Direct and alternating current differences. Difference between DC and AC voltage

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An alternating current is a current whose change in magnitude and direction is repeated periodically at equal intervals of time T.

In the field of production, transmission and distribution of electrical energy, alternating current has two main advantages over direct current:

1) the ability (with the help of transformers) to simply and economically increase and decrease voltage, this is crucial for the transfer of energy to long distances.

2) greater simplicity of electric motor devices, and therefore their lower cost.

The value of a variable quantity (current, voltage, emf) at any time t is called instantaneous value and is designated lowercase letters(current i, voltage u, emf – e).

The largest of the instantaneous values of periodically changing currents, voltages or emf is called maximum or amplitude values and are designated by capital letters with the index “m” (I m, U m).

The shortest period of time after which the instantaneous values of a variable quantity (current, voltage, emf) are repeated in the same sequence is called period T, and the totality of changes occurring during the period is cycle.

The reciprocal of the period is called frequency and is denoted by the letter f.

Those. frequency – the number of periods per 1 second.

Frequency unit 1/sec - called hertz (Hz). Larger units of frequency are kilohertz (kHz) and megahertz (MHz).

Obtaining alternating sinusoidal current.

In technology, alternating currents and voltages are sought to be obtained according to the simplest periodic law - sinusoidal. Because a sinusoid is the only periodic function that has a derivative similar to itself, as a result of which the shape of the voltage and current curves in all links of the electrical circuit is the same, which greatly simplifies the calculations.

To obtain industrial frequency currents, use alternators whose operation is based on the law of electromagnetic induction, according to which, when a closed circuit moves in a magnetic field, a current arises in it.

Circuit diagram of a simple alternator

High-power alternating current generators, designed for voltages of 3–15 kV, are made with a stationary winding on the machine stator and a rotating electromagnet-rotor. With this design, it is easier to reliably insulate the wires of the fixed winding and it is easier to divert the current to the external circuit.

One revolution of the rotor of a two-pole generator corresponds to one period of alternating EMF induced on its winding.

If the rotor makes n revolutions per minute, then the frequency of the induced emf

Because wherein angular velocity generator
, then between it and the frequency induced by the EMF there is a relationship
.

Phase. Phase shift.

Let us assume that the generator has two identical turns at the armature, shifted in space. When the armature rotates, EMFs of the same frequency and with the same amplitudes are induced in the turns, because the coils rotate at the same speed in the same magnetic field. But due to the shift of turns in space, the EMF does not reach amplitude signs simultaneously.

If at the moment the time count begins (t=0) turn 1 is located at an angle relative to the neutral plane
, and turn 2 is at an angle
. Then the EMF induced in the first turn:

and in the second:

At the time of countdown:

Electrical angles And the determining values of the emf at the initial moment of time are called initial phases.

The difference in the initial phases of two sinusoidal quantities of the same frequency is called phase angle .

The quantity for which zero values (after which it takes on positive values) or positive amplitude values is achieved earlier than the other is considered advanced in phase, and the one for which the same values are achieved later - lagging in phase.

If two sinusoidal quantities simultaneously reach their amplitude and zero values, then the quantities are said to be in phase . If the phase shift angle of sinusoidal quantities is 180 0
, then they are said to change in antiphase.

Despite the fact that electric current is an indispensable part of modern life, many users do not even know basic information about it. In this article, skipping the basic physics course, we will consider how direct current differs from alternating current, as well as how it is used in modern domestic and industrial conditions.

In contact with

Difference in current types

We will not consider what current is here, but will immediately move on to the main topic of the article. AC current is different from constant topics, what he continuously changes in the direction of movement and its size.

These changes are carried out in periods of equal time intervals. To create such a current, special sources or generators are used that produce an alternating EMF ( electromotive force), which changes regularly.

The basic circuit of the mentioned device for generating alternating current is quite simple. This is a rectangular frame made from copper wires, which is fixed to the axis and then rotates in the magnetic field using a belt drive. The tips of this frame are soldered to copper contact rings that slide directly over the contact plates, rotating synchronously with the frame.

Under the condition of a uniform rhythm of rotation, an EMF begins to be induced, which changes periodically. It is possible to measure the EMF generated in the frame with a special device. Thanks to the appearance, it is possible to determine the variable EMF and with it the alternating current.

In graphical execution, these quantities are typically depicted in the form of a wave-like sinusoid. The concept of sinusoidal current often refers to alternating current, since this type of current change is the most common.

Alternating current is an algebraic quantity, and its value at a specific time instant is called the instantaneous value. The sign of the alternating current itself is determined by the direction in which the current flows at a given time. Therefore, the sign can be positive or negative.

Current characteristics

For a comparative assessment of all possible alternating currents, criteria called AC parameters, among which:

period;
amplitude;
frequency;
circular frequency.

A period is a period of time during which a complete cycle of current change occurs. Amplitude is called maximum value. The frequency of alternating current is the number of completed periods in 1 second.

The parameters listed above make it possible to distinguish different kinds alternating currents, voltages and EMF.

When calculating the resistance of different circuits to alternating current, it is permissible to connect another characteristic parameter called corner or circular frequency . This parameter is determined by the speed of rotation of the above-mentioned frame at a certain angle in one second.

Important! You should understand the difference between current and voltage. The fundamental difference is known: current is an amount of energy, and voltage is called a measure.

Alternating current gets its name because the direction of motion of electrons changes continuously, as does the charge. It has different frequencies and electrical voltages.

This is what it is distinctive feature from direct current, Where the direction of electron movement is unchanged. If the resistance, voltage and current are constant, and the current flows only in one direction, then such a current is constant.

For the passage of direct current in metals, it is necessary that the constant voltage source be closed on itself using a conductor, which is the metal. In some situations, a chemical energy source called a galvanic cell is used to generate direct current.

Current transmission

AC power sources are regular outlets. They are located at facilities for various purposes and in residential premises. Various are connected to them electrical devices, which receive the voltage necessary for their operation.

Use of alternating current in electrical networks is economically justified because the magnitude of its voltage can be transformed to level required values. This is accomplished using transformer equipment with minor losses allowed. Transportation from power sources to end consumers is cheaper and easier.

The transfer of current to consumers begins directly at the power plant, where a variety of extremely powerful electrical generators are used. Electric current is obtained from them, which is sent through cables to transformer substations. Often, substations are located near industrial or residential electrical consumption facilities. The current received by the substations is converted into three-phase alternating voltage.

Batteries and accumulators contain direct current, which is characterized by stable properties, i.e. they do not change over time. It is used in any modern electrical products, as well as in cars.

Current conversion

Let us consider separately the process of converting alternating current into direct current. This process is carried out using specialized rectifiers and includes three steps:

The first step is to connect a four-diode bridge of a given power. This, in turn, makes it possible to specify unidirectional motion for charged particles. In addition, it lowers the upper values of sinusoids characteristic of alternating current.
Next, a smoothing filter or a specialized capacitor is connected. This is done from the diode bridge to the output. The filter itself helps correct the valleys between the peak values of the sinusoids. And connecting a capacitor significantly reduces ripple and brings it to minimum values.
Then voltage stabilizing devices are connected to reduce ripple.

This process, if necessary, can be carried out in two directions, converting direct and alternating current.

Another distinctive feature is the propagation of electromagnetic waves in relation to space. It has been proven that direct current does not allow electromagnetic waves to propagate in space, while alternating current can cause them to propagate. In addition, when transporting alternating current through wires, induction losses are much less than when transmitting direct current.

Rationale for current selection

The variety of currents and the lack of a single standard is due not only to the need for different characteristics in each individual situation. In solving most issues, the advantage is in favor of alternating current. This difference between the types of currents is determined by the following aspects:

Possibility of transmitting alternating current over long distances. Possibility of conversion to heterogeneous electrical circuits with an ambiguous level of consumption.
Maintaining a constant voltage for alternating current is two times cheaper than for direct current.
The process of converting electrical energy directly into mechanical force is carried out at much lower cost in AC mechanisms and motors.

A long time ago, scientists invented electric current. The first invention was the permanent one. But later, while conducting experiments in his laboratory, Nikola Tesla invented alternating current. There were and are many differences between them, according to which one of them is used in low-current equipment, and the other has the ability to overcome different distances with minor losses. But a lot depends on the magnitude of the currents.

AC and DC current: difference and features

The difference between alternating current and direct current can be understood based on the definitions. In order to better understand the operating principle and features, you need to know the following factors.

Main differences:

Movement of charged particles;
Mode of production.

Variable current is a current in which charged particles are capable of changing the direction of movement and magnitude in certain time. The main parameters of alternating current include its voltage and frequency.

Currently, public electrical networks and various facilities use alternating current, with a certain voltage and frequency. These parameters are determined by the equipment and devices.

Note! In household electrical networks, a current of 220 Volts is used and clock frequency 50 Hz.

The direction of movement and frequency of charged particles in direct current are unchanged. Given current various foods are used for nutrition household devices, such as televisions and computers.

Due to the fact that alternating current is simpler and more economical in its production method and transmission over various distances, it has become the basis for the electrification of objects. Alternating current is produced at various power plants, from which it is supplied to the consumer through conductors.

Direct current is obtained by converting alternating current or through chemical reactions (for example, an alkaline battery). For conversion, current transformers are used.

What voltage level is acceptable for a person: features

In order to know what values electric current are acceptable for humans, corresponding tables have been compiled, which indicate the values of alternating and direct current and time.

Electric current exposure parameters:

Force;
Frequency;
Time;
Relative humidity.

The permissible touch voltage and current that flow through the human body in various modes of electrical installations do not exceed the following values.

Alternating current 50 Hz, should be no more than 2.0 Volts and a current of 0.3 mA. Current with a frequency of 400 Hz with a voltage of 3.0 Volts and a current strength of 0.4 mA. Direct current with a voltage of 8 and a current of 1 mA. Safe exposure to current with such indicators is up to 10 minutes.

Note! If electric installation work produced at elevated temperatures and high relative humidity, these values are reduced by three times.

In electrical installations with voltages up to 100 Volts, which are solidly grounded or the neutral is insulated, the safe touch currents are as follows.

50 Hz alternating current with a voltage range from 550 to 20 Volts and a current strength from 650 to 6 mA, 400 Hz alternating current with a voltage from 650 to 36 Volts, and direct current from 650 to 40 Volts, should not affect the human body within the range of 0.01 to 1 second.

Dangerous alternating current for humans

It is believed that alternating electric current is the most dangerous for human life. But this is provided, if you do not go into details. Much depends on various quantities and factors.

Factors influencing hazardous exposure:

Duration of contact;
The path of electric current;
Current and voltage;
What is the resistance of the body?

According to the rules of the PUE, the most dangerous current for humans is alternating current with a frequency that varies from 50 to 500 Hz.

It is worth noting that, provided that the current does not exceed 9 mA, then anyone can free themselves from the live part of the electrical installation.

If given value is exceeded, then in order to free yourself from the effects of electric current, a person needs strong help. This is due to the fact that alternating current is much more capable of stimulating nerve endings and causing involuntary muscle spasms.

For example, when you touch the live part of the device with the inside of your palm, the muscle spasm will cause the fist to clench more strongly over time.

Why is alternating current more dangerous? At identical values current, alternating current has a several times stronger effect on the body.

Since alternating current affects nerve endings and muscles, it is worth understanding that this also affects the functioning of the heart muscle. From which it follows that when contacting alternating current, the risk of death increases.

An important indicator is the resistance of the human body. But when struck by alternating current with high frequencies, body resistance is significantly reduced.

What magnitude is direct current dangerous for humans?

Direct current can also be dangerous for humans. Of course, variable, ten times more dangerous. But if we consider currents in different quantities, then constant can be much more dangerous than alternating.

The effects of direct current on humans are divided into:

1 threshold;
2 threshold;
3 threshold.

When exposed to direct current at the feather threshold (the current is noticeable), your hands begin to tremble a little and a slight tingling sensation appears.

The second threshold (not releasing current), ranging from 5 to 7 mA, is the lowest value at which a person cannot free himself from the conductor on his own.

This current is considered not dangerous, since the resistance of the human body is higher than its value.

The third threshold (fibrillation), with values of 100 mA and above, the current has a strong effect on the body and internal organs. In this case, the current at these values can cause chaotic contraction of the heart muscle and lead to its stop.

The strength of the impact is also influenced by other factors. For example, dry human skin has a resistance of 10 to 100 kOhm. But if the contact occurs with a wet skin surface, then the resistance is significantly reduced.

Introduce a home modern man without electrical outlets impossible. And therefore, many want to know more about the force that brings warmth and light to civilization, making all our electrical appliances work. And they start with the question: what is the current in our outlet, direct or alternating? And which one is better? To answer the question of what current is in the outlet and what determines this choice, let’s find out how they differ.

DC voltage sources

All experiments conducted by scientists with electric current began with it. The first, still primitive, sources of electricity, similar to modern batteries, were capable of delivering direct current.

Its main feature is the constant current value at any time. Sources other than galvanic cells, are special generators and batteries. A powerful source constant voltage is atmospheric electricity - lightning discharges.

AC voltage sources

Unlike direct voltage, the magnitude of alternating voltage changes over time according to a sinusoidal law. For him, there is the concept of period - the time during which one complete oscillation occurs, and frequency - the reciprocal of the period.

In Russian electrical networks, the accepted frequency of alternating current is 50 Hz. But in some countries this value is 60 Hz. This must be taken into account when purchasing household electrical appliances and industrial equipment, although most of it works fine in both cases. But it’s better to make sure of this by reading the operating instructions.

Advantages of AC

Our outlets carry alternating current. But why exactly this, why is it better than a permanent one?

The fact is that only the magnitude of the alternating voltage can be changed using converting devices - transformers. And you have to do this many times.

Thermal power plants, hydroelectric power plants and nuclear power plants are located far from consumers. There is a need to transmit large powers over distances of hundreds and thousands of kilometers. Power line wires have low resistance, but it is still present. Therefore, the current passing through them heats the conductors. Moreover, due to the potential difference at the beginning and end of the line, less voltage reaches the consumer than it was at the power plant.

You can combat this phenomenon by either reducing the resistance of the wires or reducing the current value. Reducing resistance is only possible by increasing the cross-section of the wires, and this is expensive and sometimes technically impossible.

But you can reduce the current by increasing the line voltage. Then, when transmitting the same power, less current will flow through the wires. Reduce heating losses of wires.

Technically it looks like this. From the power plant's alternating current generators, voltage is supplied to the step-up transformer. For example, 6/110 kV. Further along the 110 kV power line (abbreviated as 110 kV power line) Electric Energy goes to the next distribution substation.

If this substation is intended to power a group of villages in the area, then the voltage is reduced to 10 kV. If it is necessary to send a significant part of the received power to an energy-intensive consumer (for example, a mill or plant), 35 kV lines can be used. At node substations, three-winding transformers are used to divide the voltage between consumers located at different distances and consuming different powers. In our example, this is 110/35/6 kV.

Now the voltage received at the rural substation is undergoing a new transformation. Its value should be acceptable to the consumer. For this purpose, the power passes through a 10/0.4 kV transformer. The voltage between phase and neutral of the line going to the consumer becomes equal to 220 V. It reaches our sockets.

Do you think that's all? No. For semiconductor technology, which is the filling of our televisions, computers, music centers this value will not work. Inside them, 220 V is reduced to an even smaller value. And it is converted to direct current.

This is the metamorphosis: it is better to transmit alternating current over long distances, but we mainly need direct current.

Another advantage of alternating current: it is easier to extinguish the electric arc that inevitably occurs between the opening contacts of switching devices. The supply voltage changes and periodically passes through the zero position. At this point, the arc goes out on its own if certain conditions are met. For constant voltage, more serious protection against burnt contacts will be required. But when short circuits On direct current, damage to electrical equipment from the action of an electric arc is more serious and more destructive than on alternating current.

Advantages of DC

Energy from AC voltage sources cannot be stored. It can be used to charge a battery, but it will only output direct current. What happens if, for some reason, the generator at the power plant stops or the village power line breaks? Its residents will have to use battery-powered flashlights to avoid being left in the dark.

But power plants also have constant voltage sources - powerful ones. rechargeable batteries. After all, in order to start equipment stopped due to an accident, electricity is needed. The mechanisms, without which it is impossible to start the power plant equipment, have electric motors powered by direct voltage sources. And also all protection, automation and control devices.

Also on constant voltage There is electrified transport: trams, trolleybuses, metro. DC motors have more torque per low speeds rotation, which is necessary for an electric train to start successfully. And the regulation of engine speed, and, consequently, the speed of movement of the train, is easier to implement using direct current.