File Name: types of relays and its working .zip
The development of relays was initiated in the period
- Types of Relays - A Thomas Buying Guide
- Types of Relays
- Working of Relays
- Different Types of Relays and Their Working Principles
The relay is an automatic protective and switching device which is capable of sensing abnormal conditions in electrical circuits. These are operated to open or close the load contacts in response to one or more electrical quantities like voltage and current. Relays are used in a wide variety of applications like electric power systems, home appliances, automobiles, industrial equipments, digital computers, etc.
A relay is usually an electromechanical device that is actuated by an electrical current. The current flowing in one circuit causes the opening or closing of another circuit. Relays are like remote control switches and are used in many applications because of their relative simplicity, long life, and proven high reliability. Relays are used in a wide variety of applications throughout industry, such as in telephone exchanges, digital computers and automation systems. Highly sophisticated relays are utilized to protect electric power systems against trouble and power blackouts as well as to regulate and control the generation and distribution of power.
Types of Relays - A Thomas Buying Guide
A relay is usually an electromechanical device that is actuated by an electrical current. The current flowing in one circuit causes the opening or closing of another circuit. Relays are like remote control switches and are used in many applications because of their relative simplicity, long life, and proven high reliability.
Relays are used in a wide variety of applications throughout industry, such as in telephone exchanges, digital computers and automation systems. Highly sophisticated relays are utilized to protect electric power systems against trouble and power blackouts as well as to regulate and control the generation and distribution of power. In the home, relays are used in refrigerators, washing machines and dishwashers, heating and air-conditioning controls. Although relays are generally associated with electrical circuitry, there are many other types, such as pneumatic and hydraulic.
Input may be electrical and output directly mechanical, or vice versa. How do relays work? All relays contain a sensing unit, the electric coil, which is powered by AC or DC current. When the applied current or voltage exceeds a threshold value, the coil activates the armature, which operates either to close the open contacts or to open the closed contacts. When a power is supplied to the coil, it generates a magnetic force that actuates the switch mechanism.
The magnetic force is, in effect, relaying the action from one circuit to another. The first circuit is called the control circuit; the second is called the load circuit. Electromechanical relays have moving parts, whereas solid state relays have no moving parts. Advantages of Electromechanical relays include lower cost, no heat sink is required, multiple poles are available, and they can switch AC or DC with equal ease.
Electromechanical Relays General Purpose Relay: The general-purpose relay is rated by the amount of current its switch contacts can handle. Most versions of the general-purpose relay have one to eight poles and can be single or double throw. These are found in computers, copy machines, and other consumer electronic equipment and appliances.
Power Relay: The power relay is capable of handling larger power loads — amperes or more. They are usually single-pole or double-pole units. Because of these high power requirements, contactors always have double-make contacts.
Time-Delay Relay: The contacts might not open or close until some time interval after the coil has been energized. This is called delay-on-operate. Delay-on-release means that the contacts will remain in their actuated position until some interval after the power has been removed from the coil. A third delay is called interval timing. Contacts revert to their alternate position at a specific interval of time after the coil has been energized. The timing of these actions may be a fixed parameter of the relay, or adjusted by a knob on the relay itself, or remotely adjusted through an external circuit.
Solid State Relays These active semiconductor devices use light instead of magnetism to actuate a switch. The light comes from an LED, or light emitting diode. On the load side of this space, a part of the device senses the presence of the light, and triggers a solid state switch that either opens or closes the circuit under control.
Often, solid state relays are used where the circuit under control must be protected from the introduction of electrical noises. The drawback to using a solid state relay is that it can only accomplish single pole switching.
Their characteristics are significantly affected by factors such as the material of the contacts, voltage and current values applied to them especially, the voltage and current waveforms when energizing and de-energizing the contacts , the type of load, operating frequency, and bounce. If any of these factors fail to satisfy a predetermined value, problems such as metal degradation between contacts, contact welding, wear, or a rapid increase in the contact resistance may occur.
For example, when the relay is used to control an inductive load, such as a motor of a lamp. The contacts will wear faster and metal decomposition between the mating contacts will occur more often as the inrush current to the contacts increases. To prolong the life expectancy of a relay, a contact protection circuit is recommended. This protection will suppress noise and prevent the generation of carbon at the contact surface when the relay is opened.
Examples of these synergistic components that provide contact circuit protection include resistor capacitors, diodes, Zener diodes and varistors. Each form factor is explained below. It is open when the coil is de-energized and closes when the coil is energized. Form A contacts are useful in applications that must switch a single power source of high current from a remote location. An example of this is a car horn, which cannot have a high current applied directly to the steering wheel.
A Form A relay can be used to switch the high current to the horn. It is closed in the de-energized position and opens when the coil is energized. Form B contacts are useful in applications that require the circuit to remain closed, and when the relay is activated, the circuit is shut off. Form C is a combination of Form A and B arrangement, sharing the same movable contact in the switching circuit.
Form C contact are useful in applications that require one circuit to remain open; when the relay is activated, the first circuit is shut off, and another circuit is turned on. An example of this is on a piece of equipment that runs continually: when the relay is activated, it stops that piece of equipment and opens a second circuit to another piece of equipment. Make-before-break Contact: a contact arrangement in which part of the switching section is shared between both a Form A and a Form B contact.
When the relay operates or releases, the contact that closes the circuit operates before the contact that opens the circuit releases.
Thus both contact are closed momentarily at the same time. The inverse of a Make-before break contact is a Break-before-make contact. Poles are the number of separate switching circuits within the relay. Both the size of the load and its type are important. There are four types of loads: 1. Resistive, 2. Inductive, 3. AC or DC, and 4. High or Low Inrush. Resistive Load is one that primarily offers resistance to the flow of current. Examples of resistive loads include electric heaters, ranges and ovens, toasters and irons.
Inductive Loads include power drills, electric mixers, fans, sewing machines and vacuum cleaners. Relays that are going to be subjected to high-inrush inductive loads, such as an AC motor, will often be rated in horsepower, rather than in volts and amps. This rating reflects the amount of power the relay contacts can handle at the moment the device is turned on or switched. AC or DC — This affects the contacts circuit of the relay due to EMF and the timing sequencing and may result in performance issues in the switching capacity of the relay for different load types i.
High or Low Inrush - Some load types draw significantly higher amounts of current amperage when first turned then they do when the circuit later stabilizes loads may also pulsate as the circuit continues operating, thus increasing and decreasing the current. An example of a high inrush load is a light bulb, which may draw 10 or more times its normal operating current when first turned on some manufacturers refer to this as lamp load. In addition to the above load parameters, you now have to determine what parameters are involved with the control circuit, or coil circuit as it is sometimes called.
These may include: Sensitivity: Coils that actuate the relay when supplied with very low voltage or low current are called sensitive. Sensitivity is a relative term that differentiates low-power coils from high-power coils. Polarized: Coils of some relays that required DC voltage are polarized. That means there are specific terminals for positive and negative voltage to power the coil. Coil Information Characteristics of coils should be understood as a part of the relay chosen.
Some important specifications include: Coil Resistance: applicable to DC-switching relays only the resistance to the flow of electrical current. This resistance is measured at a temperature, depending on the manufacturer. The coil resistance of an AC-switching relay may be given for reference when the coil inductance is specified. Maximum voltage: the maximum value of permissible over voltage in the operating power supply to the relay coil. Rated coil voltage: a reference voltage applied to the coil when the relay is used under normal operating conditions.
Power consumption: the power consumed by the coil when the rated voltage is applied to it. Single Side Stable: The switch contacts in the relay remain in a normal or stable position as long as no power is applied to the coil.
When power is applied to the coil, the contacts move to a new position, but stay in that position as long as power is applied to the coil. Single-winding, latching type: This type has one coil that serves as both the set and reset coil, depending on the direction of current flow. When current flows through the coil in a forward direction, it serves as a set coil; when the current flows in the reverse direction, it functions as a reset coil. Dual-winding, latching type: This latching relay has two coils: set and reset.
It can retain the ON or OFF states even when a pulsating voltage is supplied or when the voltage is removed. Latching relays often have one set of terminals designated for the positive voltage and another for the negative voltage used to power the coil.
Such a polarized coil allows one action to take place when the coil voltage is positive, and an opposite action when the coil voltage is reversed. The difference between a single side stable relay and a latching relay is like the difference between a momentary action switch and a maintained action switch.
Impulse Relay: A special version of the latching relay. A pulse of current to the coil results in the contact changing position.
The contact remains in that position until the coil receives another pulse of current that moves the contacts back to their original position. Polarity is not important to the impulse relay; therefore, it can be actuated by AC or DC. Stepping Relay: Each time the relay coil is energized, the switch is actuated to a new set of contacts. This is similar to a rotary switch. Internal Operation of Mechanical Relays Standard: Single Side Stable with any of the following three different methods for closing contacts: 1.
Flexure Type: The armature actuates the contact spring directly, and the contact is driven into a stationary contact, closing the circuit 2. Lift-off Type: The moveable piece is energized by the armature, and the contact closes 3.
Types of Relays
Definition: The relay is the device that open or closes the contacts to cause the operation of the other electric control. It detects the intolerable or undesirable condition with an assigned area and gives the commands to the circuit breaker to disconnect the affected area. Thus protects the system from damage. It works on the principle of an electromagnetic attraction. When the circuit of the relay senses the fault current, it energises the electromagnetic field which produces the temporary magnetic field.
Relays are electrically operated switches. They are used to control a circuit by a separate low-power signal or to control several circuits with one signal. Relays were first used in long distance telegraph circuits as amplifiers. They replicated the signal coming in from one circuit and re-transmitted it to another circuit. A simple electromagnetic relay is made up of a solenoid, which is wire coiled around a soft iron core, an iron yoke that provides a low reluctance path for magnetic flux, a movable iron frame, and one or more sets of contacts.
There are different types of relays including electromagnetic relays, latching relays, electronic relays, non-latching relays, multi-dimensional relays and thermal relays which are classified based on the function, application type, configuration or structural features, etc. Now we look at various types of relays which are more popularly used in many applications. In some applications, it is needed to limit power consumption and dissipation, for that kind of applications a latching relay is best suitable. A latching relay consists of internal magnets such that when the current is supplied to the coil, it internal magnet holds the contact position and hence it requires no power to maintain its position. So even after being actuated, removal of drive current to the coil cannot move the contact position but remains in its last position. Thus, considerable energy is saved by these relays. In one coil type relay, the armature position is determined by the direction of current flow in the coil whereas in case of two coil type, position of the armature is depending on the coil in which current flows.
Working of Relays
In this article, the basics of a relay like energized relay and de-energized relay are explained in detail. Also, the design, construction, working, applications, and also relay selection is explained in detail. We know that most of the high end industrial application devices have relays for their effective working. Relays are simple switches which are operated both electrically and mechanically.
From a small traffic signal controller to a complex high voltage switchyard, relays can be found everywhere. To put it in general, relays are just like any other switch which can either make or break a connection, that is it can either connect two points or disconnect it, therefore relays are commonly used to turn on or off an electronic load. Again this working principle of relay fits only for the electromechanical relay. Dictionary says that relay means the act of passing something from one thing to another , the same meaning can be applied to this device because the signal received from one side of the device controls the switching operation on the other side.
All the Relays react to voltage or current with the end goal that they open or close the contacts or circuits. Table of Contents. A relay completely isolates the low voltage circuit from the high voltage circuit.
In this article, the different types of relays like overload protection relay, solid state relay, solid state contactor relay, Buchholz relay, Mercury Wetted relay and many more are explained with diagrams. In my earlier post I have alreaady explained the working of relays, and their design and construction.
Different Types of Relays and Their Working Principles
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