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A relay, also known as a control relay, is a type of electrically operated switch used to make or break an electrical connection between two or more circuit points. It can be turned ON or OFF in response to the application of an electric pulse or control signal. A relay is generally used to control the opening and closing of circuit contacts of an electronic circuit, either electromechanically or electronically.
The most basic control of any electrical equipment is the ability to switch it “OFF” and “ON”. This can be done using a standard electrical switch to interrupt the electric power supply. However, conventional electrical switches have to be physically (manually) turned “ON” or “OFF”. Also, they are relatively large, with a slow response rate, and can only switch small electric currents.
In contrast, a control relay can switch a high-voltage electrical circuit “ON” & “OFF” using a low-voltage control signal. For example, a relatively low-amperage relay controlled switch, sensor, or timer can be used to turn ON and OFF a load with a much higher power capacity. In addition, with a control relay, users are not required to manually turn a switch “OFF” and “ON” in order to isolate or change the state of an electronic circuit.
Control relays are normally used in control panels, building, and manufacturing automation to switch smaller current values in a control circuit along with output power control. Usually, relays don’t control power consuming equipment except for small motors and solenoids with low amperage ratings. Nonetheless, a relay can control larger amperes and voltages if it’s supplied with an amplifying effect. Because if a low voltage is applied to the relay coil with an amplifying effect, the relay contacts can then switch to a large voltage. For this reason, relays are often used to control high-voltage electrical circuits using low-voltage signals. Similarly, they are also used to control high-current electrical circuits using low-current signals.
Essentially, relays are used where it’s imperative to control an electronic circuit with an independent low-power signal, or in cases where various electronic circuits need to be controlled by a single control signal. They ensure complete electrical isolation between the low-voltage circuit (controlling circuit) and the high-voltage circuit (controlled circuit). In addition, control relays are also used to switch pilot lights, audible alarms, starting coils, and heating elements. Currently, control relays are extensively used in power electronics of motors, power supply units, power plant control systems, etc.
Moreover, protective relays, such as thermal overload relays, can be used to prevent equipment damage by detecting and isolating electrical circuit abnormalities, including overloads, overcurrent, reverse currents, and undercurrents. Relay circuits also provide time delay functions and can be used to realize critical safety logic functions.
Relays are classified into different categories depending on their structural features and operating principles. Each type of relay is used for a specific application; it’s, therefore, necessary to select the most appropriate relay that meets the requirements of your electronic circuit. Electrically operated control relays (which are the focus of this article) are divided into two major categories:
These are mechanical action relays, which include the following key parts:
In electromechanical relays, the Normally Closed (NC) and Normally Open (NO) contacts are opened or closed mechanically by a magnetic force.
With the Normally Open (NO) contact type, no electric current flows in the secondary circuit (controlled circuit), so the connected load is normally turned OFF. But when electricity is passed through the primary circuit (controlling circuit), an electromagnetic field is induced in the relay coil. The induced electromagnetic field attracts the moving armature and pulls the moveable relay contact till it touches the contact terminals of the secondary circuit. This completes the secondary circuit, thereby allowing the flow of electric current to the connected load.
With the Normally Closed (NC) contact type, there’s current flow in the secondary circuit, so the connected load is normally turned ON. But when an electric current is supplied through the primary circuit, the induced electromagnetic field causes the armature to pull away thereby disconnecting the moveable relay contact. As a result, the secondary circuit is opened, cutting off the supply of electric power to the connected load.
Solid State Relays do not include relay contacts or any moving parts like the electromechanical relays. Instead, they switch electric current using immovable electronic devices such as semiconductor transistors, triacs, thyristors, silicon controlled rectifiers, etc. They make use of the electrical and optical properties of these solid-state electronic devices to carry out input to output isolation and their switching operations.
With Solid-State Relays, an LED light source is used on the primary circuit (controlling circuit), instead of an electromagnet. The LED light provides optical coupling in the SSR by shining a beam of light across an air gap and into the receiver of an adjacent transistor which is photosensitive. The operation of the solid-state relay type is controlled by simply turning the LED light source on and off.
Note: Although solid-state relays are becoming very popular due to their durability and faster-switching process, electromechanical relays remain the most common and widely used type of electrical relays. This is mainly because most of the functions performed by heavy-duty industrial equipment require the switching effectiveness and efficiency of electromechanical relays. However, the decision to use either solid-state relays or electromechanical relays depends on the electrical requirements, life expectancy, and cost constraints of a given application. In this article, we’ll limit our discussion to electromechanical relays.
As previously discussed, electromechanical relays apply the same working principle as a conventional electrical switch, they control one electrical circuit (high-voltage circuit) by closing or opening contacts in another circuit (low-voltage circuit). By doing so, the relay is purported to switch one or more of its poles. Most types of relays are generally made up of one or more independent switch contacts and each “switch contact” is usually referred to as a “pole”. Each one of these poles or contacts can be “thrown” or connected together by energizing the relay coils mainly in three different ways:
In addition to the standard Normally Closed (NC) and Normally Open (NO) descriptions used to describe how control relays are connected in electrical circuits, arrangements of relay contacts can also be classified by their specific actions, as follows:
Relay outputs are often used with Programmable Logic Controllers (PLCs) to control moderate DC and AC loads (of up to approx. 2 Amperes) or when a control circuit with very low resistance is required. Connections of relay contacts in PLC circuits are mainly described as three arrangements or forms: FORM A, FORM B, and FORM C.
Output units of PLC control systems are available with all three relay contact arrangements, but FORM A and FORM C are the most commonly used relay output configurations. By specifying a FORM C relay contact arrangement, one can obtain both FORM A and FORM B configurations using either the Normally Closed(NC) portion of the FORM C contact as a FORM B relay contact or by using the Normally Open(NO) portion of the FORM C contact as a FORM A relay contact.
Additionally, relay outputs in PLCs are also available with a common output terminal and as isolated relay contacts. The figure to the right shows a PLC output unit with three FORM C relay contacts and a common output terminal.
The common terminal of each of the three FORM C relays is connected to a single common output terminal of the PLC output unit designated as OUTPUT COM. Note, each FORM C relay output has two labeled outputs, NO(Normally Open) and NC(Normally Closed). Each NO and NC output terminal has a number indicating the number of the PLC output associated with that particular relay terminal.
When a given PLC output is to be turned OFF, the OUTPUT COM terminal gets connected to the associated relay NC terminal. If the PLC output is to be turned ON, the OUTPUT COM terminal will get connected to the relay NO terminal associated with that output.
Note: Since all three FORM C relay contacts have one common terminal, all the power supply units(can be one or multiple) associated with the PLC outputs to be controlled should have one common connection.
To determine if a relay output is digital or analog, let’s first understand what digital and analog outputs are in the context of PLC systems.
Analog Outputs: These are time-varying or continuous signals from a PLC control system to connected field output devices. PLC controllers output two types of analog signals–Current and Voltage. Hence, a PLC analog output can be classified as either a current or voltage signal. Voltage analog signals from a PLC output module to field output devices are normally in the standard ranges of ±10 V(Volts), ±5 V, 0…5V, 0…10 V, while the current analog signals are in the range of 4…20 mA(milliamperes), or 0…20 mA.
PLC analog outputs are mainly used to control or operate actuators, valves, motors, and other equipment in industrial settings. For instance, you can provide a speed command signal to a Variable Frequency Drive (VFD) using a PLC-system analog output. Also, a 0…5 Volts PLC analog output can be used to control the output power of a 0-2000 kW generator.
Digital Outputs: A digital PLC output provides a binary control scheme (ON or OFF/OPEN or CLOSED) to the field output device connected to the PLC system. The concept of digital output signals is based on the binary number system–where a number is represented using only two possible digits (1 and 0). Thus, for a PLC output to be classified as digital it should actuate or trigger only two possible output states. For example, a digital output signal from a PLC controller can be used to turn a light bulb ON or OFF. Similarly, a PLC-system relay output can be used to OPEN or CLOSE a PLC-controlled solenoid valve.
From the discussion above, we can say a relay output is digital. This is because the switching function of any electrically operated control relay is necessarily digital; it either connects or disconnects an electrical circuit. But the nature of the provided output signal depends on the nature of the input signal, as any type of voltage (DC or AC) can be conducted through the relay, as long as it does not exceed the flash-over voltage of the relay contacts. This means that you can provide any potential (DC or AC) to the Common (COM) terminal of the relay output.
In the diagram above, the relay output module consists of relay contacts and physical coils as with any typical control relay. The relay contacts are connected to the motor’s external power supply to turn the connected motor ON or OFF. These contacts are operated by applying either DC or AC control voltage to the relay coils.
In PLC systems, relay outputs are classified as digital PLC outputs. Because the relay output is either ON or OFF, depending on the relay contacts being Normally Open (NO) or Normally Closed (NC). So once the relay output is energized, it will accordingly allow or block the flow of current (AC or DC) to the device being controlled by closing or opening its contacts, respectively. A NO relay output will allow current flow if it’s energized while an NC relay output will allow current flow if it’s de-energized.
Note: Due to the use of mechanical contacts to switch the relay output, electromechanical relay outputs in PLCs have a higher load-current carrying capacity than other types of digital PLC outputs. However, they have a shorter service life compared to TRIAC and Transistor PLC-system digital outputs. Also, they can easily get damaged if they are constantly switched at a very high frequency.
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