What is a Relay?
A relay is an electrical device that opens or closes contacts to cause the operation of another electric control. Its purpose is to detect intolerable or undesirable conditions in a particular area and provide commands to the circuit breaker to disconnect the affected area, thus protecting the system from damage.
Relays are devices that allow low voltage signals to control higher voltage circuits. They work by using an electromagnetic mechanism to switch a set of contacts between two positions. The relay is activated when a voltage is applied to the coil, which generates a magnetic field that attracts the armature and moves the contacts from their normally open or normally closed positions.
What is the working principle of relay?
The working principle of a relay is based on electromagnetic attraction. When the circuit of the relay senses a fault current, it energizes the electromagnetic field, which produces a temporary magnetic field.
This produced a magnetic field opening or closing the relay contacts by moving the relay armature. Small-power relays have only one contact, while high-power relays have two contacts for opening the switch.
The relay inner section has an iron core, which is wound by a control coil. The power supply is given to the relay control coil through the relay load contacts and the control switch. The current flows through the control coil and produces an electromagnetic field around it.
Due to this magnetic field, the upper arm of the magnet attracts the lower arm, closing the circuit, and making the current flow through the load. If the contact is already closed, it moves oppositely and hence opens the contacts.
The pole and throw are configurations of the relay, where the pole is the switch, and the throw is the number of connections. The single pole, the single throw is the simplest type of relay, which has only one switch and one possible connection. Similarly, the single pole double throw relay has one switch and two possible connections.
Construction of relay:
The construction of a relay can be divided into four main components - contacts, bearings, electromechanical design, terminations and housing.
Contacts:
Contacts are the most crucial part of a relay that performs the switching operation. The selection of the contact material depends on various factors like the nature, magnitude, frequency, and voltage of the current to be interrupted. The contact materials used in relays are typically silver, silver-cadmium oxide, silver-nickel, gold, and tungsten.
Silver is the most common contact material used due to its high conductivity, low contact resistance, and low erosion rate. The shape and size of the contacts also play an important role in the reliability of the relay.
Bearings:
Bearings are used to support the moving parts of a relay, such as an armature. Different types of bearings are used in relays, such as single-ball, multi-ball, pivot-ball, and jewel bearings.
The selection of bearings depends on the application, where high sensitivity and low friction are required, a single ball bearing is used. Multi-ball bearings are used where low friction and greater resistance to shock are required.
Electromechanical Design:
The electromagnetic design of a relay consists of a magnetic circuit and the mechanical attachment of the core, yoke, and armature. The magnetic circuit is designed to minimize the reluctance of the magnetic path, making the circuit more efficient.
The electromagnet of the relay is made up of soft iron, and the control coil current is usually restricted to 5Amp and the control coil voltage to 220V.
Terminations and Housing:
The assembly of the armature with the magnet and the base is made with the help of a spring. The spring is insulated from the armature by molded blocks, which provide dimensional stability.
The fixed contacts of the relay are usually spot welded on the terminal link of the relay. The housing of the relay is made up of materials like Bakelite, ceramic, or plastic. The housing protects the relay from the environment and provides electrical insulation.
Types of Relays:
- Electromagnetic Relay: Uses an electromagnetic mechanism to switch a set of contacts.
- Latching Relay: Uses a mechanism that holds the switch or relay in its last state until it is changed by a subsequent operation.
- Electronic Relay: Uses electronic components to switch or amplify signals.
- Non-Latching Relay: This does not hold a switch or relay in its last state after the controlling signal is removed.
- Reed Relay: Uses a set of flexible metal reeds to control the circuit.
- High-Voltage Relay: Designed to operate at voltages above a certain threshold, typically 1000 volts or more.
- Small Signal Relay: Designed for use in electronic circuits that handle weak electrical signals.
- Time Delay Relay: Includes a delay mechanism that causes the relay to activate after a certain period of time has passed.
- Multi-Dimensional Relay: Can switch multiple circuits or signals simultaneously.
- Thermal Relay: Uses a bimetallic strip or other thermal mechanism to detect overcurrent or overheating conditions.
- Differential Relay: Compares the input and output of a circuit to detect faults or imbalances.
- Distance Relay: Measures the distance to a fault in a power transmission line and triggers a trip signal if the fault is too far away.
- Automotive Relay: Designed for use in automotive applications, typically to control lights, fans, or other electrical systems.
- Frequency Relay: Measures the frequency of a signal and triggers a trip signal if it is outside a certain range.
- Polarized Relay: Uses a polarized magnetic field to ensure that the relay switches in a specific direction.
- Rotary Relay: Uses a rotating mechanism to switch between multiple circuits or positions.
- Sequence Relay: Switches between multiple circuits in a predetermined sequence.
- Moving Coil Relay: Uses a moving coil to switch between multiple circuits or positions.
- Buchholz Relay: A type of protective relay used in oil-filled transformers that detects gas or oil faults.
- Safety Relay: Designed to ensure safe operation of machinery or equipment.
- Supervision Relay: Monitors the operation of a circuit or system and triggers an alarm or trip signal if a fault is detected.
- Ground Fault Relay: Detects ground faults in electrical systems and triggers a trip signal to disconnect the affected circuit.