The topic of contactors and relays often raises questions, even amongst professionals. We will address and answer some of these questions in this article. The most frequently discussed issue is the exact difference between a contactor and a relay and when to use either. There is also a whole number of different contactors: modular contactors, auxiliary contactors, vacuum contactors, motor protection switches, and combination switches. We will discuss each of those individually in separate articles where we will explain the differences between the individual types of contactors and where to use them.
Originally, the term 'contactor' was used for especially robust and powerful relays. Contactors are electromagnetic switches which are able to connect or disconnect at higher loads than relays. They are used when a high level of voltage resistance (230V/400V) is needed (for high-power appliances). What relays and contactors have in common is that both are used in control circuits. Contactors are controlled remotely. They can have 2 switching positions (normally, they switch as monostable). In contrary to relays contactors always have 2 disconnection points for each contact. The reasons are mostly safety and wear.
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Protect your motor with the BE Series motor protection switches from Schrack Technik.
Pre-assembled combination contactors - ready for use with Schrack Technik.
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The construction of a contactor is quickly explained; it consists of: a casing, electrical connections, a magnetic coil, a fixed coil core, a movable bobbin, and switching contacts as well as bobbin return springs.
In case of contactors differentiation is made between models with operating contacts and models with auxiliary contacts. The different models can be identified by the numbers on the contactor: If there are one-digit numbers (1-6) it is a model with operating contacts (which are used to switch drives or lighting). These operating contacts (also called main contacts) always start at number one. This means that the first contact has the numbers 1 and 2, the second contact has numbers 3 and 4, and so on. What is the reason for this kind of numbering? As a rule, the uneven numbers are used to connect the supply line and the even numbers connect with the appliance. This helps to find your way around connections even if they are very complex (or after some years have passed).
Two-digit numbers (e.g. 13/14) indicate a model with auxiliary contacts (which are used e.g. in industrial system controls). These auxiliary contacts (also called control contacts) each have an order number and a function number. The first number is used consecutively (order number) and the second number specifies the type of contact (function number).
Contactors always disconnect the NC - within milliseconds - before they connect the NO. Another characteristic is the so-called "spark extinguishing chamber". This is where sparks that may occur during switching are extinguished to prevent a fire.
The parts that usually require maintenance in contactors are the contacts. Maintenance efforts are very low with auxiliary contactors, because they handle only very low powers. Operating contactors, however, need regular inspection and maintenance of the contacts (or, ideally, the complete contactor is replaced), because (depending on how often they connect/disconnect) the contacts usually wear gradually and their correct functioning can no longer be guaranteed after some time.