will indicate the direction current. In this case, to understand the processes in the simplest generator it will be convenient to distinguish four positions of the loop relative to the magnet:

• 0 ° - the coil moves parallel to the direction of the magnetic current, so no potential difference is induced;
• 90 ° - the potential difference is maximum;
• 180 ° - the turn is again parallel to the magnetic field;
• 270 ° - maximum EMF is induced, but in the opposite direction;
• 360 ° - return to the starting point.

The shape of the variable output electrical signal can be thought of as a sinusoid. With the help of the collector, the connection of the brushes with the coil is reversed every half cycle. Due to this, the current in the external circuit of the generator moves in one direction.

Excitation windings

The DC generator device has the potential to be used only in small electrical machines. First of all, because for low-power devices, the use of permanent magnets is permissible. In other cases, only solenoids - coils with a core - or excitation windings can create a magnetic flux of sufficient strength. By the type of food they eat generators can be divided into the following classes:

• with independent excitement;
• self-excited.

For the first operation, an auxiliary current source is required. This is the main disadvantage of this type of machine, so their use is limited. In generators with independent excitation, the windings are powered from the armature. Electric machines arranged according to this scheme, are divided in turn into three types:

• shunt (with parallel excitation);
• serial (with serial);
• compound generators (with parallel and series excitation coils).

Modern dynamos

One of the features of collector generators is their voltage limitation. This is due to the need to avoid sparking between the brushes and the collector. Therefore, in some machines, the conversion of alternating current to direct current is carried out using electronic devices, for example, diode rectifiers.

Unlike the simplest design, modern generators use drum armatures, which, as a rule, consist of a large number of turns located in the longitudinal slots of the core and connected to the corresponding segments of the multiple switch.

The multi-segment collector used with the armature drum always connects the external circuit only to the turns of wire moving through the zone of maximum magnetic field intensity. As a result of this work, the current generated in the armature windings is almost constant. These generators are usually equipped with four or more electromagnetic poles in order to increase the size and strength of the magnetic field.

Large dynamos have found their application in the modern world as components of wind turbines or hydro turbines, as of a reversible machine in electric transport and in those areas of industry where their use is technologically no alternative. Their relatively complex design, as well as the suitability of alternating current for transportation, led to the fact that that DC generators have been supplanted since the invention by more economical asynchronous devices.