Types of Relays
Purpose and Characteristics:
Relays are critical components in automation and electrical control systems. These devices, operating based on switching electrical circuits, have become an integral part of modern equipment protection and control systems.
With their vast variety and extensive range of applications, understanding the specific characteristics of each type of relay is essential.
The history of relay development spans over 150 years, from the first electromechanical devices to today's solid-state solutions. Over time, relay technology has evolved, resulting in the creation of numerous specialized types of devices.
Today, there are many types of relays, differing in construction, purpose, characteristics, and operating principles:
- Electromagnetic
- Reed
- Thermal
- Intermediate
- Time
- Solid-state
Additionally, there are specialized relay types such as:
- Frequency Relays: Used to monitor the frequency of AC voltage in electrical networks, ensuring equipment protection against unacceptable frequency deviations.
- Phase Relays: Ensure correct phase sequence, the presence of all phases, and voltage symmetry in three-phase networks.
- Voltage Relays: Protect equipment from voltage surges and drops in the network.
Main Characteristics of Different Relay Types
- Electromechanical Relays
Electromechanical relays are the most common type of switching devices. They work based on the interaction of a magnetic field generated by a coil and a movable armature that mechanically opens or closes contacts. The main advantages of these relays are simple construction, reliability, and the ability to handle large currents.
The switching time for electromechanical relays typically ranges from 5 to 15 milliseconds, sufficient for most applications. They are designed to function in a wide temperature range, from -40°C to +85°C, and can last for millions of switching cycles with proper use.
Special attention should be given to the contact materials in electromechanical relays. Silver contacts are the most commonly used due to their optimal price-quality ratio, while palladium or gold contacts are used for critical applications, extending the relay’s service life.
- Solid-State Relays
Solid-state relays represent the next stage in relay technology evolution. Unlike electromechanical relays, they have no moving parts and operate based on semiconductor elements, which provides them with virtually unlimited service life and high switching speeds.
The switching time of solid-state relays is less than 1 millisecond, making them indispensable in high-speed control systems. Additionally, they operate silently and do not generate electromagnetic interference, typical of mechanical contacts.
However, solid-state relays have specific operational requirements. They need heat dissipation when switching large currents, as they experience a voltage drop across their power elements. Solid-state relays also tend to be more expensive than their electromechanical counterparts.
- Intermediate Relays
Intermediate relays are used to amplify control signals and provide galvanic isolation between control and actuator circuits. They are optimized for frequent switching and long-term operation.
Typically, an intermediate relay includes several switching contact groups and can be controlled by a small current, making it an ideal interface between microprocessor devices and power equipment. The switching time for these relays usually ranges from 10 to 20 milliseconds.
Modern intermediate relays often come with additional features such as status indication, reverse polarity protection, and built-in noise suppression circuits, making them easier to integrate into complex control systems.
- Time Relays
Time relays provide a time delay before switching electrical circuits. They can operate to either close or open contacts with adjustable delays ranging from fractions of a second to several hours.
Modern time relays are often based on microprocessors, which ensure high accuracy in time delay settings and offer broad functional capabilities. They can work in various modes, including cyclical on/off switching, delay before activation or deactivation, and pulse generation with defined duration.
A key feature of time relays is their stability, with time settings remaining accurate even when temperature and supply voltage fluctuate. The best models maintain a time accuracy of 0.1% across all operational conditions.
- Thermal Relays
Thermal relays are an essential component in protecting electric motors from overloads. They are also used in circuit breakers and differential protection devices.
Their operating principle is based on the bending of a bimetallic plate when it heats up due to the current passing through it. When the temperature reaches a certain threshold, the contacts open, disconnecting the load.
Thermal relays are selected based on the thermal characteristics of the equipment being protected. An important parameter is the tripping class, which defines the response time at various overload multiples.
Modern thermal relays are often supplemented with electronic components to enable more sophisticated protection algorithms and motor diagnostics while maintaining the high reliability of electromechanical devices.
- Reed Relays
Reed relays are a special type of switching device, where the contacts are housed in a hermetically sealed glass capsule filled with an inert gas. This design ensures high reliability and protects the contacts from external influences.
The switching time for reed relays is typically between 0.5 and 2 milliseconds, making them ideal for high-frequency applications. They also offer exceptional durability, with tens of millions of switching cycles.
Reed relays have a relatively low switching power, usually not exceeding 100 watts. However, in low-power switching applications, they remain indispensable due to their high reliability and stability.
- Polarized Relays
Polarized relays include a permanent magnet in their design, which enables them to operate based on the polarity of the control signal. This allows for more complex control and protection algorithms.
The main features of polarized relays include high sensitivity and low energy consumption in the control circuit. They are capable of operating from very low-power sources, making them ideal for use in measurement devices.
The switching time of polarized relays typically ranges from 2 to 5 milliseconds, offering high stability and repeatability of switching parameters.
Conclusion
When selecting a relay for a particular application, several factors must be considered:
- Operating conditions
- Required switching parameters
- Speed
- Reliability
- Economic feasibility
The modern market offers a wide selection of devices that can meet virtually any technical requirement. The correct selection of relay type and characteristics ensures the creation of reliable and efficient control and protection systems. However, it is crucial to remember that each relay type has its unique features and limitations that should be accounted for during design.
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