ABSTRACT
An electromagnetic neutral relay is the simplest, most ancient, and widespread type of relay. What are its basic elements? As a rule, most people asked this question would probably name the following: a winding, a magnetic core, an armature, a spring, and contacts. This all is true of course, but if you begin to analyze how a relay works, it might occur to
you that something is missing. What is the purpose of a magnetic system? Apparently it is used to transform input electric current to the mechanical power needed for contact closure. And what does a contact system do? It transforms the imparted mechanical power back to an electric signal. Don’t you think that something is wrong here? Everything will become more obvious if the list of basic components of a relay includes
one more element, which is not so obvious from the point of view of the construction of a relay, for example, a coil or contacts. Very often, it is not just one element, but several small parts, that escape our attention. Such parts are often omitted on diagrams illustrating the principle of relay operation (Figure 2.1). I am referring to an insulation system providing galvanic isolation of the input circuit (winding) from output one (contacts). If we take such an insulation system into account, it becomes clear that an input signal at the relay input and the output signal at the relay output are not the same. They are two different signals that are completely insulated from each other electrically. Note that if you use Figure 2.1, which is often used to illustrate principles of relay operation, as the
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only guide while constructing a relay, the relay will not operate properly since its input circuit (the winding) is not electrically insulated from the output circuit (the contacts). In simple constructions used for work at low voltage, insulating bobbins with winding
(not shown in Figure 2.1) provide basic insulation (apart from an insulator). In a relay with a free bobbin coil, it is necessary to use a special insulating baffle pin between the armature (3) and the contacts (not shown in Figure 2.1, but it can be seen on the blueprint of the construction of an MKU-48-type relay (see Figure 1.26). In more expensive constructions used for work at higher voltages (over 300 to 500V)
both elements are included. In high-voltage relays the insulating baffle pin usually comes in the form of a long rod (7) linking the armature with the contacts (Figure 2.2). In relatively low-voltage relays (up to 220 V), in order to simplify the construction and
to make it more compact, a coil from a magnetic core is insulated with the help of an insulating bobbin and connects a movable contact directly to the armature (Figure 2.3). Note that the contact spring and the restoring spring of the armature are the same component; a flat bent beryllium bronze plate. Since the magnetic core of the relay is not insulated from the contacts and is alive, the relay is put into a hermetic plastic case. Let us consider the basic types of electromagnetic relays.
