Alternating current and its parameters: the intensity of charge fluctuations in the electrical network, measurement methods

The directional movement of charged particles under the action of an electric driving force (EMF) is called an electric current, it is variable and constant. In the latter case, the movement of nucleons is stable in time, and in the first case, it periodically reverses the direction and magnitude. One of the main parameters of alternating current is frequency. The characteristic depends on the frequency of oscillations of electrons, it can be measured in several ways and instruments.

Alternating electric current

In English, this term corresponds to the expression alternating current - an abbreviation AC; in power engineering, the tilde sign (~) is used as a letter designation. The alternating current changes in a sinusoidal period. The sources are generators that generate EMF by means of electromagnetic induction. The speaker is characterized by the following parameters:

• network voltage U in volts;
• current strength I = Q / Δt, [A] is the number of charges that have passed through the conductor cross-section per unit of time;
• period T - time interval of a complete cycle of changes;
• frequency f is the number of oscillations during a second: f = 1 / T, [Hz] in domestic networks the standard is 50 hertz;
• current density j = I / S, [A / mm2] is a vector quantity, where S is the cross-sectional area of ​​the conductor, the direction j coincides with the course of motion of the electrons;
• phase - the state of the AU, can be single and multiphase;
• amplitude I max - the height of the sinusoid, the maximum value of the current instantaneously achieved during the period.

In the power industry, three-phase networks are mainly used: 3 separate electrical circuits with the same voltage and frequency at a shift φ = 120 °. The stability and reliability of the entire power grid depends on the stability of the vibrational motions of nucleons in the system.

Ripple period and frequency

The physical essence of alternating current lies in the movement of electrons in a conductor, first in one direction, then in the other direction. A full cycle of movements back and forth is performed for a certain period, determined by the frequency of oscillations: T = 1 / f.

Cycle intensity

For the conditions of power grids in Russia, the indicator is f = 50 Hz, and the time of one ripple is T = 1/50 = 0.02 seconds. The feedback of the two parameters allows you to determine the frequency ~ current by the duration of the signal: f = 1 / 0.02 = 50 Hz.

One hertz means 1 oscillation per second. The faster the electromotive force changes, the faster the radius vector is reversed and the period is shortened. Accordingly, when forcing the speed, the frequency increases: the values ​​of T and f are inversely proportional, the more one, the less the second. The values ​​of the characteristic f vary over a wide range, which predetermines the use of extended terminology:

Number of zeros after one	Hertz prefix
3 (thousand)	Kilo (kHz)
6 (million)	Mega (MHz)
9 (billion)	Giga (GHz)

Depending on the value, the frequency of the alternating current is divided into the following subgroups:

• industrial: 16-25 Hz on railway networks in some countries, 25 and 75 Hz in rail circuit blocking schemes, in autonomous systems of aviation and military energy - 400 Hz, in some industrial and agricultural installations 200―400 Hz;
• sound ones are in the range 20-20,000 Hz (20 kHz), in transmitting antennas - up to 1.5 GHz;
• technical: automation - the range from 1 kHz to 1 GHz is used, metallurgy and mechanical engineering: melting, welding and heat treatment of metals;
• radar stations for satellite communications, special systems GLONASS, GPS - up to 40 GHz and higher.

High frequency currents (HFC) begin at the level of tens of kHz, when the radiation is significantly manifested electromagnetic waves and skin effect: the charge moving in the conductor is distributed not over the cross section, but in surface layer.

Power generators and oscillatory circuits are used to generate HDTV. In the latter case, the device is a circuit with capacitance and inductance included.

Danger of multi-frequency charges

The AC and DC voltages equivalent to the effect on the human body are 42 V and 120 V, respectively. The hazard inequality disappears when the EMF reaches 500 V, and at high values ​​the constant becomes more dangerous. The manifestations of the adverse effect of the latter are thermal and electrolytic, and the alternating one is mainly expressed in the contraction of blood vessels, muscles, and vocal cords. In this case, the frequency of the current plays a decisive role in the danger:

• 40-60 Hz - the greatest threat of injury, the possibility of cardiac fibrillation; a further increase in the intensity of charge oscillations leads to a decrease in risk, but the likelihood of death remains within the entire range of industrial frequencies - up to 500 Hz;
• above 10 kHz, HDTV begins - they are safe up to the level of 1 MHz with respect to internal lesions, which is due to skin effect, but cause burns and the threat from them is no less than from constant or variables of the previous groups;
• high-frequency currents are accompanied by electromagnetic radiation - from this side there is a possibility of negative impact on living organisms.

Their use in medicine for physiotherapeutic procedures is based on the relative safety of HDTVs. The severity of an electric shock depends not only on the physical parameters of the charge, but also on the state of the human body.

Measuring instruments

To determine the intensity of the oscillations, an oscilloscope is used, on which you can see the frequency and shape of the signal. There are also special devices - frequency meters. They use the following methods for determining the parameter:

• capacitor overcharge - the average value of the current strength is proportional to its intensity and is measured by a magnetoelectric ammeter with a scale in hertz;
• discrete counting - fixing pulses of the required frequency for a certain period, receiving data sufficient accuracy: an error within 2%, this is a good indicator for domestic use device;
• the resonance method is based on an electrical phenomenon of the same name that occurs in a circuit with tunable elements; frequency - more than 50 Hz, is determined by the scale of the adjusting mechanism.

Another well-known method is used in oscilloscopes, based on mixing and comparing a reference parameter with a measured frequency. As a result of overlapping, beating occurs, and when aligning, a certain figure is set on the screen. The desired characteristic is calculated according to the fixed schedule by means of mathematical formulas.