Many beginners in electrical work are familiar with contactors, understanding their uses and performance. However, they often feel confused when intermediate relays are mentioned, unsure of their purpose, especially since some intermediate relays look very similar to contactors. (See image below.)
Intermediate relays and contactors
In fact, intermediate relays and contactors are basically the same in structure and principle. Their main difference lies in their contact capacity and contactor operation. Contactors have main contacts that can handle larger currents, enabling them to control the main circuit current of loads such as motors. Intermediate relays, on the other hand, generally have smaller contact capacity; in other words, they lack main contacts and consist entirely of auxiliary contacts, resulting in a larger number of contacts. Therefore, intermediate relays are typically used in control circuits.
What is the function of an intermediate relay?
Intermediate relays are used in relay protection and automatic control systems to increase the number and capacity of contacts, and to transmit intermediate signals in the control circuit 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).
For example, in a circuit, a contactor may only have two sets of normally open auxiliary contacts, but the circuit requires all three sets of these contacts, which is insufficient. What can be done? An intermediate relay with multiple normally open contacts can be added. One set of normally open contacts from the contactor controls the intermediate relay coil. When the contactor is energized and engages, the normally open contacts close, and the intermediate relay also engages. Then, the normally open contacts of the intermediate relay act as auxiliary contacts for the contactor. This increases the number of contacts.
For example, the output current of a transistor-type PLC is generally very small, around 0.3 amps. If this current is used to drive a relatively large contactor, the capacity is insufficient. In this case, a small intermediate relay can be added, and the 0.3 amp current can be used to drive the intermediate relay coil. The intermediate relay contact capacity is generally 5 amps, and the intermediate relay contact can then be used to drive the contactor coil. This conversion increases the capacity.
Intermediate relays can also be used for voltage conversion. For example, what if a PLC outputs 24V DC, but the load it needs to drive requires 220V AC, such as a contactor? An intermediate relay with a 24V DC coil voltage can be added. The PLC output controls the intermediate relay, and the relay contacts are connected to 220V AC to control the load.
