An electric capacitor is a device that can store charge and energy in an electric field. Basically, it consists of a pair of conductors (plates) separated by a dielectric layer. The thickness of the dielectric is always much smaller than the size of the plates. On electrical equivalent circuits, the capacitor is indicated by 2 vertical parallel segments (II).
Basic quantities and units of measurement
There are several basic quantities that define a capacitor. One of them is its capacity (Latin letter C), and the second is the operating voltage (Latin U). Electric capacity (or simply capacitance) in the SI system is measured in farads (F). Moreover, as a unit of capacitance, 1 farad - this is a lot - is almost never used in practice. For example, the electric charge of the planet Earth is only 710 microfarads. Therefore, the electrical capacitance of capacitors in most cases is measured in farad-derived quantities: in picofarads (pF) with a very small capacitance value (1 pF=1/106µF), in microfarads (µF) at its sufficiently large value (1 uF=1/106 F). In order to calculate the electrical capacity, it is necessarydivide the amount of charge accumulated between the plates by the modulus of the potential difference between them (voltage across the capacitor). The charge of the capacitor in this case is the charge accumulating on one of the plates of the device in question. On 2 conductors of the device, they are identical in modulus, but differ in sign, so their sum is always equal to zero. The charge of a capacitor is measured in coulombs (C), and is denoted by the letter Q.
Voltage on electrical appliance
One of the most important parameters of the device we are considering is the breakdown voltage - the potential difference between the two conductors of the capacitor, leading to electrical breakdown of the dielectric layer. The maximum voltage at which there is no breakdown of the device is determined by the shape of the conductors, the properties of the dielectric and its thickness. Operating conditions under which the voltage on the plates of the electrical appliance is close to the breakdown voltage are unacceptable. The normal operating voltage on the capacitor is several times less than the breakdown voltage (two to three times). Therefore, when choosing, pay attention to the rated voltage and capacitance. In most cases, the value of these quantities is indicated on the device itself or in the passport. The inclusion of a capacitor in the network for a voltage exceeding the nominal voltage threatens to breakdown, and a deviation of the capacitance value from the nominal value can lead to the release of higher harmonics into the network and overheating of the device.
Appearance of capacitors
The design of capacitors can bethe most varied. It depends on the value of the electrical capacity of the device and its purpose. The parameters of the device under consideration should not be affected by external factors, therefore, the plates have such a shape in which the electric field created by electric charges is concentrated in a small gap between the conductors of the capacitor. Therefore, they may consist of two concentric spheres, two flat plates, or two coaxial cylinders. Therefore, capacitors can be cylindrical, spherical and flat depending on the shape of the conductors.
Permanent Capacitors
According to the nature of the change in electrical capacitance, capacitors are divided into devices with a constant, variable capacity or trimmers. Let's take a closer look at each of these types. Devices whose capacitance does not change during operation, that is, it is constant (the capacitance value can still fluctuate within acceptable limits depending on temperature) are fixed capacitors. There are also electrical appliances that change their electrical capacity during operation, they are called variables.
What does C in a capacitor depend on
Electrical capacity depends on the surface area of its conductors and the distance between them. There are several ways to change these settings. Consider a capacitor, which consists of two types of plates: movable and fixed. The movable plates move relative to the fixed ones, as a result of which the capacitance of the capacitor changes. Variable analogs are used to adjust the analogdevices. Moreover, the capacity can be changed during operation. Trimmer capacitors are in most cases used to tune factory equipment, for example, to select capacitance empirically when calculation is impossible.
Capacitor in circuit
The considered device in the DC circuit conducts current only at the moment it is connected to the network (in this case, the device is charged or recharged to the source voltage). Once the capacitor is fully charged, no current flows through it. When the device is connected to an alternating current circuit, the processes of discharging and charging it alternate with each other. The period of their alternation is equal to the period of oscillation of the applied sinusoidal voltage.
Characteristics of capacitors
The capacitor, depending on the state of the electrolyte and the material of which it consists, can be dry, liquid, oxide-semiconductor, oxide-metal. Liquid capacitors are well cooled, these devices can operate under significant loads and have such an important property as dielectric self-healing during breakdown. The considered dry-type electrical devices have a fairly simple design, slightly less voltage loss and leakage current. At the moment, it is dry appliances that are most popular. The main advantage of electrolytic capacitors is their low cost, compact size and high electrical capacity. Oxide analogues are polar (incorrect connection leads to breakdown).
How to connect
Connecting a capacitor to a DC circuit is as follows: the plus (anode) of the current source is connected to the electrode, which is covered with an oxide film. Failure to comply with this requirement may result in dielectric breakdown. It is for this reason that liquid capacitors must be connected to a circuit with an alternating current source, connecting two identical sections in opposite series. Or apply an oxide layer on both electrodes. Thus, a non-polar electrical appliance is obtained, operating in networks with both direct and sinusoidal current. But in both cases, the resulting capacitance becomes half as much. Unipolar electrical capacitors are large, but can be included in AC circuits.
Main application of capacitors
The word "capacitor" can be heard from workers of various industrial enterprises and design institutes. Having de alt with the principle of operation, characteristics and physical processes, we will find out why capacitors are needed, for example, in power supply systems? In these systems, batteries are widely used in construction and reconstruction at industrial enterprises to compensate for the reactive power of the RFC (unloading the network from unwanted overflows), which reduces electricity costs, saves on cable products and delivers better quality electricity to the consumer. The optimal choice of power, method and place of connection of reactive power sources (Q) in networks of electric power systems (EPS) providessignificant impact on the economic and technical performance of the EPS. There are two types of KRM: transverse and longitudinal. With transverse compensation, capacitor banks are connected to the busbars of the substation in parallel with the load and are called shunt (SHBK). With longitudinal compensation, the batteries are included in the cut of the power line and are called SPC (longitudinal compensation devices). Batteries consist of individual devices that can be connected in various ways: capacitors connected in series or parallel. As the number of devices connected in series increases, the voltage increases. APC is also used to equalize loads by phases, increase the productivity and efficiency of arc and ore-thermal furnaces (when the APC is switched on through special transformers).
On the equivalent circuits of power transmission lines with voltages over 110kV, capacitive conduction to earth is denoted as capacitors. The power supply of the line is due to the capacitance between the conductors of different phases and the capacitance formed by the phase wire and the ground. Therefore, to calculate the operating modes of the network, the parameters of power transmission lines, and determine the places of damage to the electrical network, the properties of the capacitor are used.
More applications
Also, this term can be heard from railway workers. Why do they need capacitors? On electric locomotives and diesel locomotives, these devices are used to reduce the sparking of the contacts of electrical devices, smooth out the pulsating current generated by rectifiers and pulsedbreakers, as well as to create a generation of a symmetrical sinusoidal voltage used to power electric motors.
However, this word is most often heard from the lips of a radio amateur. Why does he need capacitors? In radio engineering, they are used to create high-frequency electromagnetic oscillations, they are part of smoothing filters, power supplies, amplifiers and printed circuit boards.
In the glove box of every motorist you can find a couple of these electrical appliances. Why are capacitors needed in a car? There they are used in the amplifying equipment of acoustic systems for high-quality sound reproduction.
