Characteristics, classification and selection of various types of relays

A relay is an automatic control device that switches a low-current control circuit based on an input signal, enabling remote control and protection. Currently, the basic relay research and development department has announced that relays can respond to various inputs such as current, voltage, or non-electrical quantities like temperature, time, speed, and pressure. The output of the relay typically involves contact action or changes in circuit parameters.

When the relay's input—whether voltage, current, frequency, or other non-electrical factors like temperature, pressure, or speed—reaches a specified threshold, its contacts either close or open the controlled or protected circuit.

In general, a relay consists of two main parts: an input sensing mechanism and an output actuator. The input mechanism detects the level of the incoming signal, while the output mechanism controls the circuit by turning it on or off.

Most commonly used relays have contacts that operate in two states—on or off. These states are switched through a specific mechanical mechanism.

Relays that respond to contact action and coil operating voltage are known as voltage relays. They are primarily used for voltage protection and control in electric drive systems. In practice, the coil of a voltage relay is connected in parallel with the load.

Features: The coil of a voltage relay usually has many turns and thin wire.

Classification: Based on the type of coil current, voltage relays can be divided into AC and DC types. Depending on the pull-in voltage, they can also be categorized as over-voltage, under-voltage, or zero-voltage relays.

The intermediate relay is designed to increase the number of controlled lines or enhance the contact disconnection capacity, allowing for signal distribution and power amplification in control systems.

Intermediate relays come in both AC and DC versions. They are characterized by having multiple contacts—typically three to four pairs or more—and often use bridge-type contacts similar to those found in contactor auxiliary contacts.

Relays with higher operating power resemble small contactors in structure.

They are suitable for AC control circuits below 500V. Common coil voltages include AC 12V, 36V, 127V, 220V, and 380V. Some models offer eight contact pairs with a rated current of 5A and a maximum operating frequency of 1200 cycles per minute.

For DC applications, intermediate relays are suitable for circuits below 110V. Their coil voltages can be DC 12V, 24V, 48V, or 110V, with a power consumption of no more than 3W. These relays typically have three normally open and three normally closed contacts, with a rated current of 3A per contact.

A current relay is a type of relay that responds to the magnitude of the coil current. When in use, the coil of a current relay is connected in series with the load.

Features: The coil of a current relay has fewer turns and thicker wire.

Classification: Based on the type of coil current, current relays can be AC or DC. Depending on the pull-in current, they can also be classified as over-current or under-current relays.

The key feature of a time relay is that when a control signal is received (such as the coil being energized or de-energized), the contact does not switch immediately. Instead, there is a delay before the contact opens or closes. This type of relay is also known as a time-delay relay.

A thermal relay is a protective device that uses heat generated by current flowing through a heating element to bend a bimetallic strip, which then triggers the actuator. It is mainly used for overload protection, phase failure, and current imbalance in AC motors, as well as for controlling the heating status of other electrical equipment.

Selection of Thermal Relays:

(1) For motors with poor overload capability, the rated current of the thermal component (IRT) should be between 60% and 80% of the motor’s rated current (IN).

(2) In cases of infrequent starting, if the motor’s starting current is six times its rated current and the start time is less than six seconds, the thermal relay can be selected based on the motor’s rated current.

(3) When the motor operates in repeated short-time cycles, the operating frequency of the thermal relay must be carefully considered. For high-frequency operations, thermal relays may not be suitable for overload protection.