They have many characteristics and mandatory parameters. Since contacts are one of the main structural parts of the contactor, then parameters such as opening, dip and pressure on the contacts are considered fundamental. As a result, the contacts are subject to mandatory periodic checks and, if necessary, adjustment. The figure below shows the positions of the contacts of the contactor of the KTP-6000 and KTP-6000 series, at which dips, openings, pressings and simultaneous contact of the main contacts are adjusted.

Checking the failures of contacts of contactors of the KT, KTP series.

It is impossible to measure the dip in practice, so the gap that controls the dip, that is, the gap formed when the main contacts are fully closed, between the contact holder and the adjusting screws of the lever carrying the moving contact, is checked. The failure of the main contacts is controlled in the closed position of the magnetic system of the contactor.

1 - the place of laying the paper tape when measuring the initial pressure on the contact; 2 - clearance that controls the failure of the contact; 3 - contact line of contacts; 4 - the place of laying the paper tape when measuring the final pressure on the contacts; 5 - contact solution; 6 - the direction of application of force when measuring the final pressure on the contacts; 7 - the direction of the application of force when measuring the initial pressure on the contacts; 8 - adjustment of pressing the contact; 9 - adjustment of the dip and simultaneity of contact contact.

The full amount of dip guarantees full final pressure on the contact. As the contact wears out the dip decreases accordingly and the final contact pressure becomes smaller, this can lead to overheating of the contact. The size of the gap controlling the dip must not be allowed to be less than half of its original size.

The contact system of contactors KT and KTP is designed in such a way that it allows two-fold restoration of dips without changing contacts using an adjusting screw for 100 and 160 A models, a bushing for a 400 A model and adjusting screws for 250 and 630 A models. Using a probe produced measuring the size of the gap controlling the failure. Having set the required clearance and making sure that there are no distortions of the movable contact, the adjusting screws should be tightened, and the bushings should be fixed with the petals of the plate.

Contact openings must correspond to the established size depending on the contactor model and are checked by gauge. In cases where the solutions are not in order, it is regulated by turning the stop around the axis by 90o. In contactor models KT and KTP several stop positions, which define the adjustment steps of the solution.

Contact Simultaneity Check

To check the non-simultaneity of contact contact, use a probe that controls the gap between the contacts when other contacts touch each other. It is very convenient to control the simultaneity of touching the contacts with the help of an electric light bulb (3-6 V), which is connected in series in the contact circuit, but within the limits. For new contacts, non-simultaneity of contact up to 0.3 mm is allowed. Please note that the more accurately the dips are adjusted, the less the non-simultaneity of contact contact.

Checking for contact pressure

Contact pressure is regulated by the highest values ​​depending on the contactor model so that after contact wear, the pressure does not decrease below the allowable values. Degree of contact wear determined by the magnitude of the dip. When, as a result of wear of the contacts, the failure is less than the permissible value, they should be replaced with new ones. When changing the pressure, you should pay attention, but what would the pressure line be approximately perpendicular to the plane of contact contact.

The initial pressing is nothing more than the force that is created by the contact spring at the point of initial contact of the contacts. Due to insufficient initial pressure, melting or welding of contacts may occur, and increased initial pressure leads to fuzzy switching on of the contactor or its retention in intermediate positions. The initial pressing is checked with open contacts and no current in the coil. In practice, the control of the initial pressing of the contacts is carried out not on the contact line of the contacts, but between the movable contact and the lever using a dynamometer, a strip of thin paper and a loop. The loop is superimposed on the moving contact, and a thin paper tape is inserted between the shaft protrusion and the adjusting screw (100 and 160 A contactors), between the holder and the adjusting sleeve (400 A contactors). Then the tension of the dynamometer is determined by the force at which the strip of paper is easily pulled out. This force must correspond to the initial pressing of the contact set by one or another model of the contactor. In cases where the tension does not correspond to the required value, it is necessary to change the tightening of the contact spring by turning the adjusting screws, nuts and bushings. After setting the required pressure, the adjusting devices must be firmly fixed so that the setting is not disturbed.

End Press

The final pressing characterizes the pressure of the contacts when the contactor is on. Matching end presses to tabular data is possible only for new contacts. Indeed, as the contacts wear, the magnitude of the final pressure will decrease. To measure the final pressure, it is necessary to fully switch on the contacts, for which the armature of the magnetic system is pressed against the core and wedged or the pull-in coil is connected to full voltage. A strip of thin paper is clamped between the contacts, a loop is put on the movable contact (as when measuring the initial tension). The loop is pulled back by the hook of the dynamometer until the contacts are so far apart that the paper can be moved. The dynamometer at the same time gives indications of the value of the final pressure on the contacts. The end pressure is not adjustable, but controlled. If the end pressure does not correspond to the required one, the contact spring should be replaced and the entire adjustment process should be carried out from the beginning.

Contact solution for electronic devices

In low-voltage electronic devices, the contact gap is mainly determined by the arc-extinguishing criteria, and only at significant voltages (above 500 V) does its value begin to depend on the voltage between the contacts. As experiments show, the arc leaves the contacts already at a gap of 1 - 2 mm.

More unfavorable conditions for extinguishing the arc are obtained at a constant current, the dynamic forces of the arc are so significant that the arc moves intensively and dies out already at a gap of 2–5 mm.

According to these experiments, it can be assumed that in the presence of a magnetic field to extinguish the arc at a voltage of up to 500 V, a solution value of 10 - 12 mm can be taken for a constant current, for alternating current, 6 - 7 mm are taken for all current values. An excessive increase in the solution is not necessary, because it leads to an increase in the stroke of the contact parts of the apparatus, and, as it should, to an increase in the dimensions of the apparatus.

The presence of a bridge contact with 2 breaks allows you to reduce the course of contact, while maintaining the total value of the solution. In this case, a solution of 4 - 5 mm is usually taken for each gap. In particular, excellent results for arc quenching are obtained by using an alternating current bridge contact. Excessive reduction of the gap (less than 4 - 5 mm) is usually not done, because errors in the manufacture of individual parts can significantly affect the size of the gap. As it is necessary to obtain small mixtures, it is necessary to foresee the possibility of its adjustment, which complicates the design.

In the case of contacts operating in conditions where they may be heavily contaminated, the solution must be increased.

Usually the solution increases and. for contacts that open a circuit with a large inductance, because at the moment of extinction of the arc significant overvoltages occur and with a small gap there may be a re-ignition of the arc. The solution also increases for the contacts of protective devices in order to increase their reliability.

The solution grows significantly with an increase in the frequency of the alternating current, because the rate of voltage rise after the arc is extinguished is very high, the distance between the contacts does not have time to deionize and the arc ignites again.

The magnitude of the opening on alternating current of the highest frequency is usually determined experimentally and is very dependent on the design of the contacts and the arc chute. At voltages of 500-1000 V, the size of the solution is usually taken to be 16 - 25 mm. Huge values ​​refer to contacts that switch off circuits with more inductance and huge current.

During operation, the contacts wear out. To ensure their reliable contact for a long time, the kinematics of the electronic device is made in such a way that the contacts are in contact before the movable system (the system for moving movable contacts) reaches the stop. The contact is attached to the moving system through a spring. Due to this, after contact with the fixed contact, the movable contact stops, and the movable system moves further forward to the stop, additionally compressing the contact spring.

Thus, if the motionlessly fixed contact is removed in the closed position of the movable system, then the movable contact will shift to a certain distance, called a dip. The dip determines the reserve for contact wear for a given number of operations. Other things being equal, a larger dip provides higher wear resistance, i.e. longer service life. But a larger failure usually requires a stronger drive system.

Contact pressing- the force that compresses the contacts at the point of contact. Distinguish between the initial pressing at the moment of the initial contact of the contacts, when the dip is zero, and the final pressing at the complete failure of the contacts. As the contacts wear, the failure is miniaturized, and, as it should, additional compression of the spring. The final pressing approaches the initial one. In such a way, initial pressing is one of the main characteristics in which the contact must remain operational.

The main function of the dip is to compensate for contact wear, therefore the dip value is determined first by the value of the greatest contact wear, which is usually taken: for copper contacts - for each contact up to half its thickness (total wear is the full thickness of the 1st contact); for soldered contacts - Until the soldering is completely worn out (total wear is the total thickness of the soldering of the movable and fixed contacts).

In the case of contact lapping, especially rolling, the dip value is very often significantly greater than the maximum wear and is determined by the kinematics of the movable contact, which provides the desired amount of rolling and slip. In these cases, in order to reduce the overall travel of the moving contact, it is advantageous to position the axis of rotation of the moving contact holder closer to the contact surface.

The values ​​of little allowable contact pressures are determined from the criterion for maintaining a measured contact resistance. In the case of taking special measures to maintain a measured contact resistance, the values ​​​​of small contact pressures can be reduced. So, in special compact equipment, the contact material of which does not form an oxide film and the contacts are completely protected from dust, dirt, water and other external influences, contact pressing is miniaturized.

The final contact pressure does not play a decisive role in the work of contacts, and its value at the theoretical level should be equal to the initial pressure. But the choice of failure is almost always associated with compression of the contact spring and an increase in its force, therefore it is unrealistic to constructively obtain uniform contact pressures - initial and final. Usually, the final contact pressure with new contacts exceeds the initial one and a half to two times.

Dimensions of contacts of electronic devices

Their thickness and width are very dependent both on the design of the contact connection, and on the design of the arc quenching device and the design of the entire apparatus as a whole. These dimensions in different designs can be very different and are very dependent on the purpose of the apparatus.

It should be seen that the dimensions of the contacts, which often break the circuit under current and extinguish the arc, are best increased. Under the action of a frequently broken arc, the contacts get very hot; an increase in their size, mainly due to heat capacity, makes it possible to reduce this heating, which leads to a very noticeable decrease in wear and to an improvement in the arc extinguishing criterion. Such an increase in the heat capacity of contacts can be carried out not only due to a direct increase in their size, but also due to arcing horns connected to the contacts in such a way that not only an electronic connection is made, but also a good heat removal from the contacts is ensured.

Contact vibration- the phenomenon of repeated rebound and the next closure of contacts under the influence of various circumstances. Vibration can be damped, when the amplitudes of rebounds decrease and after a certain time it stops, and undamped, when the vibration phenomenon can last at least some time.

Vibration of the contacts is very harmful, because current passes through the contacts and at the moment of rebounds between the contacts an arc occurs, causing increased wear, and from time to time, welding of the contacts.

The prerequisite for the damped vibration that occurs when the contacts are turned on is the impact of the contact on the contact and their subsequent rebound from each other due to the elasticity of the contact material - mechanical vibration.

It is unrealistic to remove 100% mechanical vibration, but it is always better that both the amplitude of the first bounce and the total vibration time are smaller.

The vibration time is characterized by the ratio of the contact mass to the initial contact pressure. In all cases, it is better to have this value smaller. It can be reduced by reducing the mass of the movable contact and increasing the initial contact pressure; but the decrease in mass should not affect the heating of the contacts.

In particular, huge values ​​of the vibration time when switching on are obtained if, at the moment of contact, the contact pressure does not increase abruptly to its own real value. This happens with an incorrect design and kinematic scheme of the movable contact, when, after touching the contacts, the initial pressure is set only after selecting the backlash in the hinges.

It should be noted that an increase in the lapping process usually increases the vibration time, because the contact surfaces, when moving relative to each other, encounter bulges and roughness, which contribute to the rebound of the movable contact. This means that the amount of lapping must be chosen in good sizes, usually determined by experimental method.

The prerequisite for the undamped vibration of the contacts, which appears when they are in a closed position, are electrodynamic forces. Because vibration under the action of electrodynamic forces occurs at huge current values, the resulting arc is very intense and, as a result of such vibration of the contacts, they usually weld. Thus, this kind of contact vibration is completely unacceptable.

To reduce the possibility of vibration under the action of electrodynamic forces, the current leads to the contacts are often made in such a way that the electrodynamic forces acting on the moving contact compensate for the electrodynamic forces arising at the contact points.

When a current passes through the contacts of such a value at which the temperature of the contact points reaches the melting temperature of the contact material, adhesion forces arise between them and the contacts are welded. Welded contacts are such contacts when the force that ensures their divergence cannot overcome the adhesion forces of the welded contacts.

A more common means of preventing contact welding is the use of appropriate materials, also expediently increasing the contact pressure.

ELECTROSPETS

ELECTROSPETS

AC contactors, contact adjustment.

The main parameters of the contact device are the contact gap, the failure of the contacts, and the pressure on the contacts of the contactors, so they are subject to mandatory periodic checks and adjustments in accordance with the data in Table. one.

Contactor type	Contact gap, mm	Gap controlling dip, mm	Initial pressing. kg (N)	Final pressure kg (N)
Table 1. Contactors series KT6000, KT7000 and KTP6000
KT6012, KT6022, KTP6012, KTP6022, KT7012, KT7022			2,2-2,4 (22,05-23,52)	2,5-2,9 (25,4-28,42)
KT5013, KT6023, KTP6013, KTP6023, KT7013, KT7023			1,5-1,6 (14,7-15,68)	1,8-2,2 (17,64-21,56)
KT6014, KT6024, KT7014, KT7024			1,1-1,2 (10,78-11,76)	1,4-1,7 (13,72-16,66)
KT7015, KT7025			0,85-0,95 (8,33-9,31)	1.1-1,4 (10,78-13,72)
KT6032, KTP6032, KTP6033, KTP6033			2,0-2,2 (19,6-21,56)	3,7-4,5 (36,26-44,1)
			1,4-1,56 (13,72-15,19)	3-3,4 (29,45-33,32)
			1.1-1,2 (10,78-11,76)	2,6-3 (25,48-29,4)
			5,3-5,5 (51.94-53,9)	7,32-8,43 (71,74-82,61)
				13,1-16,6 (128,38-162,68)
				7,32-8,43 (71,74-82,61)
				13,1-16,6 (128,38-162,68)
			4-4,2 (39,2-41,16)	6,12-7,13 (59,98-69,67)
			3,2-3,3 (31,36-32,34)	5,34-5,23 (52,33-51,25)

Continuation of table 1.

Contactor type	Contact gap, mm	Gap controlling dip, mm	Initial pressing, kg (N)	Final pressing, kg (N)
KT6052, KTP6052. KT6053, KTP6053	10 - 12,5	3,7 - 4	9,6-10,0 (94,08-98)	18 - 21 (176,4-205,8)
KT6054			6,5-6,8 (63,7-66,64)	12,5-15 (122-147)
KT6055			4,8-5 (47,04-49)	10,5-13 (102,8-127,4)
	Contactors series KT6000/2
KT6022/2	7,5-8,5	1,7-2	2.2,-2,4 (22,05-23,52)	2,5-2,9 (24,5-28,42)
KT6023/2			1,5-1,6 (14,7-15,68)	1,8-2,2 (17,64-21,56)
KT6032/2, KT6033/2		3,3-3,5	2,0-2,2 (19,6-21,56)	3,7-4,5 (36,26-44,1)
KT6042/2, KT6052/2, KT6043/2, KT6053/2	10-12,5	3,7-4	9,6-10,0 (94,08-98)	18-21 (176,4-205,8)

On the rice. 2 the on and off positions of the contactor contacts are shown, at which dips, openings, pressings and simultaneity of touches of the main contacts are adjusted.

Rice. 2. Positions (on, off) of contacts for adjusting solutions, dips, pressings and simultaneous touching of contacts of contactors of series KTP6000, KTP6000, KT7000 and KT6000/2. a - contactors KT6032/2, KT6033/2; b, c - contactors of the KTP6000, KTP6000, KTP7000 series; 1 - the place of laying the paper tape when measuring the initial pressure on the contact; 2 - gap controlling contact failure; 3 - contact line of contacts; 4 - the place of laying the paper tape when measuring the final pressure on the contact; 5 - contact solution; 6 - direction of application of force when measuring the final pressure on the contacts; 7-directional application of force when measuring the initial pressure on the contacts; 8 - adjustment of pressure on the contact; 9 - adjustment of the dip and simultaneity of touching the Contacts.

Checking for contact failures. Since it is practically impossible to measure the magnitude of the dip, the gap that controls the dip is checked, i.e. the gap formed when the main contacts are completely closed, between the contact holder and the adjusting screws of the lever that carries the moving contact (Fig. 2). Control the failure of the main contacts in the closed position of the magnetic system of the contactor. With the full value of the contact dip, a complete final pressing on the contact is ensured. As the contacts wear, the dip decreases, therefore, the final pressure on the contact also decreases, which can lead to overheating of the contact. It is not allowed that the value of the gap that controls the failure is less than 1/2 of its initial value, indicated in Table. one.
In contactors of the KT6000/2 series, the failure of the main contacts is set by turning one adjusting screw in contactors for 160 A currents or two adjusting screws in contactors for currents of 250, 400 and 630 A. The design of the contact system of contactors of the KTP6000, KTP6000 and KTP7000 series allows two dip recovery, which is performed by turning the adjusting screw (in 100 and 160 A contactors), bushing (in 400 A contactors) and adjusting screws (in 250 and 630 A contactors).
The gap that controls the dip is measured with a feeler gauge. It is desirable that the contact dips be as large as possible. Having set the required gap and making sure that there is no skew of the moving contact, the adjusting screws must be tightened, and the bushings must be fixed with the petals of the plate.
Checking the simultaneity of contacts touching. The non-simultaneity of the contact of the main contacts is checked with a probe that controls the gap between the contacts when the other contacts touch each other. It is convenient to control the simultaneity of touching the contacts using a 3-6 V electric light bulb connected in series to the contact circuit, but within the limits indicated in Table. 1. Non-simultaneity of touching new contacts is allowed up to 0.3 mm. It should be borne in mind that the more precisely the dips are adjusted, the less the non-simultaneity of contact contact.
Checking contact solutions. Contact solutions are checked by caliber and must correspond to the dimensions indicated in Table. 1. If the solution is not normal, then by turning the eccentric bar “the time of the anchor around the axis, they are brought back to normal (KT6000/2 series contactors). In contactors of the KTP6000, KTP6000, KTP7000 series (except for KTP6050), the contact gap is adjusted by turning the stop around the axis by 90°. These contactors have several stop positions that determine the degree of adjustment of the solution.
Checking contact pressure. The pressing of the main contacts is determined by the elasticity of the contact springs. Pressing contacts is regulated by the largest values ​​indicated in Table. 1, so that after wear of the contacts it does not decrease below the permissible values. The degree of wear of the contacts (crackers) is determined by the magnitude of the dip. If, as a result of the wear of crackers, the dip turns out to be less than the minimum values ​​\u200b\u200bspecified in Table 1, the contacts should be replaced with new ones. When measuring pressure, it is necessary to ensure that the tension line is approximately perpendicular to the plane of contact of the contacts.
Initial pressing- this is the force created by the contact spring at the point of initial contact of the contacts. Insufficient initial pressure leads to melting or welding of contacts, and an increased initial pressure can lead to fuzzy switching on of the contactor or its sticking in intermediate positions.
Initial Press Check produced with open contacts (no current in the coil). In practice, the control of the initial pressure of the contacts is carried out not on the contact line of the contacts, but between the movable contact and the lever using a dynamometer, a strip of thin paper and a loop (for example, from steel wire or keeper tape). The loop is superimposed on the movable contact, and a thin paper tape is inserted between the shaft protrusion and the adjusting screw - for 100 and 160 A contactors (Fig. 2, c), between the holder and the adjusting sleeve - for 400 A contactors (Fig. 2, b ), between the holder and two adjusting screws - for contactors for 250, 400 and 630 A (Fig. 2, a). Then the tension of the dynamometer is determined by the force at which the strip of paper is easily pulled out. This force must correspond to the initial contact force indicated in Table. 1. In fig. 2, the arrow indicates the direction of tension of the dynamometer. If the tension does not correspond to the table, it is necessary to change the tightening of the contact spring by turning the adjusting screws, nuts and bushings. After setting the required pressure, the adjusting devices must be firmly fixed so that the setting is not disturbed.
End push. The final pressing characterizes the pressure of the contacts when the contactor is on. Compliance of end presses with tabular ones is possible only for new contacts. As the contacts wear, the amount of final pressure will decrease. To measure the final pressure, it is necessary to fully switch on the contacts, for which the armature of the magnetic system is pressed against the core and wedged or the pull-in coil is connected to full voltage. A strip of fire paper is clamped between the contacts. A loop is put on the moving contact (as when measuring the initial tension). The loop is pulled with a dynamometer hook until the contacts are so far apart that the paper can be moved. In this case, the dynamometer readings give the value of the final pressure on the contacts. The end pressure is not adjustable, but controlled. If the final pressing does not correspond to that indicated in the table. 1, it is necessary to replace the contact spring and carry out the entire adjustment process from the beginning.

Contact solution of electrical apparatus

In low-voltage electrical apparatus, the contact gap is mainly determined, and only at significant voltages (above 500 V) does its value begin to depend on the voltage between the contacts. As experiments show, the arc leaves the contacts already at a gap of 1 - 2 mm.

The most unfavorable conditions for extinguishing the arc are obtained at direct current, the dynamic forces of the arc are so great that the arc actively moves and extinguishes already at a gap of 2–5 mm.

According to these experiments, it can be assumed that in the presence of a magnetic field to extinguish the arc at a voltage of up to 500 V, a solution value of 10–12 mm for direct current can be taken, for alternating current, 6–7 mm is taken for any current values. An excessive increase in the solution is undesirable, since it leads to an increase in the stroke of the contact parts of the apparatus, and, consequently, to an increase in the dimensions of the apparatus.

The presence of a bridge contact with two gaps allows you to reduce the course of contact, while maintaining the total value of the solution. In this case, a solution of 4 - 5 mm is usually taken for each gap. Especially good results for extinguishing the arc are obtained by using an alternating current bridge contact. Excessive reduction of the solution (less than 4 - 5 mm) is usually not done, since errors in the manufacture of individual parts can significantly affect the size of the solution. If it is necessary to obtain small solutions, it is necessary to provide for the possibility of its adjustment, which complicates the design.

In the case of contacts operating under conditions where their severe contamination is possible, the solution must be increased.

Usually the solution increases and. for contacts that open the circuit with , since at the moment of extinction of the arc, significant overvoltages appear and, with a small gap, it is possible to re-ignite the arc. The solution is also increased for the contacts of protective devices in order to increase their reliability.

The solution increases significantly with an increase in the frequency of the alternating current, since the rate of voltage rise after the arc is extinguished is very high, the distance between the contacts does not have time to deionize and the arc ignites again.

The magnitude of the opening at high frequency alternating current is usually determined experimentally and strongly depends on the design of the contacts and the arc chute. At voltages of 500-1000 V, the size of the solution is usually taken to be 16 - 25 mm. Higher values ​​refer to contacts that switch off circuits with higher inductance and higher current.

Failure of contacts of electrical devices

During operation, the contacts wear out. In order to ensure their reliable contact for a long time, the kinematics of the electrical device is designed in such a way that the contacts are in contact before the moving system (moving contact movement system) reaches the stop. The contact is attached to the moving system through a spring. Due to this, after contact with the fixed contact, the movable contact stops, and the movable system moves further forward until it stops, additionally compressing the contact spring.

Thus, if, in the closed position of the movable system, the fixed contact is removed, then the movable contact will move a certain distance, called a dip. The dip determines the margin for contact wear for a given number of operations. Other things being equal, a larger dip provides higher wear resistance, i.e. longer service life. But a larger dip usually requires a more powerful drive system.

Contact pressing- the force that compresses the contacts at the point of contact. Distinguish between the initial pressing at the moment of initial contact of the contacts, when the dip is zero, and the final pressing at the complete failure of the contacts. As the contacts wear, the failure decreases, and, consequently, the additional compression of the spring. The final pressing approaches the initial one. In this way, the initial pressing is one of the main parameters at which the contact must remain operational.

The main function of the dip is to compensate for contact wear, therefore, the value of the dip is determined primarily by the value of the maximum wear of the contacts, which is usually taken: for - for each contact up to half its thickness (total wear is the full thickness of one contact); for soldered contacts - Until the soldering is completely worn out (total wear is the total thickness of the solderings of the moving and fixed contacts).

In the case of contact lapping, especially rolling, the value of the dip is very often much greater than the maximum wear and is determined by the kinematics of the movable contact, which provides the required amount of rolling and slip. In these cases, in order to reduce the total travel of the moving contact, it is expedient to place the axis of rotation of the moving contact holder as close as possible to the contact surface.

The values ​​of the minimum allowable contact pressures are determined from the conditions of maintaining a stable contact resistance. If special measures are taken to preserve , the values ​​of the minimum contact pressures can be reduced. So, in special small-sized equipment, the contact material of which does not form an oxide film and the contacts are absolutely reliably protected from dust, dirt, moisture and other external influences, the contact pressure decreases.

The final contact pressure does not play a decisive role in the operation of the contacts, and its value should theoretically be equal to the initial pressure. However, the choice of dip is almost always associated with compression of the contact spring and an increase in its force, so it is impossible to constructively obtain the same contact pressures - initial and final. Usually, the final contact pressure with new contacts exceeds the initial one and a half to two times.

Contact dimensions of electrical apparatus

Their thickness and width depend very much both on the design of the contact connection, and on the design of the arc quenching device and the design of the entire apparatus as a whole. These dimensions in various designs can be very diverse and strongly depend on the purpose of the apparatus.

It should be noted that it is desirable to increase the dimensions of the contacts, which often break the circuit under current and extinguish the arc. Under the action of a frequently broken arc, the contacts become very hot; an increase in their size, mainly due to heat capacity, makes it possible to reduce this heating, which leads to a very noticeable decrease in wear and to an improvement in the conditions for extinguishing the arc. Such an increase in the heat capacity of the contacts can be carried out not only due to a direct increase in their size, but also due to arcing horns associated with the contacts in such a way that not only an electrical connection is made, but also a good heat removal from the contacts is ensured.

Vibration of electrical apparatus contacts

Contact vibration- the phenomenon of periodic rebound and subsequent closure of contacts under the influence of various reasons. Vibration can be damped, when the amplitudes of rebounds decrease and after a while it stops, and undamped, when the vibration phenomenon can continue for any time.

Vibration of the contacts is extremely harmful, since current passes through the contacts and at the moment of rebounds an arc appears between the contacts, causing increased wear and sometimes welding of the contacts.

The reason for the damped vibration that occurs when the contacts are turned on is the impact of the contact on the contact and their subsequent rebound from each other due to the elasticity of the contact material - mechanical vibration.

It is not possible to completely eliminate mechanical vibration, but it is always desirable that both the amplitude of the first bounce and the total vibration time be as short as possible.

The vibration time is characterized by the ratio of the contact mass to the initial contact pressure. In all cases, it is desirable to have this value as small as possible. It can be reduced by reducing the mass of the movable contact and increasing the initial contact pressure; however, the reduction in mass should not affect the heating of the contacts.

Particularly large values ​​of the vibration time when switching on are obtained if, at the moment of contact, the contact pressure does not increase abruptly to its actual value. This happens with an incorrect design and kinematic scheme of the movable contact, when, after touching the contacts, the initial pressure is set only after the selection of backlashes in the hinges.

It should be noted that an increase in the lapping process, as a rule, increases the vibration time, since the contact surfaces, when moving relative to each other, encounter irregularities and roughness, which contribute to the rebound of the movable contact. This means that the amount of lapping must be chosen in the optimal size, usually determined empirically.

The reason for the undamped vibration of the contacts, which appears when they are in a closed position, are. Since vibration under the action of electrodynamic forces appears at high current values, the resulting arc is very intense, and as a result of such vibration of the contacts, as a rule, they are welded. Thus, this kind of contact vibration is completely unacceptable.

To reduce the possibility of vibration under the action of electrodynamic forces, the current leads to the contacts are often made in such a way that the electrodynamic forces acting on the movable contact compensate the electrodynamic forces arising at the contact points.

When a current passes through the contacts of such a magnitude at which the temperature of the contact points reaches the melting temperature of the contact material, adhesion forces appear between them and the contacts are welded. Welded contacts are those when the force that ensures their divergence cannot overcome the adhesion forces of the welded contacts.

The simplest means of preventing contact welding is the use of appropriate materials, as well as an expedient increase in contact pressure.

The following requirements apply to the contact material:

1. High electrical conductivity and thermal conductivity.

2. Resistance to corrosion in air and other gases.

3. Resistance to the formation of films with high resistivity.

4. Low hardness to reduce the required pressing force.

5. High hardness to reduce mechanical wear during frequent switching on and off.

6. Small erosion.

7. High arc resistance (melting point).

8. High values ​​of current and voltage required for arcing.

9. Easy processing, low cost.

The properties of some contact materials are discussed below.

Copper. Positive properties: high electrical and thermal conductivity, sufficient hardness, which allows it to be used with frequent switching on and off, rather high values U o And I o, simplicity of technology, low cost.

Disadvantages: low melting point, when working in air, it is covered with a layer of strong oxides with high resistance, it requires rather large pressing forces. To protect copper from oxidation, the contact surface is electrolytically coated with a silver layer 20–30 µm thick. Silver plates are sometimes placed on the main contacts (in devices that are switched on relatively rarely). It is used as a material for flat and round busbars, contacts of high voltage devices, contactors, automatic machines, etc. Due to low arc resistance, it is undesirable to use in devices that turn off a powerful arc and have a large number of starts per hour.

Silver. Positive properties: high electrical and thermal conductivity, the silver oxide film has low mechanical strength and quickly collapses when the contact point is heated. Silver contact is stable, due to the low mechanical strength, small pressures are sufficient (used for pressures of 0.05 N and above). Contact stability, low contact resistance are the characteristic properties of silver.

Negative properties: low arc resistance and insufficient hardness of silver prevent its use in the presence of a powerful arc and with frequent switching on and off.

Used in relays and contactors for currents up to 20 A. For high currents up to 10 kA, silver is used as a material for the main contacts that work without an arc.

Aluminum. This material has rather high electrical conductivity and thermal conductivity. Due to the low density, the current-carrying part of the round section made of aluminum for the same current as the copper conductor has almost 48% less mass. This reduces the weight of the device.

Disadvantages of aluminum: formation in air and in active media of films with high mechanical strength and high resistance; low arc resistance (melting point is much lower than that of copper and silver); low mechanical strength; in contact with copper, a vapor is formed that is subject to severe electrochemical corrosion. In this regard, when combined with copper, aluminum must be electrolytically coated with a thin layer of copper, or both metals must be coated with silver.

Aluminum and its alloys (duralumin, silumin) are mainly used as a material for tires and structural parts of vehicles.

Tungsten. The positive properties of tungsten are: high arc resistance, high resistance to erosion, welding. The high hardness of tungsten allows it to be used for frequent switching on and off.

The disadvantages of tungsten are: high resistivity, low thermal conductivity, the formation of strong oxide and sulfide films. Due to the high mechanical strength and film formation, tungsten contacts require high pressure.

In relays for small currents with a small pressure, corrosion-resistant materials are used - gold, platinum, palladium and their alloys.

metal-ceramic materials. Consideration of the properties of pure metals shows that none of them fully satisfies all the requirements for discontinuous contacts.

The main necessary properties of the contact material - high electrical conductivity and arc resistance - cannot be obtained by alloying materials such as silver and tungsten, copper and tungsten, since these metals do not form alloys. Materials with desired properties are obtained by powder metallurgy (metal-ceramics). The physical properties of metals in the manufacture of metal-ceramic contacts are preserved. The arc resistance of ceramics is reported by such metals as tungsten, molybdenum. To obtain a low contact resistance, silver or copper is used as the second component. The more tungsten in the material, the higher the arc resistance, mechanical strength, welding resistance. But accordingly, the resistance of the contacts increases, the thermal conductivity decreases. Typically, cermets with a tungsten content above 50% are used for heavily loaded devices that break high short-circuit currents.

For contacts of high-voltage devices, cermets KMK-A60, KMK-A61, MK-B20, KMK-B21 are most widely used.

In low-voltage devices, the most widely used metal-ceramic KMK-A10 from silver and cadmium oxide CdO. A distinctive feature of this material is the dissociation of CdO into cadmium vapor and oxygen. The released gas causes the arc to quickly move over the contact surface, which significantly reduces the contact temperature and promotes deionization of the arc.

The cermet, consisting of silver and 10% copper oxide, MK-A20 is even more resistant to wear than KMK-A10.

Silver-nickel contacts are well machined and highly resistant to electrical wear. The contacts provide a low and stable contact resistance. However, they are easier to weld than contacts made of KMK-A60, KMK-B20, KMK-A10 material.

Silver-graphite and copper-graphite contacts are used as arcing contacts due to their high resistance to welding.

In conclusion, it should be noted that although the use of cermets increases the cost of equipment in operation, these "extra" costs quickly pay off, as the service life of the apparatus increases, the time between revisions increases, and reliability increases significantly.

Under the failure of contacts the value of the displacement of the movable contact at the level of the point of 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 under 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 the contacts come into contact, the movable contact rolls 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. The points of contact of the contacts during rolling 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).

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.