5-lecture. Materials used in electrical appliances

5-LECTURE. MATERIALS USED IN ELECTRICAL APPLIANCES
The materials and technologies used in the manufacture of electrical devices largely determine their quality indicators (reliability, performance, dimensions, and so on).
Various materials are used for electrical appliances. The main materials are: magnetic, conductive and insulating materials.
5.1 Magnetic materials
Magnetic materials are used for the manufacture of magnetic cores of electrical machines. As a rule, it is sheet electrical steel. Depending on the frequency at which the electric machine will operate, the thickness of the electrical steel is also selected (50 Hz - 0.5 mm and 0.35 mm; 400 Hz - 0.2 mm and 0.15 mm; 20 kHz - up to 0.05 mm).
The composition of electrical steel includes alloying additives, the main of which is silicon. Silicon additives reduce magnetic losses in steel. Typically, silicon additives are in the range of 1% 5%, and the higher the silicon content, the lower the magnetic loss in steel. With an increase in the silicon content, the brittleness and hardness of steel also increases, which introduces certain difficulties in its processing. Therefore, steel with a silicon content of 4% ... 5% (ie, high-alloyed) is used where there are no parts that are complex in their configuration, for example, for the manufacture of transformers. Sheet electrical steel with a silicon content of 1% ... 3% (ie, not highly alloyed) is used where there are parts that are complex in their configuration, for example, for the manufacture of stators and rotors of rotating machines with stamped grooves of complex shapes.
In most cases, sheet electrical steel is manufactured at metallurgical plants by the cold rolling method. An important magnetic property of such steel is that when the directions of rolling and magnetic flux coincide, the saturation induction and magnetic permeability increase, and the losses in the steel due to magnetization reversal decrease by about 2-3 times. Therefore, when designing and manufacturing magnetic circuits from cold-rolled steel, it is necessary to take into account the above property, which complicates the design and technology, since it is required to exclude the passage of the magnetic flux across the rolling, forcibly reducing the lengths of the sections where this cannot be avoided. These difficulties, up to the limited use of cold-rolled steel, are especially evident in the design and manufacture of rotating machines (electric motors, generators, etc.), where a complex configuration of magnetic circuits takes place, especially for very large electrical machines. It should be noted that cold rolled electrical steel sheet is widely used in the manufacture of transformers.
Cases of DC machines are made of soft magnetic low-alloy steel, since they are often one of the components of the magnetic circuit. Shafts of electric machines are made of steels with additives of nickel, chromium, etc., that is, high-strength structural steels.
The following ferromagnetic materials are used for the manufacture of magnetic cores in apparatus and instrument making: technically pure iron, high-quality carbon steel, gray cast iron, electrotechnical silicon steel, iron-nickel alloys, iron-cobalt alloys, etc.
Let's consider briefly some of their properties and application possibilities.
Technically pure iron
For magnetic circuits of relays, electrical measuring instruments, electromagnetic clutches, magnetic shields, etc., commercially pure iron is widely used. This material has a very low carbon content (less than 0.1%) and a minimum amount of manganese, silicon and other impurities.
These materials usually include: armco iron, pure Swedish iron, electrolytic and carbonyl iron, etc. The quality of pure iron depends on small amounts of impurities.
The most harmful effects on the magnetic properties of iron are carbon and oxygen. Obtaining chemically pure iron is associated with great technological difficulties and is a complex and expensive process. The technology specially developed in laboratory conditions with double high-temperature annealing in hydrogen made it possible to obtain a single crystal of pure iron with extremely high magnetic properties.
The most widely used steel of the Armco type, obtained by the open-hearth method. This material has a sufficiently high magnetic permeability, significant saturation induction, relatively low cost, and at the same time has good mechanical and technological properties.
The low electrical resistance of armco steel to the passage of eddy currents, which increase the response and release times of electromagnetic relays and couplings, is considered to be a major drawback. At the same time, when using this material for electromagnetic time relays, this property, on the contrary, is a positive factor, since it makes it possible to obtain relatively large decelerations of the relay operation by extremely simple means.
Manufacturing industry

alloyed cast iron have sufficiently satisfactory magnetic properties.

Electrotechnical silicon steels
Thin sheet electrical steel is widely used in electrical and hardware engineering and is used for all kinds of electrical measuring instruments, mechanisms, relays, chokes, ferroresonant stabilizers and other devices operating on alternating current with normal and increased frequency. Depending on the technical requirements for losses in steel, magnetic characteristics and the applied frequency of alternating current, 28 grades of thin sheet steel with a thickness of 0.1 to 1 mm are produced.
To increase the electrical resistance to eddy currents, a different amount of silicon is added to the steel composition and, depending on its content, one obtains: low-alloy, medium-alloy, high-alloy and high-alloy steels.
With the introduction of silicon, losses in steel decrease, magnetic permeability in weak and medium fields increases, and the coercive force decreases. Impurities (especially carbon) in this case have a weaker effect, the aging of the steel decreases (losses in the steel change little over time).
The use of silicon steel improves the stability of the operation of electromagnetic mechanisms, increases the response time for actuation and release, and reduces the possibility of the armature sticking. At the same time, with the introduction of silicon, the mechanical properties of steel deteriorate.
With a significant silicon content (more than 4.5%), steel becomes brittle, hard and difficult to machine. Small stamping results in significant rejects and rapid die wear. Increasing the silicon content also decreases the saturation induction. Silicon steels are produced in two types: hot-rolled and cold-rolled.
Cold rolled steels have different magnetic properties depending on the crystallographic directions. They are divided into textured and low-textured. Textured steels have slightly better magnetic properties. Compared to hot-rolled steel, cold-rolled steel has a higher magnetic permeability and low losses, but provided that the magnetic flux coincides with the direction of rolling of the steel. Otherwise, the magnetic properties of the steel are significantly reduced.
The use of cold-rolled steel for traction electromagnets and other electromagnetic devices operating at comparatively high inductions gives significant savings in n. from. and losses in steel, which makes it possible to reduce the overall dimensions and weight of the magnetic circuit.
According to GOST, the letters and numbers of individual steel grades denote: 3 - electrical steel, the first number 1, 2, 3 and 4 after the letter indicates the degree of alloying of steel with silicon, namely: (1 - low-alloy, 2 - medium-alloy, 3 - high-alloy and 4 - highly alloyed.
The second number 1, 2 and 3 after the letter denotes the amount of losses in steel per 1 kg of weight at a frequency of 50 Hz and magnetic induction B in strong fields, and number 1 characterizes normal specific losses, number 2 - low and 3 - low. The second number 4, 5, 6, 7 and 8 after the letter E indicates: 4 - steel with specific losses at a frequency of 400 Hz and magnetic induction in medium fields, 5 and 6 - steel with magnetic permeability in weak fields from 0.002 to 0.008 A / cm (5 - with normal magnetic (permeability, 6 - with increased), 7 and 8 - steel with magnetic permeability in medium (fields from 0.03 to 10 A / cm (7 - with normal magnetic permeability, 8 - with increased).
The third digit 0 following the letter E indicates that the steel is cold-rolled textured, the third and fourth digits 00 indicate that the steel is cold-rolled with low texture.
For example, steel E3100 is a high-alloyed cold-rolled low-textured steel with normal specific losses at a frequency of 50 Hz.
The letter A, placed after all these numbers, denotes especially low specific losses in steel.
For current transformers and some types of communication devices, the magnetic circuits of which operate at very low inductions

Nickel-iron alloys

These alloys, also known as permalloy, are mainly used for the manufacture of communication and automation devices. The characteristic properties of permalloy are: high magnetic permeability, low coercive force, low losses in steel, and for a number of grades - the presence, in addition, of a rectangular hysteresis loop. Depending on the ratio of iron and nickel, as well as the content of other components, iron-nickel alloys are produced in several grades and have different characteristics.
Iron-nickel alloys are manufactured in the form of cold-rolled, thermally untreated strips and strips with a thickness of 0.02 - 2.5 mm of various widths and lengths. Hot-rolled strips, rods and wires are also produced, but they are not standardized.
Of all grades of permalloy, alloys with a nickel content of 45-50% have the highest saturation induction and a relatively high electrical resistivity. Therefore, these alloys make it possible to obtain, with small air gaps, the required traction force of an electromagnet or relay with small losses. from. on steel and at the same time ensure sufficient performance.
For electromagnetic mechanisms, the residual traction force obtained due to the coercive force of the magnetic material is very important. The use of permalloy gives a decrease in this force.
Alloys of grades 79НМ, 80НХС and 79НМА, which have a very low coercive force, very high magnetic permeability and electrical resistivity, can be used for magnetic circuits of highly sensitive electromagnetic, polarized and other relays.
The use of permalloy grades 80НХС and 79НМА for low-power chokes with a small air gap makes it possible to obtain very large inductances with magnetic circuits small in volume and weight.
For more powerful electromagnets, relays and other electromagnetic devices operating at a relatively high N. c, permalloy has no particular advantages over carbon and silicon steels, since the saturation induction is much lower, and the cost of the material is higher.
Iron-cobalt alloys
An alloy consisting of 50% cobalt, 48.2% iron and 1.8% vanadium (known as permendur) has received industrial application. With relatively small n. from. it gives the highest induction of all known magnetic materials.
In weak fields (up to 1 A / cm) the permendur induction is lower than the induction of hot-rolled electrical steels E41, E48 and, in particular, cold-rolled electrical steels, electrolytic iron and permalloy. The hysteresis and eddy currents of the permendur are comparatively large, and the electrical resistivity is relatively small. Therefore, this alloy is of interest for the manufacture of electrical equipment operating at high magnetic induction (electromagnets, dynamic loudspeakers, telephone membranes, etc.).
For example, for traction electromagnets and electromagnetic relays, its use with small air gaps gives a certain effect. A given tractive effort can be obtained with a smaller magnetic circuit.
This material is produced in the form of cold-rolled sheets with a thickness of 0.2 - 2 mm and rods with a diameter of 8 - 30 mm. A significant disadvantage

iron-cobalt alloys are their high cost, due to the complexity of the technological process and the significant cost of cobalt. In addition to the listed materials, other materials are used in electrical devices, for example, iron-nickelecobalt alloys, which have constant magnetic permeability and very small hysteresis losses in weak fields.

Materials used to make electrical contacts
The contact material is highly dependent on its service life and reliability.
Requirements for materials of contact connections:
1. High electrical and thermal conductivity.
2. Resistant to corrosion.
3. Resistance to high r film formation.
4. Low hardness of the material, to reduce the pressing force.
5. High hardness to reduce mechanical wear during frequent switching on and off.
6. Low erosion.
7. High arc resistance (melting point).
8. High current and voltage required for arcing.
9. Easy handling and low cost.
The listed requirements are contradictory, and it is almost impossible to find material that would satisfy all these requirements.
The following materials are used for contact connections:
Copper. Satisfies almost all of the above requirements, with the exception of corrosion resistance. Copper oxides have low conductivity. Copper is the most common contact material used for both demountable and switching contacts. In dismountable joints, anti-corrosion coatings of working surfaces are used.
In switching contacts, copper is used when pressed over 3 N for all operating modes, except for continuous operation. For continuous operation, copper is not recommended, but if it is used, then measures should be taken to combat oxidation of the working surfaces. Copper can also be used for arcing contacts. At low contact pressure (P <3 N), the use of copper contacts is not recommended.
Silver. Very good contact material that meets all the requirements, except for arc resistance at high currents. It has good wear resistance at low currents. Silver oxides have almost the same conductivity as pure silver. Silver is used for main contacts in high current devices, for all contacts of continuous operation. In contacts for low currents at low pressures (relay contacts, contacts of auxiliary circuits).
Silver is usually used in the form of overlays - the entire part is made of copper or other material, onto which a silver overlay is welded (soldered) to form a working surface.
Aluminum. Compared to copper, it has significantly lower conductivity and mechanical strength. Forms a poorly conductive solid oxide film, which significantly limits its use. Can be used in demountable contact connections (busbars, field wires). For this, the contact working surfaces are silver-plated, copper-plated or reinforced with copper.
However, it should be borne in mind the low mechanical strength of aluminum, as a result of which the connections can weaken over time and the contact will be broken (contact pressure should not be overestimated). Aluminum is not suitable for switching contacts.
Platinum, gold, molybdenum. They are used for switching contacts for very low currents at low pressures. Platinum and gold do not form oxide films. Contacts made of these metals have low contact resistance.
Tungsten and tungsten alloys. With high hardness and high melting point, they have high electrical wear resistance. Tungsten and tungsten-molybdenum alloys, tungsten-platinum, and others are used at low currents for contacts with a high breaking frequency. At medium and high currents, they are used as arcing contacts for breaking currents up to 100 kA and more.

Melting points of various conductive materials

Metal-ceramics - a mechanical mixture of two practically non-alloying metals, obtained by sintering a mixture of their powders or impregnating one with a melt of the other. Moreover, one of the metals has good conductivity, while the other has high mechanical strength, is refractory and arc-resistant. The cermet thus combines high arc resistance with relatively good conductivity.
The most common cermet compositions are: silver - tungsten, silver - molybdenum, silver - nickel, silver cadmium oxide, silver - graphite, silver - graphite - nickel, copper - tungsten, copper - molybdenum, etc. silver, mainly for alternating current) for medium and large breaking currents, as well as for main contacts for rated currents

Conductor Materials

Conducting materials are used to make windings and contacts for electrical machines. As a rule, windings are made of copper or aluminum, and slip rings are made of bronze or steel.
Since copper has a higher (approximately 1.6 times) electrical conductivity than aluminum, it is used more often for the manufacture of windings. Copper windings also make it possible to reduce the size of electrical machines. Therefore, despite the significant difference in price (copper is more expensive than aluminum), copper is preferred.
When is aluminum used? In cases where the windings of an electric machine have a relatively low thermal load, as well as in those cases where the increase in size and weight due to the increase in the size of the windings are not critical factors for the operation of the machine. Therefore, aluminum is much more often used for the manufacture of transformer windings than for the manufacture of windings of rotating electrical machines.
Copper, bronze or steel is used to make collectors and slip rings. Bronze and steel are used where, in addition to electrical conductivity, the mechanical strength of the product is also important.
Insulation materials
Insulating materials are used mainly for reliable prevention of turn-to-turn short circuits of windings of electrical machines, as well as their electrical contact (breakdown) with the case. Turn-to-turn circuits cause overheating of the electrical machine and, as a result, its possible fire. Electrical contact (breakdown) of the windings with the casing, in most cases, can cause electric shock, if the protective grounding of the electrical machine has not been correctly performed before. Therefore, there should always be increased requirements for insulating materials. As a rule, malfunctions caused by insulation faults are eliminated either by major repairs or by replacing the electrical machine. The cost of insulating modern machines is quite high and can be up to 30% ... 70% of their total cost. Basic requirements for insulation: heat resistance, electrical strength, moisture resistance, thermal conductivity, mechanical strength, elasticity.
Heat resistance is an essential requirement for insulation. The duration of operation (or life) of electrical devices in the mode of temperature overloads caused by various reasons largely depends on it. Depending on the heat resistance, insulation materials in electrical engineering were divided into classes.
Heat resistance is the ability of insulation not to change its electrical and mechanical properties under the influence of temperature. The service life and properties of the insulation are strongly influenced by the effect of temperature, which is confirmed by research in this area. For example, studies have shown that an increase in temperature by only 8 Сº in the temperature range of 60 Сº -180 Сº for class A insulation reduces its service life by 2 times.
Enamel wire insulation is mainly used for the manufacture of electrical machines. Enamel insulations are of classes from A to H. Such an important parameter as heat resistance depends on the material from which the enamel is made. For example, heat-resistant enamels are considered to be made of fluoroplastic-3, fluoroplastic-4, less heat-resistant on the basis of lavsan and epoxy resins, even less heat-resistant from polystyrene, polyamide, etc.
Cast insulation is widely used in modern production. Cast insulation is very thick and as a kind of heat-resistant enamel insulation is quite promising. Enamel and cast insulations relatively easily fill the slots of the windings and increase their heat resistance, and also allow mechanizing the process of manufacturing the insulation.
Electric brushes
The brushes provide electrical contact with sliding surfaces (collector and slip rings). They are rectangular bars made of a material of complex composition on a graphite base. The types of brushes are numerous and differ in terms of coefficient of friction, hardness, voltage drop underneath, and so on. Basically, brushes are selected experimentally, while being guided by certain rules, to