What are the types of Relays?

Overview: Electrical and electronic circuits consist of many components such as fuses, relays, diodes, transistors and so on. All these components have a specific task to carry out and are equally vital for full functioning of the circuit or the equipment as a whole to function. Some components are used for protecting the circuit from damage and some others are used to maintain the integrity of the equipment. Holding on to this concept, relays are present in multiple circuits across various gadgets and equipment’s. The relay can be considered to be a type of switch to open or close a circuit by electronic or electromechanical means.  Various types of relays are present and can be chosen according to the application.

What is a relay?  The relay can be considered to be a type of switching element to open and close a circuit. Relays are able to control the opening and closing action in another circuit. Terms like normally open and normally closed can be heard frequently while discussing about relays. Relays can be found on many electrical devices. Due to the inclusion of relays no connection between the two particular circuits is established as the action can be either mechanical or magnetic.

The basic construction of a relay consists of a electromagnetic coil, armature, a spring element and multiple contacts. When the coil is energized the armature moves and the depending load is connected to the power supply. All the armature movement is made possible by means of a spring element present in the construction of the relay. Here, it can be imagined that the relay is an intermediate medium between the two circuits and hence these circuits do not come in contact with each other directly. Relays have either normally open or normally closed contacts which means that whenever the coil is energized the common contact moves between either of the two contacts and completes the circuit.  

Manufacturers like Schneider provide control relays and accessories. Relay price and other details are available online and one can buy control relays online according the requirement.

Relay and their classification:

The application tends to determine the type of relay to be used. Some relays can be classified as protective relays, regulating relays, auxiliary relays, etc. Of course, depending on the structure and principle of operation, relays are of different types for e.g. electromagnetic relays, thermal relays etc. and have a different shape as well as size.

Electromagnetic relays: Electromagnetic relays consist of electrical as well as mechanical components, mechanical contacts and coils. As mentioned earlier, the coil included in the relay is energized and due to this the contacts either open or close depending on their type. Such relays can work with both AC and DC electrical supply. As the relay works on the principle of induction (electromagnetic) it can work with both types of supplies. Even if these relays can work with AC and DC the construction in each case differs. In DC supply the relays are equipped with freewheeling diodes for de-energizing the coil where as in the AC supply the relays come equipped with cores which have a laminated form.

In AC supply, the direction of current changes for every half cycle, the relay coil cannot sustain magnetism throughout the entire cycle but loses it within the half cycle. Hence, a separate AC circuit or a coil might be included in the relay to help the relay sustain magnetism in the zero current position.

A few types of electromagnetic relays are an induction type relays, latching and non latching relays, attraction type relays.  Following paragraph summaries all these types of relays on brief.

Induction type relays consist of a conductor having the ability to move through the electromagnetic flux. A subtype of induction type relays is shaded pole relay, induction cup relay and watt hour meter relay. The conductor might be cupped or disc shaped. Shaded pole relay has pivoted aluminium disc, which is able to move in the air gap of the electromagnet. Metallic ring mostly made up of copper surrounds the pole. The metallic ring can be called as a shaded ring and the flux which is induced in this ring can be called as shaded flux. This shaded flux tends to lie behind the flux produced in the unshaded pale. Due to this difference of fluxes a much required torque can be produced so that the disc can be rotated. The induction cup type relay has a similar operation to that of an induction disc type relay. This type of relay can have multiple poles, which tend to depend on the number of windings. As a cup replaces the disc in the relay, inertia experienced in the rotating system. Similar to the shaded pole relay, the induction cup type relay develops the torque by means of two poles which, have some degree of lag in between them. The cup is standstill in at one position in normal operating condition, but if the circuit encounters a fault and high current is induced in the coil, cup moves and circuit are isolated.

Attraction type electromagnetic relays can work with both AC and DC supply. Such relays are fast acting and hence can provide instantaneous action. These relays can be made up of a plunger pulled towards a solenoid or armature pulled towards electromagnet poles. Sub types of this type of relay are solenoid type relay, attracted armature relay and balanced beam relay. Latching and non latching type relays are also a type of electromagnetic relays.  Non latching relays tend to be in a normally closed state by means of a spring action when the current is not flowing through the coil. If current ever flows through the coil the contact opens due to electromagnetic pull from the coil. When the current stops flowing the contacts revert back to the normally closed state.  Latching type relays are made up of one or couple of coils without any fixed default position. When the current flowing in the coil stops these relays maintain the position.

Thermal relays:

Thermal relays have a slightly different working principle as compared to the electromagnetic type replays. Typically, thermal relays work in tandem with ambient heat. In simple words, if the temperature around the relay changes and moves above the required limit, the relay is forced to switch to another state. Again, such relays are mainly used in protection of electric motors and similar inductive loads from overloading or an imbalance in voltage. Bimetallic elements tend to be present in the construction of the relay, which act as temperature sensing elements.

A few different type of thermal relays is melting alloy relay, temperature control relay, bi-metallic relay, solid state thermal relays.

Melting alloy relays are made up of eutectic alloy, a heater coil and some triggering mechanisms. The alloy which is used in the construction of this relay has a tendency to turn its state from solid to liquid at a certain specific temperature. A ratchet wheel is coupled with the alloy element which practically acts as a tripping mechanism. Current flows through the heating coil of the relay, which play an important part in tripping the relay. Whenever the relay faces an overload condition the heater coil heats up to a temperature so as to melt the alloy. When the alloy melts the ratchet wheel attached to the element turns tripping the relay. Temperature control relays typically make use of temperature sensitive devices like thermostats or RTD’s for applications like winding protection in an electric motor. Sensory elements are placed in the winding of the motors which detect the temperature in the circuit. Such type of relays is available in 3 pin, 4 pin or 5 pin configurations were some relays can be normally open or normally closed.

Bimetallic thermal relays consist of a bi-metallic strip which, when heats up changes its form factor, i.e. either it expands or flexes. As the bi-metallic strip is made up of 2 different metals, the strip has a different linear expansion pattern. Whenever this type of relay tends to face an overload condition, temperature rises in the windings. As the temperature rises the bi-metallic strip tends to expand, breaking the circuit in turn protecting the circuit from overloading. Solid state relays tend to have a straight forward working principle. The relay has solid state electronic components in its construction and it tends to monitor both starting current and load current as well as the load temperature. If the relay identifies a spike in temperature, it trips and isolates the circuit.

Schneider thermal overload relay and thermal overload relay from L&T are some of the relays available in the market. Details about thermal overload relay price can be easily found online.

Solid State/Electronic relays:

These types of relays have electronic devices such as power transistors or Thyristors to carry out switching operation. The control energy requirement is very much lower in comparison to the output power that needs to be controlled, as a result, these relays have a higher power gains as compared to the electro-magnetic counterparts. Some types of solid state relays are reed relays, transformer coupled SSR, Photo coupled SSR etc.

Reed relays as the name suggests, consist of reeds or in other words, a type of magnetic strip. This reed is encapsulated inside a glass tube. The reed inside the tube is movable and can act as an armature. Magnetic field, when applied around the glass tube, influences the reed into movement. When the reed moves to either side the switching action takes place. Transformer coupled relays [SSR] and Photo coupled relays [SSR] are a couple of solid state relays. Transformer coupled relays consist of an AC/DC converter, trigger circuit, control circuit and a thyristor. In such type of relays a controlling signal is connected to the primary of a transformer. A thyristor switch is triggered by means of a secondary excitation depending on the whichever circuit is being used. Photo coupled relays consist of an LED which is responsible for emitting the signal which is received by a semi-conductor device which is photo sensitive in nature. Photo detector triggers a TRIAC which in turn switches the loads.

High voltage relays like vacuum filled relays work at a higher operating voltage than other low voltage relays. The main difference between the construction of these relays relates to the design of the contacts which are obviously designed for operating at high temperatures. Insulation is also present between the earth and contacts as well as the activators and the contacts. Due to higher voltage such relays tend to be encapsulated in a ceramic or glass enclosure as a contingency against arching which takes place during switching action. Types of high voltage relays are vacuum filled relays and gas filled high voltage relays. Radio frequency [RF] typically tend to prefer vacuum filled relays due to stable and low contact resistance. In the case of a vacuum filled relays, arc generated at the time of switching cannot sustain for long and extinguishes quickly due to the presence of vacuum. Gas filled relays showcase excellent dielectric strength and low oxidation. Cable testing equipments, heart defibrillators tend to make use of gas filled relays due to the qualities mentioned earlier. The gas used in the relays is a combination of nitrogen and sulfur hexa fluoride.

Motor protection relays:

Motors used in industries require protection as motors are costly to repair, fault in the motor can cause loss of production as well as waste of man hours and similar other factors. Similar to the functions of relays, motor protection relays also tend to locate faults and if identified isolate the system from the fault. Motors can encounter a number of faults such as short circuit, locked rotors, earth faults, bearing failures, etc. Motor protection relay offer protection against such faults.

Apart from relays themselves manufacturers like Larsen & Turbro manufacture accessories for relays. Relay accessories like push button extension kits, push button units, mounting kits and various types of reset cords. All these accessories are easily available online and can be purchased easily. Summarizing this content, relays can prove to be an important and essential part of the electrical circuits used in various industrial applications as relays protect the circuits from untoward failures and mishaps. Application requirement differs an so do the relay used for the same. Variety of relays are easily available in the market and can be chosen according to the application at hand.

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