There are two fuses. Fuse material

Fuses are called electrical devices that, at currents greater than a given value, open the electrical circuit when a fusible insert burns out, which is directly heated by current until it melts. Fuses according to their purpose are divided for protection of installations with voltages up to 1000 V and fuses for protection at voltages greater than or equal to 1000 V.

Fuses are devices that protect installations from overloads and short circuit currents.

The main elements of the fuse are a fusible insert, which is included in the cut of the protected circuit, and an arc extinguishing device, which extinguishes the arc that occurs after the insert is melted.

The choice of fuses is made according to the rated voltage, the rated currents of the fuse and the fuse-link, and the maximum breaking current.

Primary requirements. applied to fuses

The fuses are subject to the following requirements:

1. The time-current characteristic of the fuse should be lower, but as close as possible to the time-current characteristic of the protected object.

2. In the event of a short circuit, the fuses must operate selectively.

3. Fuse trip time in case of short circuit should be as short as possible, especially when protecting semiconductor devices. Fuses must operate with current limiting.

4. The characteristics of the fuse must be stable. Scattering of parameters due to manufacturing deviations must not impair the protective properties of the fuse.

5. Due to the increased power of installations, fuses must have a high breaking capacity.

6. Replacing a blown fuse or fuse should not take long.

In industry, fuses of types PR-2 and PN-2 are most widely used.

Fuse box PR-2

Fuses PR-2 for currents from 15 to 60 A have a simplified design. The fusible insert 1 is pressed against the brass clip 4 by cap 5, which is the output contact. Fusible insert 1 is stamped from zinc, which is a low-melting and corrosion-resistant material. The specified shape of the insert makes it possible to obtain a favorable time-current (protective) characteristic. In fuses for currents over 60 A, the fusible insert 1 is connected to the contact blades 2 with bolts.

The fuse insert PR-2 is located in a sealed tubular cartridge, which consists of a fiber cylinder 3, a brass clip 4 and a brass cap 5.

The principle of operation of fuses PR-2

The process of extinguishing the arc in the fuse PR-2 occurs as follows. When disconnected, the narrowed isthmuses of the fuse-link burn out, after which an arc occurs. Under the action of the high temperature of the arc, the fiber walls of the cartridge release gas, as a result of which the pressure in the cartridge rises to 4-8 MPa in fractions of a half-cycle. By increasing the pressure, the current-voltage characteristic of the arc rises, which contributes to its rapid extinction.

The fusible link of the PR-2 fuse can have from one to four constrictions, depending on the rated voltage. The narrowed sections of the insert contribute to its rapid melting in the event of a short circuit and create the effect of current limitation.

Since the arc extinguishing in the PR-2 fuse occurs very quickly (0.002 s), it can be assumed that the broadened parts of the insert remain motionless during the extinguishing process.

The pressure inside the fuse holder is proportional to the square of the current at the moment the insert melts and can be high. Therefore, the fiber cylinder must have high mechanical strength, for which brass clips 4 are installed at its ends. Disks 6, rigidly connected with contact knives 2, are attached to the cartridge clip 4 using caps 5.

Fuses PR-2 operate silently, with virtually no emission of flame and gases, which allows them to be installed at a close distance from each other. Fuses PR-2 are produced in two axial sizes - short and long. Short fuses PR-2 are designed to operate on an alternating voltage not exceeding 380 V. They have a lower breaking capacity than long ones designed to operate on a network with voltage up to 500 V.

Depending on the rated current, six dimensions of cartridges of various diameters are produced. Inserts for different rated currents can be installed in the cartridge of each size. So, in a cartridge for a rated current of 15 A, inserts for a current of 6, 10 and 15 A can be installed.

There are lower and upper values ​​of the test current. The lower value of the test current is the maximum current that, flowing for 1 h, does not lead to a blown fuse. The upper value of the test current is the minimum current which, after passing for 1 h, will melt the fuse insert. With sufficient accuracy, it is possible to take the boundary current equal to the arithmetic mean of the test currents.

Fuse box PN-2

These fuses are more advanced than the PR-2 fuses. The body of square section of 1 fuse type PN-2 is made of durable porcelain or steatite. Inside the case there are tape fusible inserts 2 and a filler - quartz sand 3. Fusible inserts are welded to disk 4, which is attached to plates 5 connected to knife contacts 9. Plates 5 are attached to the body with screws.

As a filler in fuses PN-2, quartz sand is used with a SiO2 content of at least 98%, with grains of (0.2-0.4) 10-3 m and a humidity of not more than 3%. Before backfilling, the sand is thoroughly dried at a temperature of 120-180 °C. Quartz sand grains have high thermal conductivity and a well-developed cooling surface.

The fusible link of fuses PN-2 is made of copper tape with a thickness of 0.1-0.2 mm. To obtain current limitation, the insert has narrowed sections 8. The fusible insert is divided into three parallel branches for a more complete use of the filler. The use of a thin tape, effective heat removal from the narrowed sections allow you to choose a small minimum cross-section of the insert for a given rated current, which ensures a high current-limiting capacity. The connection of several narrowed sections in series helps to slow down the growth of current after the melting of the insert, as the voltage across the fuse arc increases. To reduce the melting temperature, tin strips 7 are applied to the inserts (metallurgical effect).

The principle of operation of the fuse PN-2

In the event of a short circuit, the fusible link of the PN-2 fuse burns out and the arc burns in the channel formed by the filler grains. Due to burning in a narrow gap at currents above 100 A, the arc has an increasing current-voltage characteristic. The voltage gradient across the arc is very high and reaches (2-6)104 V/m. This ensures that the arc is extinguished in a few milliseconds.

After the fuse is activated, the fuse-links together with disk 4 are replaced, after which the cartridge is covered with sand. To seal the cartridge, an asbestos gasket 6 is placed under the plates 5, which protects the sand from moisture. At a rated current of 40 A and below, the fuse has a simpler design.

Modern electrical networks and devices are very complex and require reliable protection against possible overloads and short circuits. The main protective role in such cases is played by various safety devices. Among the variety of these devices, fuses are considered the most common, having a high degree of reliability, ease of operation and relatively low cost.

Despite the widespread use of automatic protective devices, fusible links remain relevant in the protection of electronic equipment, automotive electrical networks, industrial electrical installations and power supply systems. They are still used in the switchboards of many residential buildings due to their reliable performance, small size, stable performance and quick replacement.

What are fuses used for?

In the case of connecting two wires connected to a current source, the well-known short circuit effect will occur. The reason may be damaged insulation, incorrect connection of consumers, etc. With a relatively small resistance of the wires, at this moment a very high current will flow through them. As a result of overheating of the wires, the insulation ignites, which can lead to a fire.

It is quite possible to avoid negative consequences by including fuses, also known as plugs, in fuses. If the current exceeds the permissible value, the wire inside the fuse becomes very hot and quickly melts, breaking the electrical circuit in this place.

The design of fuses can be tubular or cork. The tubular elements are manufactured in a closed fiber casing with gas generation properties. If the temperature rises, a high pressure is created inside the tube, causing the circuit to break. Plug fuses are of a standard design, equipped with a wire that melts under the influence of a high electric current.

There is another type of so-called self-healing fuses made of polymeric materials that change their structure at different temperatures. Significant heating leads to a sharp increase in resistance, as a result of which the circuit breaks. Further cooling causes a decrease in resistance, so the circuit closes again. Basically, such fuses are used in complex digital devices. In conventional power networks, they are not used because of the high cost.

Sometimes some craftsmen try to replace a blown fuse, using the so-called bugs instead, which are a piece of thick wire or thin wires twisted into a common bundle. It is strictly forbidden to use such home-made devices, since the current in the event of a short circuit will be unacceptably high. Strong heating of the wiring will cause damage, fire and fire.

Fuse device

The composition includes a body or cartridge with electrical insulating properties, and the fuse itself. Its ends are connected to terminals that connect the fuse in series to the electrical circuit, together with the protected device or electrical line. The material of the fusible insert is selected so that it can melt before the temperature of the wires reaches a dangerous level, or the consumer fails as a result of an overload.

Based on the design features, fuses can be cartridge, plate, plug and tube. The estimated current that the fusible link can withstand is indicated on the device case.

A fairly simple design for low-voltage fuses. Under the influence of high current, the fusible insert or current-carrying element is subjected to strong heating, after which, when a certain temperature is reached, it melts in the arc-extinguishing medium and evaporates, breaking the protected circuit. This is how a fuse works in an electrical circuit.

In order to prevent hot gases and liquid metal from entering the environment, a ceramic insulator is used, which is also the body of the device, resistant to high temperatures and significant internal pressure. Protective covers located along the edges of the fuse are equipped with special strips for unified handles that capture fusible inserts when replacing unusable elements. With the help of protective covers and a ceramic housing, an explosion-proof enclosure is created that limits the switching electric arc.

Sand filling the interior space limits the current strength. The material is selected with certain crystal sizes, after which it is compacted properly. As a rule, fuses are filled with quartz crystalline sand, which has a high chemical and mineralogical purity. The connection of the fusible insert with the base-holder is carried out mechanically, using contact knives. For their manufacture, copper or copper alloys coated with tin or silver are used.

Fuse characteristics

The main characteristic is the direct dependence of the melting time on the current strength. Therefore, the time it takes for a fuse link to blow out corresponds to a certain current. This parameter is better known as the time-current characteristic.

In addition to the time indicator, there are other characteristics by which the types of fuses are determined. Among them, first of all, it should be noted. This is the most permissible load current according to the conditions of heating the fuse case for a long time. When choosing a device according to this indicator, the load of the electrical circuit, as well as the operating conditions of the fuse, must be taken into account.

In some cases, the rated current may be higher than the current in the electrical circuit itself. For example, in electric motor starters, to avoid blown fuse during start-up. Please note that the rated current of the fuse must correspond to the rated current of the element being replaced.

In turn, the rated current of the element being replaced is the maximum allowable load current for a long time when this element is installed in the holder or in the contacts. In addition, there are base and fuse holder current ratings that must be taken into account when choosing a protective device. In addition, an indicator such as rated voltage is used. This parameter represents the interpole voltage, coinciding with the nominal phase-to-phase voltage of the protected electrical networks.

In order for fuses to provide reliable protection, the value of this value must be greater than or equal to the voltage of the protected object. For example, a 400 volt fuse can be used to protect 220 volt circuits, but never the other way around. Thus, this value characterizes the ability of the fuse to timely break the electrical circuit and extinguish the arc.

Therefore, when choosing a fuse as a protective agent, it is imperative to take into account the parameters that allow you to ensure reliable protection of the object.

Types of fuses

For all devices of this type, there is a general classification in accordance with their basic properties.

Fusible links can be closed in different ways, in connection with this, external effects that occur when the current is turned off also differ. Such fuses are divided into the following types:

• An open fusible link that does not contain devices to limit the volume of the arc, ejection of molten metal particles and flame.
• Semi-closed cartridge with shell open on one or both sides. It poses a certain danger to people nearby.
• Closed cartridge. It is the most reliable, since it does not have all of the above disadvantages. Almost all modern fuses are produced with a closed cartridge.

Arc quenching can be done in a variety of ways. Depending on this, fuses come with or without filler. In the first case, powdered, fibrous or granular components are used, and in the second case, due to the movement of gases or high pressure in the cartridge. The designs of the cartridges themselves are divided into collapsible and non-collapsible. The first option involves replacing the melted insert, and in the second case, you will have to change the entire element. In some cases, non-separable cartridges can be reloaded in special workshops.

Fuses may or may not be replaced while energized. In the first case, the replacement can be done directly by hand, without touching live parts. In the second case, the device is necessarily disconnected from the voltage.

Fuse marking

Each fuse in the diagram is indicated by a certain symbolism. Standard marking consists of two alphabetic characters. The first letters define the guard interval: a - partial (protection only against short circuits) and g - full (protection against short circuits and overloads is provided).

The second letter indicates the types of protected devices:

• G - protects any equipment.
• F - only low current circuits are protected.
• Tr - protection of transformers.
• M - electric motors and disconnecting devices.

For more information on fuse markings, see reference books for electrical professionals.

The body of fuse links is made of high-strength grades of special ceramics (porcelain, steatite or corundum-mullite ceramics) to ensure their high breaking capacity. Some foreign firms (USA, Japan) fuse cases are made of fiberglass impregnated with organosilicon resin. Analysis of the mechanical barrels of cast resins confirms that they can be used to make fuse cases. The tensile strength of the hulls produced in this way is higher than that of a similarly sized ceramic hull with steel roofs. The main factor preventing the use of resins is their aging at elevated temperatures. At case temperatures not exceeding 30 0 C, aging is not detected, but at higher temperatures, the mechanical and electrical properties of the resins deteriorate over time. Due to the fact that significant overheating of the fuse body is possible both in the nominal mode (up to 120 0 C) and in the area of ​​current overloads, the use of insulating resins for the manufacture of cases and other structural elements of fuses will become possible only after the creation of casting resins with a sufficiently large thermal resistance in various operating modes of fuses.

Fritz Driescher (Germany) manufactured fuses with a spherical body made of epoxy resin, which greatly simplified the mass production of fuses. Fibrous material is added to the epoxy resin to increase the mechanical strength. There are no threaded connections in this fuse. These fuses are waterproof. But such fuses are designed only to cut off large short-circuit currents, since with small current overloads unacceptable overheating of the resin body occurs.

For fuse cases with low rated currents, special glasses are usually used.

DESIGN OF FUSED ELEMENTS.

All varieties of fusible elements can be divided into two groups: a fusible element with a constant cross-section along the length and a variable one. Fusible elements of constant section are usually made of wire, and fusible elements of variable section are made of metal foil or thin metal film.

The ratio of the cross section of the wide part of the fusible element to the cross section of the narrow isthmus determines the type of protective characteristic. For example, high-speed fuses typically use fuses with a ratio greater than five. Characteristics for slow-acting and normally operating fuses are obtained with a ratio of less than five.

Fusible elements of constant cross section usually have a current density much less than in fusible elements of variable cross section. When triggered, fuses with fusible elements of constant cross section have large values ​​of melting current and melting integral, large overvoltages, but the duration of the arc and the ratio of the maximum value of the transmitted current to the melting current in these fuses is much less.

With an increase in the rated voltage of the fuse in fusible elements of variable cross section, the number of narrow isthmuses connected in series increases, which is necessary so that when the fuses operate on each isthmus, a separate arc lights up. As a result of an increase in the number of consecutively burning arcs, the voltage across the fuse rises faster than in cases where the fusible element has only one narrow isthmus.

The creation of several relatively narrow parallel channels for burning an electric arc improves the conditions for extinguishing it by using a larger amount of filler materials and reducing the current in each of the parallel arcs, therefore, when designing, fusible elements are preferred to be divided into a number of parallel branches. The number of parallel branches is limited by technological difficulties in manufacturing narrow isthmuses of small dimensions.

The temperature of the fusible elements in various operating modes of the fuses varies significantly. As a result, there is a greater or lesser elongation of the fusible element. A certain variation in the sizes of fuse-link bodies also leads to a spread in the lengths of fusible elements from fuse to fuse, therefore, in fuse elements, several bends are provided along the length to compensate for the difference in the lengths of the body and fusible element as a result of various factors.

The quality of fuses largely depends on the values ​​of transient electrical resistances. Studies have shown that with a poor contact connection of the fusible element with the contacts of the fusible insert, the transient resistance can reach 50% of the electrical resistance of the fusible element. Because of this, the fuses overheat in the nominal operating mode, and their service life is reduced. In addition, if the contact connection is poor, the reproducibility of test results from one sample to another is impaired. All fusible elements of fuses with high rated currents are connected to the contact terminals by welding, which ensures a good quality of the contact connection. For fuses with low rated currents, soft soldering is sometimes used, but more often mechanical crimping. In collapsible fuses, the fusible element is connected to the fuse-link terminals with a bolt clamp.

DESIGN OF OPERATION INDICATIONS OF FUSIBLE LINKS

The fusible elements of modern fuses are inside an opaque case, and the state of the fusible element cannot be visually determined. It is especially important to have an idea of ​​the condition of the fuse element for fuses with high current ratings due to the significant difficulties associated with installing and removing the fuse. In this regard, various types of indicators are used that show whether the fuse element has blown.

There are a large number of patents for pointer designs. The most widely used is the trip indicator, which uses the same principle as the main fusible element - melting under the action of overcurrent. To create such a pointer, a thin metal wire with sufficient mechanical tensile strength is electrically connected in parallel with the main fusible element. When overcurrent flows through the fuse, the main fusible element and the indicator wire burn out. The actuation indicator wire is fixed tightly on one side, and on the other it is connected to a pin, which is pulled with a spring into a special hole. The indicator wire is in quartz sand. Its length is usually approximately equal to the length of the fusible element, which is necessary for reliable arc quenching at the rated voltage of the fuse.

Trip indicators of this type are manufactured in two types: autonomous - in the form of a small fuse-link with a high-resistance fusible element and a filler, installed in their own housing outside the fuse-link and built into the fuse-link body. Independent operation indicators are sometimes mounted directly on the fusible link, and sometimes they are installed completely away from the fuse, having only electrical connection with it. The latter is typical for the fuses of the English Electric Company (Great Britain).

When the indicator wire burns out, the spring is released, which pushes out the brightly colored pin, which is a visual indicator that the fuse has blown. Sometimes the pin also serves as a striker that acts on the auxiliary contacts of the fuse. As a result, a fuse blown signal is transmitted to the appropriate controls.

Depending on the ratio of electrical resistances and thermophysical parameters of the main fusible element and the pointer, three different cases can be observed when the fuse is triggered:

1) initial melting of the main fusible element, arc burning on it. The active resistance of the pointer shunts the arc of the main fusible element, helping to reduce the rate of voltage rise across the gap and reduce the voltage peak;

2) initial melting of the pointer wire, and then melting of the main fusible element. Due to the fact that the main fusible element has a low active resistance, it will bridge the gap formed after the pointer wire melts and prevent the arc in the pointer from burning for any long time;

3) almost simultaneous melting of the main fusible element and the wire of the operation indicator. The arc burning on the pointer can occur before the end of the arc burning on the main fusible element in some cases, and in others, the arc burning on the pointer will stop much earlier than in the main fusible element

Unfortunately, the pointers of the type in question are unstable. At low voltages and at low current overloads, the wire burns out in a small area. If this area is at a great distance from the spring and if the packing density of the sand filler in the indicator body is large, the frictional forces of the wire on the sand filler may exceed the spring force and the operation indicator may not work. The disadvantage of these indicators is also that in the event of an accidental mechanical breakage of the fusible element during assembly or for any other reason, the operation indicator does not show the actual state of the fuse without turning on the voltage.

Discharge lamps and LEDs connected in parallel with the fusible link are also used as visual indicators of operation. But the cost of such operation indicators is higher, and their reliability in operation is lower than that of the operation indicators described above.

CLOSED FUSES

Closed fuses are usually made in the form of a fiber tube, closed at the ends with brass caps. Fusible inserts inside the tube. The electric arc formed during the combustion of the insert burns in a closed volume. When the arc burns, the walls emit gas, the pressure in the tube rises, and the arc goes out.

Closed fuses of the PR-2 series (collapsible) have rated currents from 100A to 1000 A, the maximum interrupted currents at a voltage of 380V and cosj³0.4 range from 6 kA to 20 kA. Inserts mostly with isthmuses.

FUSES WITH FILLER (LOCK)

Fusible inserts are placed in a medium of fine-grained solid filler (for example: chalk, quartz sand), placed in a porcelain or plastic case. The electric arc that occurs during the melting of the inserts is in close contact with the fine grains of the filler, is intensively cooled, deionized, and therefore quickly extinguished.

Load fuses of the PN-2 series have rated currents from 100 A to 600 A, the maximum breaking current at a voltage of 500 V () is in the range from 25 kA to 50 kA. Series PP31 for rated currents from 63 A to 1000 A, maximum breaking current up to 100 kA at a voltage of 660 V.

In such fuses, parallel inserts are used, which makes it possible to obtain a large cooling surface with the same total cross section of the inserts.

TIME SAFETY

Characteristic on the site b-c is provided by a normal insert of an increased cross section, and in the area a-b another element.

IP series for voltage 30 V and currents from 5 A to 250 A.

LIQUID METAL– current up to 250 kA at a voltage of 450 V AC. Fuses operate repeatedly with a large current limit. (Consider the device yourself; Chunikhin, pp. 514-515).

HIGH-SPEED FOR PROTECTION OF SEMICONDUCTOR DEVICES. PP-57 for rated currents (40-800) A, PP-59 for rated currents (250-2000) A. Rated voltages are up to 1250 V AC and 1050 V DC.

BLOCK FUSE-SWITCH. BPV rated current up to 350 A at alternating voltage up to 550 V.

FUSE SELECTION

Fuses choose

1. according to the condition of start-up and long-term operation;

2. by the condition of selectivity.

1 During long-term operation, the fuse heating temperature should not exceed the permissible values. In this case, the stability of the time-current characteristics of the fuse is ensured. To fulfill this requirement, it is necessary that the cartridge and the fusible link are selected for a rated current equal to or slightly greater than the rated current of the protected installation.

The fuse should not turn off the installation during overloads that are operational (for example, the starting current of an asynchronous motor with a squirrel-cage rotor can reach seven times the rated current. As it accelerates, the starting current drops to a value equal to the rated current of the motor. The duration of the start depends on the nature of the load).

For motors with easy starting conditions (motors of pumps, fans, machine tools)

,those. the rated current of the insert is selected according to the starting current of the load.

For difficult starting conditions, when the engine turns slowly (centrifuge drive, cranes, crushers), or in intermittent mode, when starts are carried out with a high frequency, inserts are selected with an even greater margin

If the fuse is in a line that feeds several motors, the fusible link is selected according to the formula:

where is the calculated rated current of the line, equal to .

Difference taken for the engine, which has the largest.

For welding transformers, the fuse selection conditions are as follows: , where PV is the on-time.

2 The choice of fuses according to the condition of selectivity.

Between the energy source and the consumer, several fuses are usually installed, which should switch off the damaged sections as selectively as possible.

Fuse, passing a larger rated current, has an insert of a larger cross section than a fuse, installed at one of the consumers.

In case of a short circuit, it is necessary that the damage be switched off by a fuse located at the place of damage. All other fuses located closer to the source should remain operational. Such coordination of fuse operation is called selectivity or selectivity. To ensure selectivity, the total operating time () of the fuse must be less than the heating time of the fuse to the melting temperature of its insert, i.e. t pl1 ³t p2. To ensure selectivity, the shortest actual fuse operation time (for a longer current) must be greater than the longest fuse operating time (for a lower rated current): , where and - fuse operating time for higher and lower rated currents corresponding to the nominal characteristic.

The operating time of the fuse due to manufacturing tolerances may deviate from the nominal value by . Then the above inequality can be written as .The multipliers 0.5 and 1.5 take into account that the fuse is taken with a negative operating time tolerance, and the fuse is taken with a positive one. As a result, we obtain the necessary selectivity condition: ,those. for selective operation, the operation time of a fuse with a higher current should be 3 times longer than that of a fuse with a lower current. For fuses of the same type, it is enough to check the insert with a lower rated current at the highest current to check the selectivity.

For different types of fuses, the selectivity check is carried out over the entire range of currents: from a 3-phase short circuit at the end of the protected section to the rated current of the fuse-link.

10 AUTOMATIC SWITCHES (AUTOMATIC)

Circuit breakers, as a rule, are intended to turn off the damaged section of the network when an emergency occurs in it (short circuit, overload current, low voltage). Thermal and electrodynamic (in the event of a short circuit) effects of increased currents can lead to failure of electrical equipment. In undervoltage conditions, if the mechanical torque of the load on the shaft remains unchanged, an increased current will also flow through the running motors.

The machine, unlike the contactor, has a node of protection elements that automatically detects the appearance of abnormal conditions in the network and gives a signal to turn off. If the contactor is designed only to cut off overload currents that reach several thousand amperes, then the machine must turn off short-circuit currents that reach many tens and even hundreds of kiloamperes. In addition, the machine rarely turns off the electrical circuit, while the contactor is intended for frequent operational switching of rated load currents.

There are several types of machines: universal(work on direct and alternating current), installation(intended for installation in public premises and are made according to the type of installation products), fast acting DC and magnetic field suppression powerful generators.

Figure - Structural diagram of the machine

The figure shows a conditional structural diagram of a universal automaton in a simplified image. The machine switches the electrical circuit connected to terminals A and B. In the indicated position, the machine is turned off and the power circuit is open. To turn on the machine, it is necessary to manually rotate the handle 3 clockwise. A force is created that, by moving the levers 4 and 5 to the right, will rotate the main bearing part 6 of the machine around the fixed axis O clockwise. They close and turn on the current circuit, first the arcing 8 and 10, and then the main 7 and 11 contacts of the machine. After that, the entire system remains in the extreme right position, fixed by a special latch, and is held by it (not shown in the figure).

The opening spring 2 is charged when the machine is turned on. When a shutdown command is given, it turns off the machine. When a short circuit current flows through the coil of the electromagnetic release 1, an electromagnetic force is created on its armature, moving the levers 4 and 5 upwards beyond the dead point, as a result of which the machine is automatically turned off by the spring 2. In this case, the contacts open, and the arc arising on them is blown into the arc chute 9 and extinguished in it.

The system of levers 4 and 5 performs the functions of a free release mechanism, which in real machines has a more complex device. The free-release mechanism allows the machine to be switched off at any time, including during switching on, when the closing force acts on the machine's moving system. If levers 4 and 5 are moved up beyond the dead point, then the rigid connection between the drive and movable systems is broken. The dead point corresponds to such a position of the levers when the straight lines and connecting the axes of rotation coincide in direction with each other. The machine is immediately switched off due to the action of the return spring 2, regardless of whether the closing force acts on the drive system of the machine or not.

The free trip mechanism prevents the possibility of following one after another cycles of “off-on” of the machine (“machine jump”) when it is possible to turn it on to a short circuit existing in the circuit. Let us imagine that when the contacts of the switched-on machine come into contact, a short-circuit current passes through the circuit. In this case, the maximum release 1 will work and move the levers of the free trip mechanism 4 and 5 upward beyond the dead point. The machine will turn off and will not turn on again, since the mechanical connection between the closing force and the machine's moving system is broken. If there were no free-release mechanism, then after the automatic disconnection of the machine, it would be immediately re-enabled under the influence of the force of the enabling device, which by this time could not have been removed. There would be multiple switching off and on of the machine in a heavy short circuit mode quickly following one after another, which could lead to the destruction of the machine.

When the machine is turned off, the main contacts 7 and 11 open first, and all the current will go into a parallel circuit of arcing contacts 8 and 10 with linings made of arc-resistant material. On the main contacts, the arc should not occur so that these contacts do not burn. The arcing contacts open when the main contacts diverge a considerable distance. An electric arc appears on them, which is blown up and extinguished in the arc chute 9.

When the machine is turned on, the arcing contacts are closed first, and then the main ones. The electric arc, which is possible due to vibration of the contacts, occurs and is extinguished only at the arcing contacts.

Fast automata designed to protect DC installations (transport, converter). Their own response time is fractions of a millisecond, conventional automata are tenths of a second.

The fast opening of contacts in the event of an emergency mode in the network determines the characteristic feature of these machines. The resistance of an electric arc that appears early on the contacts, connected in series to the disconnected circuit, limits the short-circuit current, preventing it from rising to a steady value. The speed of the device is achieved by using polarized electromagnetic devices in the drive, intense arcing devices, magnetic systems in which changing magnetic fluxes do not interlock with closed windings and pass through the laminated part of the magnetic circuits (combating the slowing effect of eddy currents), etc., as well as the maximum simplification of the kinematic scheme of the device and the elimination of intermediate links between the measuring body (release device) and contacts.

AUTOMATIC RELEASES

Releasers in automatic machines are measuring bodies. They control the value of the corresponding parameter of the protected circuit and give a signal to turn off the machine when it reaches a predetermined value, called setting(trip current, trip voltages, etc.). The releases provide for the possibility of adjusting the setting in a fairly wide range. This is necessary for the implementation selective(selective) protection of the electrical network in which the machine is connected.

The selectivity of protection is achieved primarily due to the different response times of the previous and subsequent protection stages. The difference in the response time of these stages is called time selectivity. There is also current selectivity.

In an extensive network, an increase in the time delay from one protection stage to another can lead to an unacceptably large value of this delay at the last protection stages. Long-term flow of a high short-circuit current (10 kA) can lead to unacceptable heating of the wires in the circuit. Therefore, at high currents, it is advisable to carry out an instant shutdown of the machine (located close to the place of which the circuit is closed) using the current cutoff release.

In addition to the electromagnetic current, a thermal release can respond to the magnitude of the current, the device of which is similar to a thermal relay. This release is not used for protection against short-circuit currents, since it creates in this case unacceptably high time delays, however, it makes it possible to obtain the long time delays required under operating conditions for overload currents. Thermal releases have disadvantages: their protective characteristics (dependence of operating time on current) are unstable and change with ambient temperature; the release time to return to its original position after tripping is long.

The machines also use undervoltage releases that give a command to turn off the machine when the voltage drops below a predetermined level. Such releases are usually built on the electromagnetic principle. When the voltage drops below a predetermined level, the electromagnetic force is less than the force of the return spring. The armature of the electromagnet is released and through the intermediate link (roller) acts on the latch of the machine, as a result of which the latter is turned off.

Unlike electromagnetic, semiconductor releases, which have been widely used recently, do not have such a large number of moving mechanical elements. But their main advantages lie in the improvement of operational characteristics: wide ranges of regulation of currents and operation time, which allows to unify products and produce a smaller range of them, finer and more accurate adjustment of the operation time at high short-circuit currents, etc. In the measuring elements of such releases, current transformers are used, and one of their main nodes is the time delay unit. They also include an output relay that transmits a signal to the tripping electromagnet. The time delay in such releases is carried out through the use of RC circuits in the transistor control circuits and the use of magnetic storage devices and non-contact impulse counters.

ARC-FREE CONTACT DEVICES

The AC circuit can be disconnected without arcing by opening the contacts at sufficient speed just before the current passes through zero. At this time, the electromagnetic energy stored in the circuit approaches zero.

Figure Half wave current

The figure shows a half wave of alternating current. If point A corresponds to the moment of opening the contacts and the formation of an arc, then the arc in this half-cycle will burn for a time. During this time, an amount of electricity will pass through it, determined by the area, and the energy released in the arc will be relatively large. When the contacts of the apparatus open just before the current passes through zero (point B), much less energy will be released in the arc, since the time of its existence and the instantaneous values ​​of the currents will be much less. When the contacts of the apparatus diverge before the current passes through zero, the amount of electricity in the gas discharge stage is determined by the area and the arc column does not have time to accumulate a significant supply of thermal energy in its volume. This heat is rapidly dissipated near the current zero crossing, and the restoring strength of the intercontact gap acquires high values ​​and rapidly increases with time. Conditions are created under which the arc goes out without having time to develop. Turning off the AC circuit becomes almost arc-free. Breaking devices with a fixed moment of contact divergence immediately before the zero value of the alternating current are commonly called synchronous switches.

The main difficulty in creating synchronous switches is to achieve the required accuracy of operation of the device immediately before zero current and to separate the contacts by the required isolation distance in a very short time before the current passes through zero. To overcome these difficulties, the current pause is artificially extended to one half-cycle (with at) using diodes.

COMMANDERS AND NON-AUTOMATIC SWITCHES

Command devices include limit and limit switches, control buttons, multi-circuit devices - control keys and controllers, numerous pairs of contacts of which are switched in a certain sequence when the handle is turned from one position to another.

Limit and limit switches carry out switching of control and automation circuits on a given section of the path traversed by a controlled mechanism. Limit switches are installed, for example, in the mechanisms of hoisting and transport devices, in the calipers of metal-cutting machines. In the first case, they limit the height of lifting loads, in the second - the stroke of the caliper, giving a signal at the end of the controlled stroke of the mechanism to turn off the engines (and in the lifts, also a signal to trigger the brake electromagnet).

controller- a multi-position device that controls the coils of contactors, the main contacts of which are included in the power circuits of electrical machines, transformers and resistors. The controller is also a multi-position device designed to control electrical machines and transformers by switching directly the power circuits of the windings of machines, transformers, and resistors. With the help of controllers (and controllers) start, speed control, reversal and stop of motors can be carried out.

Batch switches- Closed devices. The arc arises and is extinguished in a limited volume, as a result, the pressure in this volume increases. With increasing pressure, the resistance of the arc and the voltage across it increase. Physically, this is explained by the fact that with increasing pressure, the distances at which elementary particles of gas interact decrease. This leads, firstly, to an increase in the intensity of heat transfer between gas particles and an improvement in the conditions for heat transfer from the arc, and, secondly, to a decrease in the mean free path of electrons in the gas. Ceteris paribus, this reduces the intensity of ionization processes, since an electron with a shorter mean free path is able to acquire less energy by moving in an electric field. This leads to an increase in resistance and arc voltage.

11 ELECTRO-MECHANICAL SWITCHING DEVICES

CONTACTORS AND MAGNETIC STARTERS

The contactor is a two-position self-resetting device designed for frequent switching of currents not exceeding overload currents and driven by a drive. This device has two switching positions corresponding to its on and off states. In contactors, the electromagnetic drive is most widely used. The return of the contactor to the off state (self-return) occurs under the action of the return spring, the mass of the moving system, or the combined action of these factors.

Actuator- This is a switching device designed to start, stop and protect electric motors without removing and introducing resistors into their circuits. Starters carry out protection of electric motors from overload currents. A common element of such protection is a thermal relay built into the starter.

Overload currents for contactors and starters do not exceed (8-20)-fold overloads in relation to the rated current. For the starting mode of motors with a phase rotor and braking with countercurrent, (2.5-4)-fold overload currents are characteristic. The starting currents of electric motors with a squirrel-cage rotor reach (6-10)-fold overloads compared to the rated current.

The electromagnetic drive of contactors and starters, with the appropriate choice of parameters, can perform the functions of protecting electrical equipment from undervoltage. If the electromagnetic force developed by the drive, when the voltage in the network is reduced, is not enough to keep the device in the on state, then it will spontaneously turn off and thus protect against undervoltage. As you know, a decrease in voltage in the supply network causes the flow of overload currents through the windings of electric motors if the mechanical load on them remains unchanged.

Contactors are designed for switching power circuits of electric motors and other powerful consumers. Depending on the type of switched current of the main circuit, direct and alternating current contactors are distinguished. They have main contacts equipped with an arcing system, an electromagnetic drive and auxiliary contacts. As a rule, the type of current in the control circuit that feeds the electromagnetic drive is the same as the type of current in the main circuit. However, cases are known when the coils of AC contactors are powered by a DC circuit.

Figure 1 - Structural diagram of the contactor

On fig. 1 shows a structural diagram of a contactor that turns off the motor circuit. In this case, there is no voltage on the coil 12 and its movable system, under the action of the return spring 10, which creates the force F in, will return to its normal state. The arc D that occurs when the main contacts diverge is extinguished in the arc chute 5.

The fast movement of the arc from the contacts to the chamber is provided by the system magnetic blow. The main current circuit includes a series coil 1, which is located on a steel core 2. Steel plates - poles 3, located on the sides of the core 2, bring the magnetic field created by the coil 1 to the arc burning zone in the chamber. The interaction of this field with the arc current results in forces that move the arc into the chamber.

The contactor will turn on the circuit with current I 0 if voltage is applied U on the coil 12 drive electromagnet. The flux F, created by the current flowing through the coil of an electromagnet, will develop a traction force and attract an armature 9 electromagnet to the core, overcoming the forces F in counteracting the return 10 And Fk contact 8 springs.

The core of the electromagnet ends with a pole piece 11, the cross section of which is greater than the cross section of the core itself. By installing the pole piece, a slight increase in the force generated by the electromagnet is achieved, as well as a modification of the traction characteristic of the electromagnet (the dependence of the electromagnetic force on the size of the air gap).

Contact contact 4 And 6 with each other and the closing of the circuit when the contactor is turned on will occur before the armature of the electromagnet is completely attracted to the pole. As the armature moves, the moving contact 6 will, as it were, "fall through", resting its upper part against a fixed contact 4. It will rotate some angle around the point A and will cause additional compression of the contact spring 8. will appear contact failure, which means the amount of displacement of the movable contact at the level of the point of its contact with the fixed contact in case the fixed one is removed.

The failure of the contacts provides a reliable circuit closure when the thickness of the contacts decreases due to the burnout of their material underneath. the action of an electric arc. The value of the dip determines the supply of contact material for wear during the operation of the contactor.

After contact, the contacts roll over the movable contact over the fixed one. The contact spring creates a certain pressure in the contacts, therefore, when rolling, oxide films and other chemical compounds that may appear on the surface of the contacts are destroyed. When rolling, the points of contact of the contacts move to new places of the contact surface that were not exposed to the arc and are therefore more “clean”. All this reduces the contact resistance of the contacts and improves their working conditions. At the same time, rolling increases the mechanical wear of the contacts (contacts wear out).

At the moment of contact, the moving contact 6 immediately makes a fixed contact 4 pressure due to contact spring preload 8. As a result, the contact resistance of the contacts at the moment of their contact will be small and the contact pad will not heat up to a significant temperature when turned on. In addition, the pre-contact pressure created by the spring 8, allows you to reduce vibration(rebounds) of a movable contact when it strikes a fixed contact. All this protects the contacts from welding when the electrical circuit is turned on. The contacts have contact pads, made of a special material, such as silver, to improve the conditions for the continuous passage of current through closed contacts in the on state. Sometimes linings made of arc-resistant material are used to reduce the wear of contacts under the influence of an electric arc (cermet "silver-cadmium oxide", etc.). Flexible connection 7 (for supplying current to the moving contact) is made of copper foil (tape) or thin wire.

contact solution is the distance between the moving and fixed contacts in the off state of the contactor. The contact spacing usually ranges from 1 to 20 mm. The lower the contact gap, the shorter the stroke of the armature of the drive electromagnet. This leads to a decrease in the working air gap in the electromagnet, magnetic resistance, magnetizing force, power of the electromagnet coil and its dimensions. The minimum value of the contact gap is determined by: technological and operational conditions, the possibility of forming a metal bridge between the contacts when the current circuit is broken, the conditions for eliminating the possibility of closing contacts when the moving system rebounds from the stop when the device is turned off. The contact gap must also be sufficient to provide conditions for reliable arc quenching at low currents.

Figure 2 - Direct Throw Starter

Shown in fig. 1 rotary type contactor circuit is quite typical. Typically, such contactors are intended for heavy duty operation (high frequency of switching cycles, inductive circuits) at relatively high rated currents (tens and hundreds of amperes). Another common type of contactors and starters is straight-throw; it is calculated mainly for lower rated currents (tens of amperes) and lighter working conditions. The forward starter (fig. 2) has bridge contacts 2 And 3, from which the arc is blown into arc chutes 1. Force Fk contact spring creates pressure in closed contacts, return spring F p returns the mobile system of the device to the off state when the voltage is removed from the coil. The device is switched on by an electromagnet when voltage is applied to its coil. 5. Short-circuited coils are installed on the poles of an alternating current electromagnet 4, eliminating the vibration of the armature in the on position of the device.

Unlike a DC contactor, an AC contactor uses laminated magnetic circuits and short-circuited turns at the poles to reduce armature vibration to reduce eddy current losses. AC contactors are often made three-pole, direct current - single-pole and two-pole. Slotted chambers are more often used as an arc extinguishing device in direct current contactors, and an arc chute is more often used on alternating current.

To extinguish the arc, chambers with an arc chute are also used. The arc chute is a package of thin metal plates 5 (Fig. 1). Under the action of electrodynamic forces created by the magnetic blast system, the electric arc hits the grate and breaks into a series of short arcs. The plates intensively remove heat from the arc and extinguish it, but the plates of the arc quenching grid have a significant thermal inertia - with a high frequency of switching on, they overheat and the efficiency of arc extinguishing drops.

Powerful AC contactors have main contacts equipped with an arc extinguishing system - magnetic blast and an arc chute with a narrow slot or arc chute, like DC contactors. The design difference lies in the fact that AC contactors are multi-pole; they usually have three main NO contacts. All three contact units operate from a common valve-type electromagnetic drive that rotates the contactor shaft with movable contacts installed on it. Bridge-type auxiliary contacts are installed on the same shaft. Contactors are quite large overall dimensions. They are used to control electric motors of considerable power.

To increase the service life, the design of the contactors allows the change of contacts.

There are combined AC contactors in which two thyristors are connected in parallel with the main closing contacts. In the on position, current flows through the main contacts, since the thyristors are in the closed state and do not conduct current. When the contacts open, the control circuit opens the thyristors, which shunt the circuit of the main contacts and unload them from the tripping current, preventing the occurrence of an electric arc. Since thyristors operate in short-time mode, their power rating is low and they do not need cooling radiators.

Our industry produces combined contactors of the KT64 and KT65 types for rated currents exceeding 100 A, made on the basis of the widely used KT6000 contactors and equipped with an additional semiconductor unit.

Switching wear resistance of combined contactors in the normal switching mode is at least 5 million cycles, and switching wear resistance of semiconductor blocks is approximately 6 times higher. This allows them to be reused in control systems.

To control AC motors of small power, direct contactors with bridge contact assemblies are used. Double break of the circuit and facilitated conditions for extinguishing the AC arc make it possible to do without special arc chutes, which significantly reduces the overall dimensions of the contactors.

Direct throw contactors are usually produced by the industry in a three-pole version. In this case, the main closing contacts are separated by plastic jumpers 1.

Along with low-current reed switches, hermetically sealed power magnetically controlled contacts (gersikons) have been created that are capable of switching currents of several tens of amperes. On this basis, contactors were developed to control asynchronous motors with a power of up to 1.1 kW. Hersicons are distinguished by an increased contact gap (up to 1.5 mm) and increased contact pressure. To create a significant force of electromagnetic attraction, a special magnetic circuit is used.

The scope of electromagnetic contactors is quite wide. In mechanical engineering, AC contactors are most often used to control asynchronous electric motors. In this case, they are called magnetic starters. A magnetic starter is the simplest set of devices for remote control of electric motors and, in addition to the contactor itself, often has a push-button station and protection devices.

Figure 1 (a, b) shows, respectively, the mounting and circuit diagrams of the connections of a non-reversible magnetic starter. On the wiring diagram, the boundaries of one apparatus are outlined with a dashed line. It is convenient for hardware installation and troubleshooting. These diagrams are difficult to read because they contain many intersecting lines.

Figure 1 - Schemes of a non-reversing starter

On the circuit diagram, all elements of one apparatus have the same alphanumeric designations. This allows not to link together the conventional images of the contactor coil and contacts, achieving the greatest simplicity and clarity of the circuit.

The non-reversing magnetic starter has a KM contactor with three main NO contacts (L1-C1, L2-C2, L3-C3) and one auxiliary NO contact (3-5).

The main circuits through which the current of the electric motor flows are usually depicted in bold lines, and the power circuits of the contactor coil (or control circuit) with the highest current are thin lines.

To turn on the electric motor M, you must briefly press the button SB2 "Start". In this case, a current will flow through the circuit of the contactor coil, the armature will be attracted to the core. This will close the main contacts in the motor power circuit. Auxiliary contact 3-5 will close at the same time,

which will create a parallel supply circuit for the contactor coil. If the "Start" button is now released, the contactor coil will be switched on via its own auxiliary contact. Such a scheme is called a self-locking scheme. It provides the so-called zero motor protection. If during the operation of the electric motor the voltage in the network disappears or drops significantly (usually by more than 40% of the nominal value), then the contactor is switched off and its auxiliary contact opens. After the voltage is restored, to turn on the electric motor, press the "Start" button again. Zero protection turns an unexpected, spontaneous start of the electric motor, which can lead to an accident.

Manual control devices (knife switches, limit switches) do not have zero protection, therefore contactor control is usually used in machine drive control systems.

To turn off the motor, just press the SB1 "Stop" button. This leads to opening of the self-supply circuit and disconnection of the contactor coil.

In the case when it is necessary to use two directions of rotation of the electric motor, a reversible magnetic starter is used, the schematic diagram of which is shown in Figure 2, a. To change the direction of rotation of an asynchronous electric motor, it is necessary to change the phase sequence of the stator winding. The reversible magnetic starter uses two contactors: KM1 and KM2. It can be seen from the diagram that if both contactors are accidentally turned on simultaneously, a short circuit will occur in the main current circuit. To prevent this, the circuit is equipped with a blocking. If, after pressing the button SB3 "Forward" and turning on the contactor KM1, press the button SB2 "Back", then the opening contact of this button will turn off the coil of the contactor KM1, and the normally open contact will supply power to the coil of the contactor KM2. The motor will reverse.

Figure 2 - Schemes of a reversing starter

A similar diagram of the control circuit of a reversing starter with blocking on auxiliary break contacts is shown in Figure 2, b. In this circuit, the inclusion of one of the contactors, for example KM1, leads to the opening of the power circuit of the coil of the other contactor KM2. To reverse, you must first press the button SB1 "Stop" and turn off the contactor KM1. For reliable operation of the circuit, it is necessary that the main contacts of the contactor KM1 open before the closing of the breaking auxiliary contacts in the circuit of the contactor KM2. This is achieved by appropriate adjustment of the position of the auxiliary contacts along the armature.

In serial magnetic starters, double blocking is often used according to the above principles. In addition, reversing magnetic starters can have a mechanical interlock with a toggle lever that prevents the contactor electromagnets from simultaneously operating. In this case, both contactors must be installed on a common base.

Magnetic starters of open design are mounted in electrical cabinets. Dust-proof and dust-splash-proof starters are provided with a casing and mounted on a wall or rack as a separate device.

Electromagnetic contactors choose according to the rated current of the electric motor, taking into account the operating conditions. GOST 11206-77 establishes several categories of AC and DC contactors. AC contactors of categories AC-2, AC-3 and AC-4 are designed for switching power supply circuits of asynchronous electric motors. Category AC-2 contactors are used for starting and switching off motors with a phase rotor. They operate in the lightest mode, since these motors are usually started using a rotary rheostat. Categories AC-3 and AC-4 provide direct starting of squirrel-cage motors and must be rated for six times the starting current. Category AC-3 provides for the shutdown of a rotating asynchronous motor. Category AC-4 contactors are designed for countercurrent braking of electric motors with a squirrel-cage rotor or switching off stationary electric motors and operate in the most difficult mode.

Contactors designed for operation in AC-3 mode can be used in conditions corresponding to category AC-4, but the rated current of the contactor is reduced by 1.5-3 times. Similar utilization categories are provided for DC contactors.

Contactors of category DS-1 are used for switching low-inductive loads. Categories DS-2 and DS-3 are designed to control DC motors with parallel excitation and allow switching current equal to. Categories DC-4 and DC-5 are used to control DC motors with series excitation.

These categories define the normal switching mode, in which the contactor can operate continuously for a long time. In addition, there is a mode of rare (random) switching, when the switching capacity of the contactor can be increased by about 1.5 times.

If the asynchronous motor operates in the intermittent mode, then the selection of the contactor is carried out according to the value of the rms current. The choice of contactor is influenced by the degree of protection of the contactor. Protected contactors have worse cooling conditions and their current rating is reduced by about 10% compared to open contactors.

CONTACT - ARC EXTINGUISHING SYSTEMS OF CONTACTORS

Lever (Fig. 1, a) and bridge (Fig. 1, b) contacts are usually used in contactors. In lever contacts, one break (one arc) is formed when disconnected, in bridge contacts - two (two arcs). Therefore, ceteris paribus, the possibilities for disconnecting electrical circuits in devices with bridge contacts are higher than in devices with lever (finger) contacts.

Picture 1 - Lever and bridge contacts

Bridge contacts, compared with lever contacts, have the disadvantage that in the closed state two contact current transitions are created in them, in each of which a reliable touch must be created. Therefore, the force of the contact spring must be doubled (compared to lever contacts), which ultimately increases the power of the electromagnetic drive of the contactor.

In AC contactors for interrupted currents up to 100 A at a mains voltage of up to 100-200 V, arc chutes can not be used, since the arc is extinguished by stretching it in atmospheric air (open break). Insulating barriers are used to prevent overlapping of electric arcs at adjacent poles. Contactors with open arc breaking also exist on direct current, but the interrupted currents for them are much less.

At high values ​​of currents and voltages to be switched off, the devices are equipped with arc chutes, of which the most common slit cameras And arc chutes. The slit chamber (Fig. 2, a) forms inside a narrow gap (slit) between the walls of arc-resistant insulating material (asbestos cement, etc.). An electric arc 1 is driven into it and there it is extinguished due to increased heat removal in close contact with the walls.

The arc chute (Fig. 2, b) is a package of thin (mm) metal plates 2, on which the arc is blown. The plates act as heat sinks, intensively removing heat from the arc column and contributing to its extinction.

The most important characteristic of the arc chute is the volt-ampere characteristic. Using it, it is possible to calculate the processes of extinguishing the arc when the circuit is turned off.

Figure 2 - Arc chutes

As experience has shown, the arc chute is unsuitable for frequent circuit interruptions at relatively high currents. With a high frequency of shutdowns, its plates are heated to high temperatures and do not have time to cool down. They are unable to cool the arc column, and the grate fails to work. For the mode of frequent shutdowns of the circuit, slotted arc chutes are more suitable. , m, between plates 3 in fig. 3, a) in accordance with the law of total current for a uniform field (HL=Iw) field strength (A/m)

.

Substituting this value into (*), we get:

,

where is the number of coil turns.

Since in a system with a coil of sequential magnetic blowing, the force is proportional to the square of the current, it is advisable to use this type of blowing in contactors designed for relatively high rated currents. To reduce the consumption of copper for the manufacture of the coil, the cross section of which should be selected according to the rated current of the contactor, it is desirable to have as few coil turns as possible. However, this number of turns should provide such a magnetic field strength in the zone of its interaction with the arc current, which will create conditions for reliable arc quenching in a given range of interrupted currents. It is usually measured in units at rated currents of hundreds of amperes, and at currents of tens of amperes it reaches ten or more.

The advantage of series magnetic blow coil systems is that the direction of the force is independent of the direction of the current. This allows you to use the specified system not only on direct, but also on alternating current. However, at alternating current, due to the appearance of eddy currents in the magnetic circuit, a phase shift can occur between the arc current and the resulting magnetic field strength in the arc burning zone, which can cause the arc to “throw back” into the chamber.

The disadvantage of a system with a series magnetic blowing coil is the low magnetic field strength created by it at small interrupted currents. Therefore, the parameters of this system must be chosen so as to ensure the maximum possible magnetic field strength in the arc burning zone in the region of these currents, without resorting to a significant increase in the number of turns of the magnetic blowing coil, so as not to cause excessive consumption of copper for its manufacture. At low currents, the magnetic circuit of this system should not be saturated. Then almost the entire magnetizing force of the coil is compensated by the drop in the magnetic potential in the air gap and the magnetic field strength in it will be the maximum possible. At high currents, the magnetic circuit, on the contrary, it is advisable to enter into saturation when its magnetic resistance becomes large. This will reduce the magnetic field strength in the arc location area, reduce the strength and intensity of the arc extinguishing, and reduce the overvoltage during its extinction.

There is a system with a coil of parallel magnetic blowing, when coil 1 (see Fig. 3), containing hundreds of turns of thin wire and calculated on the full voltage of the power source, creates a magnetic field strength (A / m) in the arc burning zone

.

Electrodynamic force acting on the arc (N) (see Fig. 3, b)

,

Where

In this system, the force acting on the arc is proportional to the current to the first power. Therefore, it turns out to be more appropriate for contactors for small currents (up to about 50 A).

A contactor with a parallel coil of magnetic blowing reacts to the direction of the current. If the direction of the magnetic field remains unchanged, and the current changes its direction, then the force will be directed in the opposite direction. The arc will not move to the arc chamber, but in the opposite direction - to the magnetic blowing coil, which can lead to an accident in the contactor. This is a shortcoming of the system under consideration. The disadvantage of this system is also the need to increase the insulation level of the coil, based on the full voltage of the network. A decrease in the mains voltage leads to a decrease in the magnetizing force of the coil and a weakening of the intensity of the magnetic blast, which reduces the reliability of arc extinguishing.

In a magnetic blowing system, a permanent magnet can be used instead of a voltage coil. In terms of properties, such a system is similar to a system with a parallel magnetic blast coil. Replacing the voltage coil with a permanent magnet eliminates the waste of copper and insulating materials that would be required to build the coil. At the same time, the properties of the permanent magnet should not be violated in the system during operation.

Systems with a parallel magnetic blow coil and alternating current permanent magnets are not used, since it is practically impossible to match the direction of the magnetic flux with the direction of the arc current in order to obtain the same direction of force at any given time.

With an increase in the field strength of the magnetic blast, the conditions for the arc to leave the contacts on the arcing horns improve and its entry into the chamber is facilitated. Therefore, with growth, the wear of contacts from the thermal impact of the arc also decreases, but up to a certain limit.

Large field strengths create significant forces that act on the arc and eject molten metal bridges from the intercontact gap into the atmosphere. This increases contact wear. With optimal field strength, contact wear is minimal.

Contact wear is an important technical factor. Therefore, serious measures are taken, such as reducing the vibration of the contacts when the machine is turned on, in order to reduce wear and increase the life of the contacts.

An important characteristic of an AC arcing device is the pattern of growth restoring strength intercontact gap after the current zero crossing.

12 RELAY. INTEGRATED MICROCIRCUITS - A TECHNICAL BASIS FOR CREATING RELAY PROTECTION EQUIPMENT

Relay protection of any electrical installation contains three main parts: measuring, logical and output. The measuring part includes measuring and starting protection elements that act on the logic part when the electrical parameters (current, voltage, power, resistance) deviate from the values ​​previously set for the protected object.

The logical part consists of separate switching elements and time delay elements, which, upon a certain action (operation) of the measuring and starting elements, in accordance with the startup program embedded in the logical part

A fuse is a single-use power electronics component that performs a protective function. The fuse is the weakest section of the protected electrical circuit, operating in emergency mode, thereby breaking the circuit and preventing the subsequent destruction of more valuable elements of the electrical circuit by high temperatures caused by excessive currents.

In an electrical circuit, a fuse is a weak section of an electrical circuit that burns out in emergency mode, thereby breaking the circuit and preventing subsequent destruction by high temperature.

Fuses are divided into the following types:

1. low current inserts(to protect small electrical appliances up to 6 amps)

• 3x15 (the first number means the outer diameter, the second - the length of the insert)
• 10x30

2. forklift(for protection of electric circuits of cars)

• miniature
• conventional forklifts

3. cork(found in the residential sector, up to 63 amperes)

• DIAZED (the most common in the USSR)
• NEOZED

4. knife(up to 1250 amps)

• size 000 (up to 100 amps)
• size 00 (up to 160 amps)
• size 0 (up to 250 amps)
• size 1 (up to 355 amps)
• size 2 (up to 500 amps)
• size 3 (up to 800 amps)
• size 4a (up to 1250 amperes)

5. quartz

6. gas generating

Also, fuses differ in their operating characteristics relative to the rated current. Due to the inertness of operation of fuses, in a professional environment of electricians, they are quite often used as selective protection in tandem with circuit breakers. Selectivity between the fuses themselves is achieved by a ratio of 1:1.6 [ibid], the time-current characteristic of the fuses is set by the dependence, respectively, I²t; The PUE regulates the protection of overhead conductive lines in such a way that the fuse operates in 15 seconds (the short-circuit current at the end of the line must be equal to three rated currents of the fuse). An essential value is the time during which the destruction of the conductor occurs when the set current is exceeded. To reduce this time, some fuses contain a preload spring. This spring also spreads the ends of the destroyed conductor, preventing the occurrence of an arc.

40-amp fuses with "gG" tripping characteristic, equivalent to the Soviet "PPN" characteristic

• fusible link - an element containing a discontinuous part of an electrical circuit (for example, a wire that burns out when a certain current level is exceeded)
• a mechanism for attaching a fusible link to contacts that ensure the inclusion of a fuse in an electrical circuit and the installation of a fuse as a whole.

Fuse cases are usually made of high-strength grades of special ceramics (porcelain, steatite or corundum-mullite ceramics). For fuse cases with low rated currents, special glasses are used. The body of the fusible link usually plays the role of a base part, on which the fusible element with the contacts of the fusible link, the operation indicator, free contacts, devices for operating the fuse link and a rating plate are mounted. At the same time, the housing performs the functions of an electric arc extinguishing chamber.

Fuse marking

The first letter means the protection range:

• a - partial range (short-circuit protection only)
• g - full range (protection against both short circuit currents and overload)
• h - high breaking capacity (tubes are made of white or gray ceramics)

The second letter means the type of protected equipment:

• G - universal fuse for protecting various types of equipment: cables, electric motors, transformers
• L - protection of cables and switchgears
• B - mining equipment protection
• F - protection of low-power circuits
• M - protection of circuits of electric motors and disconnecting devices
• R - semiconductor protection
• S - fast combustion in case of short circuit and average combustion time in case of overload
• Tr - transformer protection