You must have heard the term relay before; Right? For those who don’t; An electrical or electromechanical relay is a device most used in our daily lives. It’s everywhere; in our homes, washing machine, car, air conditioner, aircraft, fridge, microwave, computers, television set, protective equipments and even on ship. But more than that; these relay’s are most used in power plant and power grids as an switch or a protective device. In past they were mostly used in telegraph and mores code communication; where signals were send in the form of 0 and 1 represented by on and off of light bulb.
So, A relay is an electromechanical device which acts as an switch with help of an electromagnet; a spring, armature and two contact points. We can use it as an electromagnetic switch; by applying a small current of 5 to 12 volts to open and close high voltage circuits electrically. This small current makes or break the armature connection with the circuit; therefore opens and close the circuit upon given signal. When current is provided; the armature make contact with the circuit against the spring tension. And when the relay is deactivated armature contacts are pulled back to cut off the power supply.
At home usually we have one or another kind of fuse to protect against electrical faults and short circuits. But in large industries / complex with high electricity demand; circuit breaker is preferred over others. They even have preferential trips to maintain essential power supply during over load condition. To achieve such efficient automatic power control; a relay is used in the circuit. It measures the voltage and current in the power circuit and send trip command in event of an abnormality.
Different Components Of Electromechanical Relay
A relay is nothing but a simple electrical device consisting of eight small parts; a pair of fixed contacts, one movable contact, armature, yoke, electromagnet, control switch and a spring. Together they make up the most simple yet most used equipment in electrical industry. A relay have this solid iron core with wire wound to it as an energizer. The fixed part of this solid iron core is called yoke while the freely moving part as armature. A spring is connected in between yoke and armature to reset its contact.
1 ) Relay Contacts
An electromechanical relay have these two fixed contacts; “NO” normally open and “NC” normally close. They share a common movable contact called “COM” or common. Under normal condition a relay can be either open or close. In open position contacts are made only when it is energized.; and return to their normal position i.e normally closed once De-energized.
You can consider these contacts as a metal conductor made of elastic material such as beryllium or phosphor bronze alloy. They have special contact strap soldered onto it. These contact straps are made of high conducting material with resistance to damage due to electric spark. This property provides somewhat protection against the spark current due to sudden high current discharge. These metal contacts upon contact close the electric circuit; allowing current to flow.
Special care must be taken to ensure the right condition of these conductors. Frequent spark discharge can damage the contact straps; increasing its resistance. Resistance and conductivity of these contact surface thus must be measured time to time. If the resistance increase to 1Ω, necessary measures must be taken to rectify; if not new contacts be installed. A conductor strap of an electromechanical relay can be made of silver, silver copper, silver tungsten, silver nickel, platinum and gold alloys.
2 ) Electromagnet
Magnetic Field is produced when the current is passed through a conductor. You can easily check its presence with the magnetic field sensor on your mobile; or by deflection of compass needle. Similarly when current is passed through a coil wounded on a soft iron core; a magnetic field is created perpendicular to its direction of current. This magnetic field when flow through this soft iron core; it turn it into an electromagnet.
An electromagnet holds its magnetic property until current is disconnected; so can be turned off and on playing a major role in relay working principle. Conventionally relays use to have latency in magnetic field. This means; they retain some of their magnetic property even when the current is removed. This is due to the magnetic lag created due to hysteresis; a hysteresis is an magnetic phenomenon in which magnetic induction on an product lags behind its own magnetic field / force.
If you wish you can check more about hysteresis and its effect on magnetism on Britannica.com. To nullify this effect and bring it back to its non magnetized state; an additional small magnetic field is applied in the opposite direction. This small magnetic force applied on the soft iron core to nullify residual magnetic property is known as corrective force of a relay. Thus, the yoke establish the magnetic circuit which then create necessary magnetic field to operate armature with the help of core and energizing coil.
3 ) Movable Armature
An armature in relay is that moving conductor which makes or break contact based upon the magnetic flux of the iron core. When energized these armature are pulled against the spring tension to make or break contact based on normally closed / normally open relay type. Care must be taken to maintain spring tension as it avoids unintentional trigger of armature due to smallest fault current. Once De-energized these movable armature return to their initial position.
Small and simple relay with regular plastic cover tends to have basic unbalanced armature. The disadvantage of having such relay type is that they are hyper sensitive; and can be easily affected by external factors. Sudden shock or acceleration can result in unintentional contact switch; provoking serious trouble. To counter such effects professional and large relay came with balanced armature. So the rotational axis of balanced armature along the pivot / stand is taken along their center of mass.
Theoretically one can use the same armature for both alternating as well as direct current. In practice alternating current change each half cycle resulting into pulsating magnetic field. This cause the armature to make and break contact each half cycle. This phenomenon can be seen in the form of spark and audible buzz. To solve this problem shader ring is placed on top of the iron core. It make sure that there is some magnetism left in between the half cycle to hold the armature in place.
4 ) Yoke
A yoke in electromechanical relay is that metal piece attached to the soft iron core which do the job of holding and attracting the armature. They are very small metal piece attached on top of the core element. In many design the armature is hinged to the yoke connected with a conducting wire. It insures the connectivity / continuity of current in between contacts and the moving armature. The other major role of yoke is to provide the low reluctance path for the magnetic flux to flow through.
Ideally when we look at an electromagnetic relay; we find an energizing coil called primary circuit. A primary circuit have two key parts; one is movable and is loaded with a spring. This part is called armature while the other part that remain fixed is called yoke. When current is supplied to the energizing circuit; yoke pull the movable armature towards itself closing the air gap in between. Modern yokes are made of ferromagnetic material with composite material as a layer in between.
5 ) Spring
Not all relays have a spring attached; but those who do have it attached to the armature for its easier movement. This spring allows the armature to move freely within the generated magnetic field to make or break contact with electrical connection.These springs are usually made from flat sheet metal cut and then drawn in form of a spring; while some high performance relays have spring made of nickel silver. Any damage or reduction in its spring tension can make a relay UN-useful; and thus care must be taken to keep it in good condition.
The spring pull back the armature to its initial condition once relay is DE-energized. This spring force is half the force of the magnetism which keep the armature fixed to its final contact. Once the armature make itself to its final point much less magnetic field is required to keep it in place. So efficient springs are used which can pull back the armature in a quick interval of time. Such a quick operation not just reduce noise and sparks but also increase its overall efficiency.
How A Relay Work And Function – Explained
An electromechanical relay works on the simple principle of electromagnetism. When a low voltage direct current is given to the energizing coil; electrical contacts are made in effect of the magnetic field. You can consider this phenomenon as a simple switch where circuit is completed on a push of a button. A simple relay is a two way switch making connection with different circuit on its either side; with three contacts NC, COM and NO.
Relays with one common and two main contacts on a single pole arrangement are called single pole double throw type. Similarly single throw single pole have one connection NO and double pole double throw have two NO and NC respectively. Initially when no current is supplied to the energizing coil; contact is made with NC and COM. If you connect a bulb with COM and NC at that time it will glow; similarly when the relay is energized you can connect your bulb with COM and NO to glow.
While you look at an relay upside down; you will find five contact points. Three at the one side while two on the opposite; the two contacts on the opposite side is for NO and NC while the others are Coil +, COM and Coil – respectively.When we supply current to these two points coil + and Coil – magnetic flux is produced; and the position of armature is altered. Similarly when we disconnect the supply the armature goes back to its position and NC contact is closed.
To better understand in the given diagram; the load is connected to a 220V 50Hz power supply through a switch actuated by relay. The relay is controlled using a separate 12 volt D.C supply. Now when the load in form of motor is needed to be start; we manually switch on the supply to relay. This in turn automatically close the high voltage circuit and the motor start. This arrangement is very helpful and most used where we have to use very high voltage and manual operation is impossible.
Different Type Of Relays And Their Application On Ship
1 ) Earth Fault Relay
Majority of electrical accidents such as electric shock and short circuit is the result of earth fault / leakage. A leakage current is the result of insulation breakdown and can happen due to moisture, wear, tear and heat. In fact once the earth leakage start; it create enough heat to sustain further degradation of wire insulation. So to keep ship’s operation safe and meet regulation guidelines; insulation resistance of different system is monitored constituently.
If the insulation resistance fall below a critical value the circuit can be tripped using an earth leakage circuit breaker. A circuit breaker use the function of a relay to trip the circuit. They have their actuating terminals connected to the earth and relay coil. When the leakage current reach a certain preset value it actuates the relay coil and thus breaks the circuit. As a minimum leakage current of say 1 mA always flow through; circuit breaker must be set to actuate relay coil only if it pass a critical value.
2 ) Under Voltage Relay
An under voltage relay provide protection from very low voltage ( under voltage ) to the bus bar. It protect the equipment against any damage by tripping the circuit under low voltage condition. A low voltage condition is a situation in which; average voltage of a three phase supply drops below its critical limit. Usually a timer delay is installed along with the under voltage relay to avoid circuit trip on momentary voltage fluctuation.
Generally they just act as an additional safety measure in case other safety switch and circuit breaker fail to operate in time. An under voltage relay is installed on board ship in auxiliary engine power output. It is intended to break the circuit in between the alternator (auxiliary engine) and the bus-bar under low excitation condition. In case of any abnormality it keeps the plunger under trip position avoiding one to accidentally close the breaker.
Above all these under voltage relays also helps avoid accidents and damage to engine and load as it prevent to closure of breaker for the alternator that is coming under load in parallel operation.
3 ) Reverse Power Relay
When two generator runs in parallel both share the load based on their KvAr value. When one of the generator excitation decrease; its power began to fall. Now it no longer supply power but run in series with the other generator as an induction motor. If remained unchecked it will draw all power from the main bus bar; resulting into system overload and then blackout.
Thus to protect equipment and machineries connected to the bus bar and the generator itself; reverse power relay is installed on each generator on ship. When current start to flow in opposite direction; it actuates the relay energizing coil. This results into breaker trip thus avoiding blackout. A time delay of five to ten seconds is given in the relay operation with reverse power setting of 10-20 percent. A reverse power relay do its job when the prime mover fail accidentally.
4 ) Thermal Overload Relay
Overload of a circuit or machinery results into inefficient operation of the equipment. It also leads to the loss of insulation, overheating, insufficient torque and damage to motor windings. Thus thermal overload relay are installed on board to trip the main breaker in event of prolonged overload current. An overload relay works on the principle of heat generation due to electric transmission. A nickel alloy metal with high thermal resistivity is used for this purpose.
Under normal condition the alloy strip remains straight making no contact with the actuating circuit. But when overload current pass through the load circuit; excessive heat is produced in the process. This excessive heat then lead to bending of nickel metal strip; thus making contact with the relay actuating circuit. This allow the relay to trip the circuit breaker protecting on board equipment and machinery.
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