The electric relay is one of the most frequently used devices in modern technological systems. It can be found in cars, washing machines, microwave ovens, and medical equipment, as well as in tanks, aircraft, and ships. Practically no industry would function without relays. In some complex automatic control systems in the industry, the number of relays is estimated in hundreds and even thousands. In the power-generation industry, no power device is allowed to operate without special protection relays. Certain electrical equipment, such as power transformers, may be protected by several different kinds of relays, each controlling different functions.
Although relays are so widely used and there are so many types, the broad population of engineers is unfamiliar with most of them. After reading the article, you will have a basic knowledge of the relay types.
Relays have various types. Some of them are listed below:
An electromagnetic relay is the simplest, most ancient, and widespread type of relay. Its basic elements are a winding, a magnetic core, an armature, a spring, and contacts. The magnetic system is used to transform the input electric current to the mechanical power needed for contact closure. And the contact system transforms the imparted mechanical power back to an electric signal. An insulation system provides galvanic isolation of the input circuit (winding) from output one (contacts).
A latching relay is one that picks up under the effect of a single current pulse in the winding and remains in this state when the pulse stops affecting it, that is when it is locked. Therefore, this relay plays the role of a memory circuit. Moreover, a latching relay helps to reduce power dissipation in the application circuit because the coil does not need to be energized all the time.
Temperature or thermal relays belong to the second (or even probably to the first) most popular type of special-purpose electric relays. There are two basic types of such devices: relays with input energizing quantity in the form of heat and relays with an input quantity in the form of electric current. Relays of the first type are applied for direct temperature control of different units. Relays of the second type are used as protective relays from current overload, for various electrical customers. In the latter case, the electric current is transformed to the heat inside the relay first, and when the temperature of the internal thermal element reaches a certain value (and the relay is energized)—to an output electric signal.
Many engineers have come across original contact elements contained in a glass shell. However, not everyone knows that reed relays differ from ordinary ones not because of the germetic shell (sealed relays are not necessarily reed ones), but because of the fact that in a reed relay, a thin plate made of magnetic material functions as contacts, magnetic system, and springs at the same time. One end of this plate is fixed, while the other end is covered with some electroconductive material and can move freely under the effect of an external magnetic field. The free ends of these two plates, directed towards each other, are overlapped for from 0.2 to 2mm and form a basis for a new type of switching device.
High voltage relays
The rapid development of electric technologies applying high voltages (power lasers, industrial accelerators, high-frequency metal, and dielectric heating, etc.), the use of power electronic equipment operating under high voltages (radar, TV, and radio transmitters), and also the need for systems for testing insulation of electrical devices of different voltage levels, were the causes that bought about the spread of high-voltage (HV) relays, operating under voltages from 5 to 300 kV and higher. Such relays can be divided into two groups: relays with HV insulation for all current-carrying elements switching high voltages, and relays with low-voltage (LV) contacts and high voltage insulation between the input elements (control coil) and the output ones (contacts).
One of the most widespread types of relays (after electrical relays) is ‘‘time relays.’’ Usually, these are relays operating with a certain delay with regard to the signal applied to the relay input, which is why frequently the term ‘‘time-delay relay’’ is used. As the change of state of a relay is accompanied by a certain delay with regard to the signal applied to its input, one can say for sure that, apart from its other functions, every relay also functions as a time relay. Sometimes standard electromechanical relays are used to enhance the stability of complex automatic control systems. Their only function is to provide a certain signal delay, the value of which equals its own make delay. In terms of engineering, ‘‘time relays’’ or ‘‘time-delay relays’’ are usually defined as relays in which the time-delay function dominates, and in which the characteristics of that function are enhanced, by one means or another.
Current and voltage relays
These relays are specially designed for current or voltage level control in electric circuits of high and low voltage, and for the generation of certain output signals, when the current or voltage level deviates from a preassigned value. Such relays are also called ‘‘measuring relays,’’ since in the process of operation they constantly measure the level of the actuating value. Very often the output signal of such relays affects the power shutdown device, de-energizing the load and thus protecting it (or the main supply) from damages in emergency modes, which is why such relays are also called ‘‘protective relays.’’
Differential protection compares two (or more) currents to locate a fault; which actually makes current protection. In comparison with other types of protection, differential current protection possesses an absolute selectivity in the sense that it operates smartly only in those cases where the fault is within the protected zone and does not operate at all if the fault is out of its zone. The zone of the differential relay is limited by a part of the electric circuit between the current transformers (CTs), to which the relay is connected. Due to such high selectivity of protection, there is no need to activate a delay for the relay pick-up, which is why all differential relays are high speed. That being so, extraordinarily high selectivity and high speed of operation are the distinguishing features of differential protection.
A relay that functions when the circuit admittance, impedance, or reactance increases or decreases beyond predetermined limits. If each of the relays installed along the line have time delays depending on impedance (distance), the relay which picks up first will always be the one that is nearest to the point of short-circuiting. This is the main purpose of distance protection. In circuits with a two-way supply, distance protection is directional.
The frequency decreases because of power system overload, while a frequency increase is evidence of a power excess. Power excess occurs in the system when one or several hard-loaded lines are suddenly disabled. Surplus power is directed to other lines, causing dangerous power flows that can lead to a power system breakdown. That is why it is so important to control voltage frequency. Like all other parameters of electric circuits, frequency is controlled by special relays.
A polarized relay is a sort of direct current (DC) electromagnetic relay with an additional source of a permanent magnetic field affecting the relay armature. This additional source of the magnetic field (called ‘‘polarizing’’) is usually made in the form of a permanent magnet.
The microprocessor-based relay is a small computer in which the output circuits have matched parameters with external current and voltage transformers, with a program stored in memory, allowing processing of input signals in such a way that operation of this or that type of protective relays can be modeled. With the help of a basic universal microprocessor, one can create any relay by just making certain changes in the program, at least that is how it used to be at the initial stage of development of microprocessor-based equipment.
A sequence relay is sometimes called an alternator, stepper, step-by-step, flip-flop, or impulse relay. The relay has the ability to open and close its contacts in a preset sequence. All sequence relays use a ratchet or catch mechanism to cause their contacts to change state by repeated impulses to a single coil. Usually, but not always, one pulse will close a set of contacts, the next will open them, and so on, back and forth.
Rotary or motor-driven relays are relays in which forward movement of the armature and contacts is replaced by rotary movement. In fact, this is a standard multi-contact rotor switch with an electromagnetic drive instead of a manual one.
Relays of this type have a quite unusual external design, sometimes resembling a vacuum tube or a measuring device. It is only natural that such a relay resembles a measuring device because, in fact, it is a highly sensitive measuring mechanism, with very sensitive contacts. The functioning of this device is based on the interaction of the magnetic field of the permanent magnet with the current in the winding. The winding is wound around a light aluminum bobbin of rectangular shape (a frame) placed in the gap between the permanent magnet and the core ring.
Annunciator target relays
An annunciator relay (target relay, signal relay, flag relay) is a nonautomatically reset device that gives a number of separate visual indications of the functions of protective devices and which may also be arranged to perform a lock-out function. In other words, target relays are used in relay protection and automation systems as an indicator of pick-up of other relays.
Flashing-light relays (or Flashers) are used to produce the flashing light of signal lamps that, due to that flashing, attract more attention than permanently switched-on lamps. Such relays are widely used to control single signal lamps and as a part of multivalve signal boards.
Buchholz relay is used to protect equipment immersed in liquids by monitoring the abnormal flow or its absence or an abnormal formation of gas by the equipment (most faults in an oil-filled power transformer are accompanied by a generation of gas). These relays are normally used in transformers with expansion tanks. They collect the gas that is gradually released due to small internal problems such as bad connections, small arcs, etc. until the volume of gas operates a switch, which then gives an alarm signal. The gas can then be collected and analyzed to determine the nature of the problem.
Electric relays contain many parts, which are subject to dynamic, electrical, or thermal wear. There are many applications where safety is very critical and it is important to use electrical equipment, ensuring that dangerous machine movement cannot occur when a fault is detected with the moving relay contacts during the cycle in which the fault is indicated. In order to assure safe function, especially in the event of a failure, appropriate controls are built into the circuits of safety devices. Safety relays with forced guidance contacts play a decisive role in preventing accidents in machines and systems.
Ground fault relays
A ground fault relay is a device that is intended to trip out an electricity supply in the event of a current flow to earth. As such, it can provide protection from harmful electric shocks in situations where a person comes into contact with a live electrical circuit and provides a path to earth. Typical examples of this occurring are the use of faulty electrical leads and faulty appliances.
The main purpose of such relays is continuous monitoring of serviceability of important units (or important electric parameters of power applied to such units). The trip coils and the power supply of high-voltage circuit breakers in electrical networks; power supply circuits for sensors of fire-alarm systems; phase sequence and phase losses in power supply for electric motors; insulation level of electric equipment, etc., concern for such units and parameters. Supervision relays also detect interruptions, too high resistances caused by galvanically bad connections, increased transfer resistance in the contacts, welding of the control contact, disappearing control voltage, and voltage failures in the relay itself.
It is an electronic relay, designed as single epoxy molded solid-state module (usually, opto-coupled). It is used in fast switching applications.
Power factor relays
A relay that operates when the power factor in an AC circuit rises above or falls below a predetermined value. It is used in power factor correction applications.