Contactors and relays are usually considered interchangeable. However, if you specify a relay in a situation where a contactor is required, you will see the drawbacks. While both are electric switches used to control and switch loads, contactors are better suited to handle currents of 10A or higher, while relays are best suited to operate at less than 10 amps.
Contactor vs. Relay – the Differences
The Institute of Electrical and Electronics Engineers (IEEE) defines a relay as “a device by which contacts in one circuit are operated by changes in conditions in the same circuit or in one or more related circuits.”
At the same time, the Institute defines a contactor as “a device that repeatedly turns on and interrupts a circuit under normal conditions.”
Delving further, Electrical Classroom noted the following contrasts between the two：
|Relatively smaller in size||Larger compared to relays|
|Used in circuits with lower ampacity||Used in circuits with low and higher ampacity|
|Mainly used for control and automation circuits, protection circuits and switching small electronic circuits.||Used in the switching of motors, capacitors, lights, etc.|
|Consists of at least two NO/NC contacts||Consists of a minimum of one set of three phase power contacts. Auxiliary contacts are also provided in some cases|
Attributes of Contactors
These devices are designed to operate in a normally open condition, which means there’s no connection when it’s de-energized. Better contactors, like those supplied by Schneider Electric use spring-loaded contacts to ensure the circuit is broken when it’s de-energized.
Contactors also provide arc suppression and they incorporate overload protocols to interrupt the circuit if a current exceeds a pre-selected threshold for a prescribed time period. Chosen based upon the amp ratings of the load they’re deployed to manage; contactors also require an additional power supply (either AC or DC depending upon the type) for excitation.
Three-phase applications require contactors, while relays should be reserved for single-phase. This is because a relay only has a common contact, connecting to a neutral position. Conversely, a contactor joins two poles without a common circuit between them. While relays are fine in situations up to 250 volts AC, contactors are good for situations up to 1000 volts.
It is also important to consider the function you require the component to provide in your system.Contactors excel in situations in which an overload could happen and failure to de-energize the circuit will put people and/or the system itself in danger. Relays are incapable of providing this protection. However, for low-power applications where a contactor added margin of safety isn’t warranted, specifying relays will reduce costs.
Thus, contactors are typically used to switch high-current motors, capacitors and slighting systems. Contactors can also be equipped with auxiliary contacts that allow them to operate in normally closed conditions. In this configuration, switching can occur regardless of whether the contactor’s coils are energized or de-energized.
As we mentioned above, the main situation in which you want to use a contactor is in any high-power circuit where an overload situation may occur. This is especially true if subsequently leaving the circuit energized would create a hazardous environment for personnel and/or equipment.
On the other hand, in situations where you only need to switch low power capabilities, choosing a contactor may not be a wise choice. In those instances, going with a relay is going to be just fine. Ultimately though, you’ll have to make the call based upon the circumstances, the nature of the installation, and of course, the budget.