A relay is an automatic control electrical device that connects or disconnects a small-current control circuit based on a certain input signal, enabling remote control and protection. Its input can be electrical quantities such as current and voltage, or non-electrical quantities such as temperature, time, speed, and pressure, while its output is the action of contacts or the change of circuit parameters.
A relay is an electrical control device that, when the input quantity changes to a specified degree, causes a predetermined step change in the controlled quantity in the electrical output circuit. Relays have an interactive relationship between the control system (also known as the input circuit) and the controlled system (also known as the output circuit). They are commonly used in automated control circuits, using a small current to control a large current, and serve functions such as automatic adjustment, safety protection, and circuit switching.
Definition and basic principle of relay
A relay generates physical quantities (such as displacement, pressure, and heat) through electromagnetic or electrothermal effects, thereby enabling the controlled system to achieve a predetermined target. It mainly consists of an electromagnetic system, a contact system, and a release spring. The electromagnetic system includes an iron core, a coil, and a yoke; the contact system includes moving contacts and stationary contacts. When the input quantity changes, the magnetic field generated by the electromagnetic system causes the contacts to close or open, thus controlling the circuit.
When the relay receives electrical quantities such as voltage, current, and frequency, or non-electrical quantities such as temperature, pressure, and speed, reaching specified values, the relay contacts will connect or disconnect the circuit being controlled or protected.
A relay generally consists of two parts: an input sensing mechanism and an output actuator. The former is used to reflect the level of the input quantity, and the latter is used to connect or disconnect the circuit.
Commonly used relays have contacts, which have two states: on and off. The change of state is driven by a certain mechanism.
1. Voltage relay
A relay whose contact operation is related to the magnitude of the coil's operating voltage is called a voltage relay. It is used for voltage protection and control in electric drive systems. When in use, the coil of the voltage relay is connected in parallel with the load.
Features: It has a large number of coil turns and a thin wire diameter.
Classification: According to the type of coil current, relays can be divided into AC and DC voltage relays. According to the magnitude of the pull-in voltage, relays can be divided into overvoltage, undervoltage, and zero voltage relays.
2. Intermediate circuit breaker
The function of an intermediate relay is to increase the number of controlled circuits or increase the allowable breaking capacity of the contacts in order to achieve the purpose of distributing control signals and amplifying power.
Intermediate relays come in both AC and DC versions. A key characteristic of intermediate relays is the large number of contacts, typically 3-4 pairs or more, often using a bridge-shaped contact configuration (similar to contactor auxiliary contacts).
Intermediate relays with higher operating power have the same structure as small contactors.
Suitable for control circuits below AC 500V, with coil voltages of 12V, 36V, 127V, 220V, and 380V AC. The relay has eight pairs of contacts, a rated current of 5A, and a maximum operating frequency of 1200 times/hour.
Suitable for control circuits with DC voltage below 110V, the coil has four rated voltages: DC 12V, 24V, 48V, and 110V, the coil power consumption is no more than 3W, and the relay has three normally open and three normally closed contacts with a common contact, and the rated contact current is 3A.
3. Current relay
A relay whose contact operation is related to the magnitude of the coil's operating current is called a current relay. When in use, the coil of a current relay is connected in series with the load.
Features: Its coil has few turns and is thick.
Classification: According to the type of coil current, relays can be divided into AC and DC current relays; according to the magnitude of the pull-in current, they can be divided into overcurrent and undercurrent relays.
4. Time relay
The characteristic of a time relay is that when a control signal is received (such as when the relay coil is turned on or off), the state of its contacts does not change immediately. Instead, the contacts close or open only after a certain period of time. Therefore, this type of relay is also called a time delay relay.
5. Thermal relay
A thermal relay is a protective electrical device that uses the heat generated when current flows through a heating element to bend a bimetallic strip, thereby actuating an actuator. It is mainly used for overload protection of AC motors, protection against phase loss and current imbalance, and control of heating conditions in other electrical equipment.
Selection of thermal relays:
(1) For motors with poor overload capacity, the rated current IRT of the thermal element is 60% to 80% of the rated current IN of the motor.
(2) In infrequent starting situations, if the starting current of the motor is 6 times its rated current and the starting time does not exceed 6 seconds, the thermal relay can be selected according to the rated current of the motor.
(3) When the motor is in a repetitive and short-time duty system, it is important to determine the operating frequency of the thermal relay. For motors with high operating frequencies, thermal relays should not be used as overload protection.
In modern electrical engineering and automation control, relays are crucial electrical components. They control the switching of one or more main circuits via a single control signal and are widely used in various automatic control and protection systems. Based on different working principles and application scenarios, relays can be categorized into several types. This article will analyze five common relay types and their application scenarios.
Electromagnetic relay
Electromagnetic relays are the most widely used type of relay. They primarily achieve control through the principle of electromagnets. When a control signal is input, the electromagnet generates magnetic force, attracting the armature and causing the contacts to close or open, thereby controlling the on/off state of the main circuit. Electromagnetic relays have advantages such as simple structure, reliable operation, and long lifespan, and are widely used in automatic control, communication, automotive, and other fields.
Temperature relay
A temperature relay is a type of relay that uses a thermistor to detect and control temperature. It typically consists of a thermistor and a relay. When the temperature reaches a set value, the resistance of the thermistor changes, triggering the relay to operate and control the circuit. Temperature relays are widely used for temperature control and protection in industries such as home appliances, motors, and automobiles.
Pressure relay
A pressure relay is a type of relay that uses a pressure-sensitive element to detect and control pressure. It mainly consists of a pressure-sensitive element, a relay, and a control system. When the pressure reaches a set value, the pressure-sensitive element generates a signal, triggering the relay to operate and control the circuit. Pressure relays are widely used for pressure protection and safety control in hydraulic and pneumatic control systems.
Liquid level relay
A liquid level relay is a type of relay that uses a liquid level sensing element to detect and control the liquid level. It mainly consists of a liquid level sensing element, a relay, and a control system. When the liquid level reaches a set value, the liquid level sensing element generates a signal, triggering the relay to operate and control the circuit. Liquid level relays are widely used for liquid level control and safety protection in industries such as chemical, water conservancy, and food processing.
Time relay
A time relay is a type of relay that uses a single component to control the on/off state of a circuit. It mainly consists of a single component, a relay, and a control system. By setting the timing parameters of the single component, timed control of the circuit can be achieved. Time relays are widely used in automated production lines, lighting control, timed switches, and other applications.
Summarize
There are many types of relays, and choosing the right one requires considering the specific requirements of the circuit, such as load type, voltage, current, and response time. Selecting the appropriate relay type based on the actual application scenario and needs can improve the reliability and safety of the system.
