A relay is an electromechanical switch. There are different classifications of relays. The more common ones refer to the controllable quantity: current relays, voltage relays, power relays, pressure relays (pneumatic pressure switch), insulation control relays, etc., according to the principle of operation: electromagnetic relays, magnet electric relays, electrodynamic relays, induction relays. There is also a separate class of semiconductor devices which in case of alternations in certain physical parameters of the environment switch circuits which are also called relays. For example opto relays (optrons) which switch circuits in case of light alternation, reed ampoules, switching in function of the magnetic field, solid state semiconductor contactless relays etc., which we will not discuss in details. Here we will pay special attention to electromagnetic relays.
Electromagnetic relays are composed of an electromagnet (a magnetic core made of ferromagnetic material and a coil wound around the magnetic core), armature of active magnetic material and a switch mechanism. Electromagnetic relays are mainly of two types – current and voltage ones. They can be identified by the structure of the coil. Current relays have coil windings which are made of thick wire and are few in number. They are used in different types of circuit protections. Voltage relays have a coil, whose windings are made of thin wire and they are a lot so that there is high impedance and when there is parallel connection with the consumer, the current that flows through the coil is relatively small.
When we supply voltage to the terminals of the relay winding, the current flowing through the coil induces a magnetic field in the magnetic core. The magnetic field attracts the armature and the armature in turn commutates the switch mechanism. If the relay contacts are normally open (NO), they close and if they are normally closed (NC), they open. Thus there is a change of the state of the electrical circuit at the output. In case of dropping out of the input voltage – the armature resets, being pushed by a spring. These are standard relays with two stable states.
The downside of each relay is the limited electrical life of the contact system. Each commutation arouses micro electric arcs and sparks that accumulate soot on the contact bodies. When the soot increases over thousands of commutations, it begins to impede the passage of electric current through the switch mechanism and the relay cannot operate.
There are relays incorporating electronic components, manufactured with the purpose of avoiding this disadvantage. For example, a resistor connected to the coil of the relay for better operation or (and) a capacitor connected parallel to the contacts to reduce arcing and interference. Mercury relays, in which the switching element is mercury, are another constructive resolution.
The main difference between a contactor and a relay is that relays have much simpler contact system. A contactor may have a significant number of contacts, some of which close and others open when current flows through the control circuit.
Specifics of operation and usage of relays
Relays have two essential features which determine their application.
1. Relays electrically (galvanically) divide circuits into control and controlled ones, which makes possible their usage as a form of protection.
2 . By means of very weak currents in the control circuit, relays can manage much stronger currents in the controlled circuit. In this way they become discrete amplifiers of current, voltage and power in electric circuits. At the input the relay may receive a control signal from various sensors (light, pressure, temperature).
The most common usage of relays is connected with controlling large currents. When currents are too large (hundreds of amps) the entire relay is immersed in engine oil and the contact areas are substantially larger.
Relays are widely used in households for switching refrigerators, washing machines, dishwashers, cookers,
boilers, when starting various electric motors, in cars (automotive relays), etc. Relays are also used when it is necessary to manage multiple circuits by a single control signal. Relays with calibrated operating characteristics and sometimes having multiple operating coils are used to protect electrical circuits from overload or damage; in modern electrical systems these functions are performed by digital tools still called „protective relays“.