In the design of electrical control systems, relay selection is far from a simple parameter comparison. It is a comprehensive decision that balances system performance, safety, and cost-effectiveness.
Relays are a core control and protection component in electrical systems. The correct selection of relays directly impacts the operational reliability of the entire system. It also impacts its response speed and service life. Whether for power system protection or industrial automation control, choosing the appropriate relay requires systematic consideration and professional methodology.
A relay is essentially an electrically controlled switch that uses a low-current control signal to operate a high-current working circuit. The fundamental principle is that when the coil is energised, it generates a magnetic field which magnetises the core and attracts the armature, causing it to move and changing the contact state. Depending on the design, the contacts can open, close or switch circuits.
Electromagnetic Relay: It operates using electromagnetic effects and features a simple structure and strong anti-interference capability. Includes clapper-type and balanced rotary armature structures, with the latter offering better vibration resistance.
Solid State Relay (SSR): Uses semiconductor devices to achieve contactless switching. The advantages of this type of memory are that it has a long lifespan, it is very fast and it does not bounce when it is switched on.Disadvantages include turn-off leakage current. There is also a need for heat dissipation measures. The cost is higher. There is an output voltage drop.
Digital/Microprocessor Relay: It integrates a microprocessor. This offers powerful, programmable functionality. It is often used in complex power system protection.
Time Relay: Provides precise delay control. Selection should be coordinated based on system requirements (e.g., accuracy, delay range, operating power supply).
Auxiliary/Intermediate Relay: Increases contact number and capacity, transmits intermediate signals in control circuits.
Protection Relay: It is specifically designed for power system protection and includes overcurrent relays, undervoltage relays and distance protection relays.
Coil Rated Voltage and Resistance: Coil voltage must be compatible with the control circuit. Most relay coils require more current than the drive capability of microcontroller I/O ports, so you must drive them via transistors or dedicated driver chips.
Pull-in Voltage: The minimum coil voltage required for reliable relay operation.
Release/Drop-out Voltage: This is the maximum coil voltage. The relay reliably releases at this voltage.
Rated Power: Power consumed by the coil during normal operation.
Note: The relay coil is an inductive load. The generation of a back electromotive force (back-EMF) is the result of switching it off. A flyback/freewheeling diode must be connected in parallel across the coil to protect the driving circuit.
Contact Rating (Load Capacity): Refers to the voltage and current values the contacts can safely switch. This must be determined based on the voltage and current of the controlled circuit. The rated values must never be exceeded.
Impact of Load Type:
Resistive Loads (e.g., incandescent lamps, heaters): Relatively easy to switch.
Inductive Loads (e.g., motors, transformers, solenoids): Generate voltage spikes that are based on induction, during the switching process.This puts more pressure on the contacts. When dealing with large inductive loads, such as air conditioner compressors, it is important to use relays that are specifically designed for these applications. Additionally, ensure that the rated current has sufficient margin.
Capacitive Loads (e.g., switched-mode power supplies): May generate high inrush current upon turn-on.
DC vs. AC Load Differences: DC loads are more demanding on contacts than AC. This is because AC arcs naturally extinguish at each zero crossing, whereas DC arcs can be sustained.
Climatic Stress Factors: Include temperature, humidity, altitude, etc. High temperatures accelerate aging of internal relay materials, while low temperatures can increase contact "cold welding" effects. Under low atmospheric pressure, relay heat dissipation worsens, leading to higher coil temperatures.
Mechanical Stress Factors: Mainly vibration, shock, impact. For vibrations caused by circuit breaker operation in high-voltage switchgear, small auxiliary relays with balanced armature mechanisms should be selected.
Pollution Degree: In dusty or corrosive environments, fully sealed relays should be used.
Incoming Feeder Protection: Should include at least: Instantaneous Overcurrent (ANSI 50), Time Overcurrent (ANSI 51), and Time Earth Fault (ANSI 51N). For feeders rated equal to or above 630A, instantaneous and time overcurrent protection is required.
Transformer Protection: Buchholz relay and oil temperature trip commands for transformers should be applied to the tripping mechanism of the incoming circuit breaker.
Short-circuit protection: The provision is made either by fuses or the magnetic trip of a circuit breaker.
Overload protection: Provided by thermal overload relays or Motor Protection Circuit Breakers (MPCBs).
Phase-loss protection: Use thermal relays with phase-loss protection. The current setting should be adjusted according to the motor's starting conditions.
Determine the required number and configuration of contacts (NO and NC combinations).
Distinguish between resistive and inductive loads and clarify the load voltage and current.
Consider the mounting method (e.g. DIN rail or PCB solder).
In harsh environments, prefer fully sealed types.
Situations requiring high-frequency switching.
Hazardous environments with explosion-proof requirements.
Situations requiring silent operation.
Contacts in Parallel: Does not increase load current rating but can reduce the probability of "fail-to-open" failures. Used in critical locations, e.g., for three-pole electrical interlocking in high-voltage circuit breakers.
Contacts in Series: Can increase the load voltage withstand capability. The increase factor is the number of contact sets in series. This improves reliability against "welded/fail-to-open" failures.
Contact Connection Polarity: Where possible, it is preferable to use Normally Open (NO) contacts rather than Normally Closed (NC) contacts in power automation control, as NO contacts tend to have fewer contact bounces during operation.
Coil Parallel Connection Issue: In DC control circuits, connecting different types of relay coils in parallel may cause delayed release, contact malfunctions, etc. It is recommended to connect an individual auxiliary NC contact in series with each relay's coil circuit.
Operating Voltage Range: The applied operating voltage should not exceed the relay coil's maximum operating voltage, nor should it fall below 90% of the rated voltage, as this can jeopardize coil life and operational reliability.
Sealed vs. Unsealed Relays: Unsealed relays offer visible operation and easier failure analysis but have poor long-term tolerance to climatic conditions. Fully sealed relays offer excellent performance in harsh environments and stable, reliable contact performance, making them the preferred choice for power automation control systems.
Rated Thermal Current vs. Rated Operational Current: Some product datasheets or nameplates may state the relay's rated thermal current rather than its rated operational current, so it is important to check which current is specified. During selection, it is crucial to make this distinction, as failure to do so can have a negative impact on relay lifespan or even lead to contact burnout.
Analyze System Requirements: Determine load characteristics (type, operating voltage, current, and peak current) and protection requirements.
Determine Relay Type: Select the relay type and series. This should be based on the application scenario. The scenarios are control, protection and signal conversion.
Select Contact Parameters: Choose contact ratings with sufficient margin based on load current and voltage, especially increasing margin substantially for inductive loads.
Determine Coil Parameters: The coil's voltage and resistance must be selected based on the voltage of the control circuit and its drive capability.
Evaluate Environmental Suitability: Choose the appropriate protection type and structural features based on the ambient temperature, humidity, altitude, vibration conditions and degree of pollution.
Confirm Compliance and Standards: Ensure the selected relay complies with relevant industry standards. Ensure the relay complies with relevant safety regulations.
Verification and Testing: Verify relay performance under actual or simulated conditions to ensure all application needs are met.
Relay selection is a systems engineering task requiring comprehensive consideration of technical performance, environmental suitability, and economics. Selecting the correct product ensures the electrical system operates stably, significantly enhancing its safety and service life. With technological advancements, relay products continue to evolve. Staying informed about new technologies and products, and regularly reviewing and optimizing selection criteria, is an ongoing task for every electrical engineer.
By following this comprehensive guide, you will be able to select the most suitable relay for various electrical systems, ensuring safe, reliable, and economical operation, whether in simple control loops or complex power protection systems.
