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. According to the latest updates from the basic relay research and development department, relays can respond to various inputs such as current or voltage, or non-electrical quantities like temperature, time, speed, and pressure. The output of a relay typically involves contact action or changes in circuit parameters.

When the relay receives an input—such as voltage, current, frequency, or other non-electrical factors like temperature, pressure, or speed—and this value reaches a set threshold, the relay’s contacts either close or open the controlled or protected circuit.

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

Most commonly used relays have contacts that switch between two states—on and off. These state changes are triggered by internal mechanisms designed to respond to specific conditions.

A voltage relay is a type of relay that responds to the coil's operating voltage and the contact action. It is primarily used for voltage protection and control in electric drive systems. In practice, the voltage relay coil is connected in parallel with the load.

Features: The coil has a large number of turns and thin wire, which allows it to respond effectively to small voltage changes.

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

The intermediate relay serves to increase the number of controlled circuits or enhance the contact disconnection capacity, allowing for better signal distribution and power amplification. It is often used in complex control systems where multiple signals need to be processed simultaneously.

There are two main types of intermediate relays: AC and DC. They are characterized by having a large number of contacts—typically 3 to 4 pairs or more—and often use bridge contacts, similar to those found in contactors.

Relays with higher operating power may have a structure similar to small contactors. They are suitable for AC control circuits below 500V, with coil voltages available in five standard options: 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 can operate at voltages up to 110V, with coil voltages ranging from 12V to 110V. They consume no more than 3W and typically include three normally open and three normally closed contacts, with a rated current of 3A.

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

Features: The coil has fewer turns and thicker wire, making it suitable for handling higher currents without overheating.

Classification: Current relays can be divided into AC and DC types based on the coil current. They can also be categorized as over-current or under-current relays depending on the pull-in current threshold.

The defining feature of a time relay is its delayed response. When a control signal is received (such as the coil being energized or de-energized), the contacts do not change state immediately. Instead, there is a pre-set delay before the contacts close or open. This makes them ideal for timing and sequence control applications.

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 activates the mechanism. It is primarily used for overload, phase failure, and unbalanced current protection in AC motors, as well as for monitoring 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. If the motor starts infrequently and the starting current is six times the rated current with a start time less than six seconds, the thermal relay can be selected based on the motor’s rated current.
3. In cases of repeated short-time operation, the operating frequency of the thermal relay must be considered. For high-frequency operations, thermal relays may not be the best choice for overload protection.