Contactor (switch)
The contactor , also contactor , is an electrically or electromagnetically operated switch for high electrical power and is similar to a relay . The contactor knows two switch positions and normally switches to monostable without special precautions .
Electromechanical contactor
If a control current flows through the magnetic coil of an electromechanical contactor, the magnetic field pulls the mechanical contacts into the active state. Without electricity, a spring restores the state of rest, and all contacts return to their original position. The connections for the control current for the solenoid as well as the contacts for auxiliary circuits (if present) and the currents to be switched are isolated from one another in the contactor: There is no conductive connection between the control and switching contacts. Basically, a contactor is a relay with a significantly higher switching capacity. Typical loads start at around 500 watts up to several hundred kilowatts.
Special designs, which depend on the application and structure, are the heating contactor and the coupling contactor .
history
Contactors have been developed so that a consumer with a high power consumption (e.g. motor) can be switched remotely using a manually operated switch with a low switching capacity. Contactors enabled faster and safer switching operations than is possible with purely mechanical or manually operated switching designs. The cable lengths of load circuits with large cable cross-sections can thus be reduced.
Areas of application
With a contactor, as with a relay, remote switching operations are possible via control lines with a relatively small conductor cross-section. The typical application areas of the contactor therefore include control and automation technology . Specific application examples include motor control, control of electrical heating elements and the switching of lighting systems as well as the safety shutdown of machines. Logical functions can be implemented using auxiliary contacts. Examples are the self-holding circuit or the star-delta circuit . The possible areas of application of a contactor are specified in the standard through utilization categories.
Difference to the relay
Contactors differ from relays in the following features :
- Relays are designed for lower switching capacity, they usually do not have any spark extinguishing chambers .
- The switching contacts of relays are single-break , while they are always double-break in contactors .
- Relays often use hinged armatures, while contactors usually use tie rods for the purpose of greater mechanical switching force, which is necessary for the higher switching capacities and more solid contacts.
However, none of the above distinguishing features are mandatory; a clear delimitation is not possible. A generally valid distinguishing feature is that contactors only have NC and NO contacts , whereas relays can also have changeover contacts (changeover switches).
Installation
Contactors are available for different types of mounting, for example for top hat rail mounting , mounting plate or in housings with holes for individual mounting.
Because of the high switching capacities and the massive contacts required, their rapid actuation and the high contact force of the strong electromagnet, a contactor causes mechanical shocks. The actuating magnets are often spring-mounted so that the structure-borne noise is dampened somewhat. The installation position is usually arbitrary.
In contrast to solid-state contactors, mechanical contactors do not require a heat- dissipating heat sink assembly. Mechanical contactors cause lower power losses than solid-state contactors.
variants
A distinction is made between power contactors (contactors with a high switching capacity) and auxiliary contactors for implementing logical links, for controlling power contactors or for switching displays or small consumers.
AC and DC voltage contactor
The operating coils of contactors can be designed for operation with alternating voltage or direct voltage. For AC voltage operation, their electromagnets have a core, part of which is enclosed by a short-circuit winding and is referred to as a split pole . This causes a phase shift and thus a time-delayed magnetic flux in a part of the iron core, which applies the holding force during the time in which the force of the main field is insufficient to hold the armature.
This is not necessary in the case of direct voltage contactors; here the restoring force of the spring can be supported by a permanent magnet. DC contactors often have intermediate layers or a non-magnetic rivet to prevent sticking due to the residual magnetization. Auxiliary contacts and series resistors are sometimes used to reduce the current flow after tightening.
In the case of a DC voltage contactor, there is no inductive resistance, as is the case with an AC voltage contactor. Since both have roughly the same (apparent) resistance (magnetic force depends on the current), this is compensated for by increasing the wire resistance. The wire of the coil must therefore be longer, and this is noticeable by an increased number of turns. However, this increases the level of the self-induction voltage, and a corresponding protective circuit should be used.
The choice of protective circuit for the actuating coil is often dependent on the type of voltage and must be selected accordingly.
Auxiliary contactor
As an inductive consumer, the actuating coil causes a disruptive voltage peak when switched off by self-induction . To protect the control electronics and to avoid interference emissions, a protective circuit against this cut-off overvoltage may therefore be necessary in the control circuit . In AC contactors, this usually consists of a series connection of a resistor with a capacitor , which are attached parallel to the armature coil (see snubber ). A freewheeling diode can be used with direct current contactors to protect controlling contacts or the control electronics.
In both cases, a varistor or a bidirectional suppressor diode can be used for interference suppression , and in the case of DC voltage, a Zener diode or a unidirectional suppressor diode can also be used. This reduces the switch-off time compared to freewheeling diodes, especially when using DC voltage, but the control circuit must be able to withstand a higher switching voltage.
Some contactors have a plug-in device for easy installation, for which suitable suppressors are supplied.
Avoidance of the switching arc
When the contacts are separated, tear-off sparks or a switching arc occur - especially when inductive loads or direct current are switched. This leads to contact wear and electrical interference emissions . Air contactors have arc extinguishing chambers or deion chambers into which the arc spreads due to its magnetic field and is cooled there so that it goes out. In AC contactors, the blow magnets are coils through which the switching contact current flows, but in DC contactors with a predetermined current direction they are permanent magnets . In special areas of application ( potentially explosive areas ) it may be necessary to encapsulate the contacts completely. A solid state relay can then also be used.
There are also vacuum contactors for systems that meet high availability requirements. The switching contacts are located here in an evacuated switching tube. Since a vacuum has a very high dielectric strength, arcs tear off very quickly when there is a small contact distance. This means that there is less wear on the switch contacts.
Oil contactors whose switching contacts work in an oil bath are rarer today.
Suppressors can be used in order to avoid sparks and switching arcs from the outset. Typical are RC combinations (see Boucherot element ) that are switched via the contacts or the consumer and briefly take over the current flow when the contact is interrupted. This RC element should not be confused with the one which is often used to compensate for the self-induction of the coil and is connected in parallel to the coil and not in parallel to the contacts.
Basically, these are all criteria for the usage category of a contactor. Basically, it can be said that the switching arc with direct voltage causes more erosion than that with alternating voltage, where the arcing itself is more easily extinguished due to the zero crossing. However, an inductive phase shift due to the connected load (e.g. motor) reduces this advantage, since the voltage - when the current crosses zero - is already at a corresponding level, which makes it easier to reignite. Self-extinguishing is favored by a (rather rare) capacitive load, because when the leading current is at 0 (and thus the arc is extinguished), the voltage drops even further to 0 and it takes significantly longer to reach a voltage level at who can re-ignite the arc. In the course of time, however, the ions have been distributed again in such a way that reignition is made significantly more difficult, and therefore usually does not occur.
Contact types
Main contacts:
- Normally open (normally open ) contacts (NO for short )
- NC contact (normally closed contacts; short: NC from Normally Closed )
- Changeover contacts / changeover contacts (combination of an NC contact with a NO contact)
- Sequence changeover contact (changeover contact in which all three contacts are briefly connected when switching.)
Auxiliary contacts for contactor control and signal display
- also normally open, normally closed and changeover contacts
- leading NO contacts and delayed NC contacts , among others
Terminal designations
The contacts are divided into two groups: main contacts for the power to be switched and auxiliary contacts as signaling lines.
The main current contacts of a contactor are designated with single-digit numbers. The odd digits (1, 3, 5) usually lead to the power grid, the even digits (2, 4, 6) lead to the consumer. The contactor itself often says 1 L1 3 L2 5 L3 or 2 T1 4 T2 6 T3 . The L stands for live wire / load or line , i.e. for the (current - /) voltage-carrying line. The T for throw , that is to say throw off / press the exit. Depending on the structure, this can also be used the other way around. In the case of NC contacts as the main current contact, some manufacturers put an R in front of the terminal designations. The auxiliary or control contacts have a two-digit designation. In the first position is the ordinal number with which the auxiliary contacts are numbered consecutively. The second position is the function number that indicates the task of the respective auxiliary contact (e.g. 1–2 for normally closed (NC) , 3–4 for normally open (NO) ). On the component itself there is usually also the abbreviation, e.g. B. 31 NC or 32 NC or 53 NO or 54 NO . The additional designations (L1-3, T1-3, NO / NC) are usually not used in the circuit diagram; only the digits are used there.
There are also the designations 5–6 and 7–8. These are intended for contacts with a special function (e.g. time-delayed opening or closing).
Examples (marked in color in the picture):
- 1–2: main power contact, closing (red)
- R3 – R4: main current contact, breaking
- 13–14: 1st auxiliary contact (1 ×), closing (yellow) (x3 / x4) -> 13/14
- 21–22: 2nd auxiliary contact (2 ×), opening (violet) (x1 / x2) -> 21/22
- A1 – A2: coil connection
- T1 – T3: Motor connections (red below)
Switching types
A : normally open contact (main contacts ), e.g. B. contacts 1 and 2
B : NC contact (auxiliary contact), e.g. B. 11 and 12
C : normally open contact (auxiliary contact), e.g. B. 13, 14
D : Late break contact
E : Early make contact, e.g. B. 57 and 58
F, G : Overlapping opener and closer
The contacts can switch either overlapping ( MBB , from English: make before break ) or non-overlapping (according to standards). Overlapping means: The normally open contact closes during the changeover process while the normally closed contact has not yet disconnected; the input and both outputs are briefly connected to one another. In contrast to the non-overlapping switching type, in which the NC contact disconnects before the NO contact makes contact, uninterrupted switching operations are possible. Overlapping contactors are referred to as Ü contactors, non-overlapping contactors as E contactors.
Function monitoring
Safety-relevant contactors are designed with forcibly guided contacts: NC and NO contacts can never be closed at the same time. That means z. For example, a normally open contact that is welded due to overload, i.e. that does not open when the coil is de-energized, means that no opener closes. Such a contactor can therefore be monitored on the basis of its break contact, whether it has dropped out. With a further redundant contactor and a safety switching device, it can be ensured that a system will nevertheless shut down safely. In the event of a sticking (defective) contactor, it cannot be switched on again by the start circuit leading via the NC contacts of both contactors (see also emergency stop switchgear).
An auxiliary relay can also be used for function monitoring (protection against hanging or burned-in contacts), which is connected behind the respective power contact of the contactor and thus switches an auxiliary current as soon as the contactor has reliably performed the switching process. The auxiliary relay can be integrated in the housing of the contactor, but is mechanically independent.
Characteristics
Data of the switching contacts:
- Switching capacity
- the power that an electromagnetically actuated contact can switch on or off
- Switching or nominal voltage
- the voltage that can be switched
- Switching current / nominal current
- the current that a contact can switch
- Utilization category
- Type of load that may be switched, on which the switching capacity, voltage and current also depend. (e.g. AC1 for purely ohmic load ( incandescent lamp , heating) or AC3 for squirrel cage motors (inductive load))
- Continuous contact current
- the current that can flow across the contacts without switching under normal operating conditions. It is also called the nominal thermal current , as it is only limited by the thermal losses of the contact resistance.
Actuating coil data:
- Nominal coil voltage
- the nominal value of the actuating voltage for which the winding of the coil and the magnetic circuit are dimensioned . There are contactors that can be operated with DC or AC voltage.
- Response voltage
- the minimum voltage from which the relay or contactor reliably picks up
- Holding voltage
- the minimum voltage at which the relay or contactor just remains safely closed
The response voltage is greater than the holding voltage.
Self-holding and bistable contactors
If a contactor is to remain in the closed switch position after a control current pulse (e.g. a button press) instead of falling back into the rest position, a self-holding circuit is used that requires an auxiliary contact on the contactor. Such auxiliary contacts can usually be mounted on the side or on top of the contactor or are already integrated. It is also possible to use a power contact that is not required as an auxiliary contact. The self-holding circuit enables the use of a pushbutton switch instead of an off switch for control. It is switched off with another but opening button. Further break contacts can be connected in its circuit, for example bimetal switches for temperature monitoring. The self-holding circuit has the advantage over a mechanical switch that a machine does not restart by itself after a power failure.
There are also bistable contactors and impulse switches that do not require a permanent holding current for the electromagnet.
Pneumatic contactor
The pneumatic contactor , also known as compressed air contactor , has the same effect as the electromechanical contactor, but it is operated with compressed air instead of an electromagnet: The electromagnet is replaced by pneumatic actuators (pressure cells) which act on the switching contacts via the armature. Instead of applying a control current, switching to the active state takes place here by increasing the pressure. Compressed air contactors are often used in medium-voltage technology, since a large distance between the working and control circuit can be maintained. In addition, in contrast to their magnetic counterparts, pneumatic contactors are able to bridge larger switching paths (contact distances). This is elementary in medium voltage technology. In high voltage technology, pneumatically operated contact shears are used here.
Solid state contactor
In order to avoid wear and tear (contact erosion, wear of moving components, etc.) when operated frequently, contactors based on power semiconductors have been developed (see solid state relays ). In contrast to the mechanical contactor, the solid-state contactor does not provide reliable separation of the power contacts in the open switch position. A small residual current flows and the dielectric strength is often lower than that of open mechanical contacts.
However, the control circuit is usually galvanically isolated from the load circuit by means of an optocoupler , so that safe isolation is also provided for the semiconductor contactor. The control takes place with safety extra-low voltage . 3 to 30 V.
Solid-state contactors must be selected more carefully with regard to the load case:
- there are zero voltage switches for resistive loads; they cause fewer interference emissions
- For inductive loads, there are semiconductor relays that can withstand or dampen the voltage peaks that occur when switching off
Solid-state contactors with rated current require mounting on a suitably dimensioned heat sink - their power loss is higher than that of mechanical switches.
Solid-state contactors do not safely separate the switching path (no galvanic separation ). Therefore, switching off a solid-state contactor does not count as activation in the sense of the first of the five safety rules .
Further advantages and disadvantages can also be found in the article about solid state relays .
Individual evidence
- ↑ Expertise in electrical professions . Bildungsverlag EINS, Troisdorf 2006, p. 231
- ^ Schaltungsbuch.de of Moeller GmbH
- ↑ Electrical engineering book of tables . Verlag Europa-Lehrmittel, Haan-Gruiten 2005, p. 342.
- ↑ moeller (PDF; 508 kB): Projecting with mechanical auxiliary contacts in compliance with standards and functionally reliable , 4-pole switching with DILMP contactors , manufacturer itself
literature
- Walter Baumann et al .: LV switchgear practice: function, selection, use. (Edited by Roland Werner), VDE-Verlag, Berlin 1984, ISBN 3-8007-1353-5 .
