Resistor, transistor, relay and now also a contactor?! With so many components, it’s easy to lose track. What exactly is this contactor all about? How is it structured and what does a contractor have to do with a relay? Find out everything about the contactor and how it works. We also explain complicated-sounding circuits such as the contactor lock and the reversing contactor circuit in an understandable way – we promise!

  • What exactly is a contactor?
  • How is a contractor constructed?
  • When does a contractor pick up?
  • What types of shooters are there?
  • What can I switch with the contactor?
  • What is the difference between a contractor and a relay?
  • What is a contactor lock?
  • How does a reversing contactor circuit work?
  • Why does a contactor hum?
  • Conclusion: small magnetic fields for large loads

What exactly is a contactor?

A contactor: The somewhat different switch with a magnet trick

A contactor: The somewhat different switch with a magnet trick
A contactor or contactor is an electromechanical switch. Its functionality is similar to that of a relay, but it is specially designed as a switch for high power.
In the contactor, there is a control circuit and a load circuit. If current flows through the control circuit, the contactor switches and current also flows through the load circuit.
The contractor, therefore, knows two switching states: on and off. It usually switches monostable, which means that the contactor only stops by itself in the off position. It goes into the on state only as long as the current is flowing in the control circuit, after which it returns to the quiescent state and thus back to the off position.
One can visualize the monostable way of working with an infinitely tired and sleepy person in bed. If an alarm clock that is placed out of reach of the sleeper rings, the person suddenly wakes up and sits bolt upright in bed. As soon as the alarm clock stops ringing, fatigue causes the sleepy person to fall back onto the pillows and slumber on.

How is a contactor constructed?

The best way to understand how a contactor works is to first understand its construction. The essential components for the operation of a contactor are:

  • housing
  • magnetic coil
  • coil core
  • Metallic Anchor
  • switching contacts
  • armature return spring

The switching contacts of the contactor are arranged on a so-called contact slide. In the off state, the contacts in the contactor do not touch. The contact carriage is moved by a lever in the on the state so that the contacts touch and are actively connected.
This lever is attached to a metallic anchor. The metallic armature in the contactor is movable and positioned so that it can move in a magnetic field generated by the coil.
An armature return spring ensures that the armature is always pushed to the off position. It, therefore, requires a certain amount of force to move the metallic armature. There is an iron core in the coil so that the magnetic effect is strong enough to actually move the metallic armature. The iron core amplifies the magnetic field generated by the coil.

When does a contactor pick up?

As with the relay, the coil is also particularly attractive for the contactor
When voltage is applied to the contactor, a control current flows through the coil. This creates a magnetic field in the iron core. The contactor attracts due to the magnetic effect: The armature is attracted to the coil against the spring force and ensures that the switching contacts are switched to the active state. The contactor is in the on the state.
Here you can also see why a contactor is called an electromechanical switch. The connections for the coil and the switching contacts for the currents to be switched are isolated from one another. There is a control circuit and a load circuit, but no conductive connection between the two circuits.
The connections for the coil, the coil core and the magnetic coil are part of the control circuit, the metallic armature enables the connection between the control circuit and the load circuit with the switching contacts for the consumers to be switched.
Note: A switched contactor is not only referred to as an energized contactor, but also as an energized contactor. A dropped-out contactor, on the other hand, means a contactor that is not switched.

What types of shooters are there?

Even if the functional principle is always the same, there are numerous types of contactors. These can be distinguished in many ways and according to various criteria.
The main distinguishing features of shooters are:

  • Type of current: AC and DC contactor
  • Installation: For mounting plates, ⦁ DIN rails or in housings
  • Contacts: main and auxiliary contacts
  • Variants: Power contactor and auxiliary contactor

Note: Snipers, archers & Co. are the less peaceful types of archers. They have very little to do with tension and magnetic effects, but they are particularly accurate.
 Difference between power contactor and auxiliary contactor

The power contactor lets small control currents The power contactor lets small control currents make a big impact!
make a big impact!
power contactor

Depending on the application, a distinction is made between the following variants of contactors according to their contact rating

  • Load or power contactor
  • Control of auxiliary contactor

Power contactors are contactors with a high switching capacity that can switch the load circuit of powerful consumers such as three-phase motors. The contacts of power contactors are equipped with arc quenching devices, among other things, which enables them to switch larger loads. Power contactors are marked with the letter “Q” in circuit diagrams.
While power contactors can also be of great help, the term contactor relays refer to those contractors that are primarily used to switch control voltages. Auxiliary contactors are designed for applications with lower loads, such as the implementation of logical operations, for controlling power contactors and for switching small loads or displays. Auxiliary contactors are marked with the letter “K” in circuit diagrams.

Note: There are also certain power contactors that are provided with additional auxiliary contacts and therefore also have “auxiliary modules”.  

contact types

Contactors are usually provided with different types of contacts. A distinction is made between main contacts and auxiliary contacts.
The main contacts of a contactor are designed for the power to be switched and are therefore suitable for connecting large loads. The auxiliary contacts only serve as signalling lines for contactor control and signal display.

  • NC contacts: interrupt the circuit when actuated (normally closed contacts, NC for Normally Closed)
  • Normally open: close the circuit when actuated (make contacts, NO for normally open)
  • Changeover contacts/changeover: a combination of make and break contacts

Auxiliary contacts also have leading make contacts and delayed break contacts.
Note: The contact designation for contactors is easy to understand: main contacts are designated with one-digit numbers and auxiliary contacts with two-digit numbers. The two-digit numbers of auxiliary contacts are made up of a sequence number for sequential numbering and a function number such as 1-2 for normally closed (NC) and 3-4 for normally open (NO).

What can I switch with the contactor?

Whether in production, in the shopping centre, in the office building or in your own home. A contactor is hidden in most electrical systems. Contactors have a wide range of applications because they can basically be used wherever control and automation technology is required.
Contactors were not only developed to enable faster switching operations but above all for additional safety. With contactors, systems with high power consumption can be controlled and switched remotely, without any direct manual operation or purely mechanical constructions.
Typical switching tasks of power contactors are the activation of motors, the switching and control of electrical heating elements, ventilation, pumps, heat pumps, lighting systems and the safety shutdown of machine systems.
Auxiliary contractors, on the other hand, are not used for switching larger loads, but primarily for the implementation of logical functions.

What is the difference between a contactor and a relay?

Relays are mentioned again and again in connection with contactors. In fact, the two electro-mechanical switches are quite similar in their functional principle.
In both switching devices, a coil is excited via a control circuit, which in turn builds up a magnetic field. The switching contacts in the load circuit are closed by this magnetic field. Both the contactor and the relay are galvanically isolated, there is no conductive connection between the control and load circuits.

  • The differences between contactors and relays are mainly in the fields of application :
  • Switching capacity: Relays switch control signals and smaller loads, while contactors are designed for switching consumers with large loads. With contactors, not only single-phase loads but also three-phase loads such as motors can be switched.
  • Spark quenching chamber: In contrast to relays, contactors are usually equipped with a spark quenching chamber that extinguishes the switching arc that occurs when switching large loads.
  • Switch contacts: With relays, the switch contacts are single-break. There are single and double-break contactors. Double break means that two contacts are arranged in series. This property is important for use in safety functions.
  • Armature: Relays are implemented with hinged or rotating armatures. With contactors, on the other hand, a tie rod is used, which requires a greater mechanical switching force and enables higher switching capacities.

Note: The exception proves the rule and this also applies to relays and contactors. The distinguishing features mentioned do not necessarily apply, but apply in most cases.

What is a contactor lock?

Contactor locking works without a padlock
Contactor locking

You read that right, this is not about a protective interlock, but about a contactor interlock. However, the two words are not that alien to each other. Because the contactor lock is ultimately used for protection and security.
A contactor interlock is required when two contactors cannot both pull in at the same time when voltage is applied. The contactors and circuits must therefore be interlocked.
Where do you need something like that? For example, when controlling motors such as that of a circular saw. Suppose contactor 1 supplies the motor with reverse run voltage when energized and contactor 2 supplies the motor with forwarding run voltage when energized.
If voltage is now applied to contactors 1 and 2 at the same time, the motor receives voltage for left and right rotation at the same time. You’ve probably guessed it already: This isn’t the beginning of a magic trick with a circular saw rotating to the right and left at the same time. In fact, in this case, there is a short circuit.
A contactor lock can be used to ensure that two contactors do not pick up at the same time and to prevent the contactors from picking up at all.
A contactor lock can be implemented in various ways. The circuit can be implemented using auxiliary contacts or buttons as well as PLC programming.

How does a reversing contactor circuit work?

While the contactor lock is primarily a safety measure, the so-called reversing contactor circuit has a practical background above all.
As is so often the case with shooters, it’s all about motors. Namely about motors that run on three-phase current. Three-phase motors (3-phase motors) have the advantage that their direction of rotation can be changed quite easily. If you swap two outer conductors (phases), the motor suddenly turns to the left instead of to the right, or vice versa.
If you want to swap the outer conductors manually, this is not only time-consuming but can only be done when the engine is switched off. And this is where the contactor comes into play: A reversing contactor circuit reverses the outer conductors in the three-phase motor and thus reverses the direction of rotation.
Typical applications for reversing contactor circuits are gates, conveyor belts and cranes that need to be able to move up and down.

Note: For safety reasons, a contactor lock is always installed with reversing contactor circuits. This prevents two outer conductors from coming together without resistance and causing a short circuit.

Why does a contactor hum?

The opposite of a troublemaker: a hum-free contactor
hum-free contactor

If contactors have been in use for a long time, a humming noise often develops at some point. This is usually harmless but can be annoying and above all raises the question: Why does a contactor buzz?
The humming can be easily explained by remembering how a contactor works: when voltage is applied, a magnetic field builds up in the iron core surrounding the coil. This magnetic field attracts the other iron core of the contactor to close the main circuit. However, it also attracts all other ferrous elements.
Depending on the frequency, the magnetic field is built up and broken down several times a second. The frequency of our mains voltage is, for example, 50 Hertz, ie the magnetic field is built up and broken down 50 times per second.
In order to ensure optimal magnetic flow, the iron core is actually made up of numerous thin sheets of metal held together with rivets. These connections can loosen over time. In addition, the iron core is surrounded by a short-circuit ring that can tear, and dust and dirt particles can get between the two cores.
Under these circumstances, the oscillating magnetic field creates a hum . In addition, the contactor also heats up a little more. The extra heat and humming noise pose no threat.
Note: For applications such as bedrooms, living rooms, hotels and hospital rooms, hum-free contactors guarantee noiseless operation over the entire service life.

Conclusion: small magnetic fields for large loads

Contactors belong to the family of electro-mechanical switches and are fundamental elements of electrical engineering. Although contactors and relays are very similar, they are used in different applications.
Even among the shooters themselves there are differences in terms of design, installation and use. Power contactors are particularly useful for switching and controlling large loads, while auxiliary contactors are used to switch smaller control voltages.
A lot can be played and tinkered with contactors and circuits such as the reversing contactor circuit are hidden in numerous systems with three-phase motors. Necessary protective measures can also be implemented according to the motto “It’s the contactor yourself”: The contactor interlock prevents the risk of a short circuit caused by two contactors being activated at the same time.



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