Contactor

The contactor, the contactor also is an electrically or pneumatically operated or electronic switch for large electrical power, and is similar to a relay. The contactor has two switch positions and monostable switches without special precautions normally.

Electromechanical contactors

A control current flows through the solenoid coil of an electromechanical contactor, the magnetic field pulls the mechanical contacts in the active state. Without electricity represents a spring hibernation restore all contacts return to their original position. The connections for control current for the magnetic coil and the contacts for auxiliary circuits (if present) and the switching currents are run in isolation in the contactor against each other: There is no conductive connection between the control and switch contacts. Basically a contactor is a relay with a much higher switching performance. Typical loads start at about 500 watts to several hundred kilowatts.

History

Sagittarius been developed to allow a consumer with great power can be switched remotely via a manually operated switch with small switching power ( eg motor). Sagittarius enabled faster and more reliable switching operations than is possible with purely mechanical or manually operated switch construction was possible. The line length of the load circuits with large cable cross-section could be reduced.

Areas of application

With a contactor are possible as with the relay switching operations remotely via control lines with a relatively small conductor cross-section. Typical application areas of the contactor therefore include the control and automation technology. Specific application examples include motor control, control of electric heating elements and the switching of light installations and the safety shutdown of machines. By means of auxiliary contacts are logical functions realized. Examples are the self-holding circuit or star-delta circuit. The possible applications of a contactor are enshrined in the norm by use categories.

Contrast to a relay

Contactors differ in the following characteristics of relay:

• Relays are designed for lower switching performance, they usually do not have arc chutes.
• The switching contacts of relays are simply interrupting while they are usually double break contactors.
• Relays often use folding anchor, contactors, however, usually tie rods for greater mechanical switching power which is necessary for higher switching capacity and massive contacts.

However, all the above distinguishing features are not mandatory, a clear distinction is not possible. A generally valid distinguishing feature is that Sagittarius may possess only NC and NO contacts, relays, however, also changeover contacts (change-over ).

Installation

Contactors are available for different types of installation, for example, for DIN rail mounting, mounting plate or in enclosures with holes for single mounting.

Due to the high switching capacity and the requisite massive contacts whose faster operation and high contact force of the powerful electromagnet, causing a contactor mechanical vibrations. Often, the actuating magnets are spring mounted so that the body sound is a bit muffled. The installation position is usually arbitrary.

In contrast to solid-state contactors mechanical contactors do not need heat-dissipating heat sink assembly. Contactors cause lower power losses than solid-state contactors.

Variants

A distinction is made between contactors ( relays with high switching capacity ) and auxiliary contactors for the realization of logical operations, for the control of contactors or for switching displays or small consumers.

AC and DC contactor

The operating coils of contactors can be designed for use with AC or DC voltage. For AC operation, their electromagnet having a core, one part of which is enclosed by a shorted turn and is referred to as a split pole. This causes a phase shift, and thus a time-delayed in a part of magnetic flux of the iron core of the holding force is applied during the period in which the power of the main field to hold the armature is insufficient.

At DC contactors it is not necessary here, the restoring force of the spring may be supported by a permanent magnet. Often DC contactors have liners or non-magnetic rivet to prevent sticking due to residual magnetization. Partial auxiliary contacts and series resistors are used to reduce the current flow after tightening.

Protection of the control circuit

The operating coil causes an inductive load when switching off by self-induction a disturbing voltage spike. To protect the control electronics and to avoid spurious emissions may therefore be necessary in the control circuit, a protective circuit against these turn-off voltage. In AC contactors this usually consists of a series connection of a resistor and a capacitor, which are mounted parallel to the armature coil (see snubber ). For DC contactors a flywheel diode can be used to protect or contacts controlling the driving electronics.

For suppression, in both cases, a varistor or a bi-directional suppressor diode in DC also a zener diode or a unidirectional TVS diode serve. Especially with DC voltage actuation, thereby compared with free-wheeling diodes reduces the turn-off time, the control circuit must ensure, however, tolerate a higher switching voltage.

Some contactors have for easy mounting on a plug-in device, be delivered to the appropriate suppressors.

Avoidance of the arc

Upon separation of the contacts occur breaking sparks or a switching arc on - especially when inductive loads are switched. This leads to contact burning and electrical interference emissions. Air contactors have arc - extinguishing chambers in propagating the arc due to its magnetic field where it is cooled so that it goes out. In special use areas ( Hazardous areas ), it may be necessary to completely encapsulate the contacts. A semiconductor contactor ( solid state relay ) can then be used.

There are also vacuum contactors for systems, which place high demands on availability. Here are the switch contacts in an evacuated interrupter. Since a high vacuum has a very large dielectric strength, tear arcing when opening even at small contact distance very quickly. Thus, less combustion occurs at the switch contacts.

Less common are today oil contactors, whose switching contacts operate in an oil bath.

To avoid breaking sparks and switching arcs from the start, suppressors can be used. Typical RC combinations (see Boucherot element), which are switched by the contacts or the consumer and briefly take over the current flow during the onset of breaking the contact.

Contact types

Main contacts:

• NO ( normally open contacts; short: NO Normally Open from engl. )
• NC ( normally closed contacts in short, of Engl NC Normally Closed. )
• Changeover / changer ( combination of an opener with a contact)
• Follow- changer ( change-over, in which all three contacts are briefly connected during switching. )

Auxiliary contacts for contactor control and signal indicator

• As normally open, normally closed and changeover
• Early-make contacts and delayed opener, among other things,

Connection designations

The contacts are divided into two groups: the main contacts for the power to be switched and auxiliary contacts as signaling line.

Main contacts of a contactor are designated by single-digit numbers. This usually lead the odd numbers ( 1, 3, 5) to the power grid, the even numbers (2, 4, 6) lead to the consumer. This can also be the other way around depending on the design though. In the case of normally closed contacts to the terminal designations is doing some manufacturers preceded by an R. The auxiliary and control contacts have a two-digit designation. At the first priority is the order number with which the auxiliary contacts are numbered consecutively. In the second place (3-4, for example 1-2 for NC, for contact) is the function number that specifies the duties of the respective auxiliary contact.

There are also the names of 5-6 and 7-8. These are great for contacts with special functions (for example, open a delaying or close) provided.

Examples (in the picture color-coded ):

Switching types

The contacts can be either overlapping ( MBB, of Engl. Make before break ) or non-overlapping ( compliant) switch. Cascade means that the contact closes during the switching already, while the NC has not yet separated; the input and both outputs are briefly connected with each other. This uninterruptible switching operations are in contrast to the non-overlapping switching mode, in which the opener separates before the NO contact makes possible. Overlapping contactors are non-overlapping referred to as OB Sagittarius as e- shooter.

Function monitoring

Relevant safety contactors are designed with forcibly guided contacts: NO and NC contacts can never be closed simultaneously. This means for example that a welded due to overload, that is, not in -energized coil opening normally causes no opener closes. Therefore, such a contactor can be monitored by the break contact, if it is dropped. With another redundant contactor and a safety switching device can be ensured so that investment yet sure off. You can at an adhesive ( defective ) contactor will not be turned on again by the reset circuit on the openers of both contactors leads (see also emergency stop switch box).

For function monitoring (protection from hanging or burnt- contacts) and an auxiliary relay can be used which is connected behind the respective output contact of the contactor, and thus an auxiliary current switches when the switching process from the contactor was carried out reliably. The auxiliary relay can be integrated in the housing of the contactor, but is mechanically independent.

Characteristics

Data of the switching contacts:

• Switching power - the power contact, a solenoid-operated on or off
• Switching or voltage - the voltage which can be connected
• Switching current - the current that can switch a contact

Switching power, switching voltage and switching current from each other and from the load (resistive load - AC1, inductive load - AC3) dependent.

• Contact time power - the current which can flow at normal operating conditions without any switching operation of the contacts. It is also called thermal rated current, since it is limited only by the thermal losses of the contact transfer resistance.

Data of the operating coil:

• Coil voltage - of the nominal value of the operating voltage for which the winding of the coil and the magnetic circuit are measured. There is with DC or AC operated contactors.
• Response voltage - the minimum voltage at which the relay or contactor attracts sure
• Hold voltage - the minimum voltage at which the relay or contactor remains closed just yet sure

The operating voltage is higher than the holding voltage.

Latching and bistable Sagittarius

If a contactor for a control current pulse ( for example, a button press ) remain in the closed switch position, instead of falling back to the rest position, is a self-holding circuit for use that requires an auxiliary contact on the contactor. Such auxiliary contacts can usually be side or top mounted on the contactor or are already integrated. Also the use of a non- contact power needed as an auxiliary contact is possible. The latch circuit enables the use of a push button switch instead of a circuit breaker to control. Switching off is done with another, but opening button. In the circuit of which more openers can be connected, for example, bi-metal switch for temperature monitoring. The self-holding circuit has the advantage that a machine will not start by itself after a power failure compared to a mechanical switch.

There are also bistable impulse relay and contactors that do not require continuous holding current for the electromagnet.

Pneumatic contactor

The pneumatic relay (also compressed air contactor) is the electro-mechanical contactor is equal In effect, it is, however, operated by compressed air instead of an electromagnet: The solenoid is replaced by pneumatic actuators ( pressurized cans ) which act on the switching contacts on the armature. Instead of by applying a control current, the switching to the active state takes place by increasing the pressure here. Pneumatic contactors commonly found in medium-voltage technology their application, as a large distance between the working and control circuit can be maintained. In addition, pneumatic contactors are unlike their magnetic counterparts in a position to bridge large switching distances ( contact distances ). This is elementary in the medium-voltage technology. In the high-voltage technology is here set to also pneumatically operated scissors contact.

Semiconductor contactor

To wear ( contact wear, wear on moving components, etc. ) to avoid, with frequent use, contactors were developed on the basis of power semiconductors (see solid state relay). Unlike the mechanical contactor no reliable isolation of the power contacts is given in the open switch position in the semiconductor contactor. Flowing a small leakage current and the withstand voltage lower than that is often open to mechanical contacts.

The control circuit is, however, usually separated by opto from the load circuit, so that the semiconductor contactor reliable separation is given. The unit is controlled with low voltage protection. Are usual 3 to 30 V.

Semiconductor contactors must be carefully selected with respect to the load case:

• For resistive loads, there is zero voltage switch; they cause less interference emissions
• For inductive loads, there are semiconductor relays that absorb or dampen the voltage spikes at switch-off

Solid-state contactors require at rated current mounting on a suitable sized heat sink - their power dissipation is higher than that of a mechanical switch.