August 29, 2019
Contactors and relays are often thought of interchangeably. However, if you should ever specify a relay in a situation calling for a contactor, you’ll be very sorry. While both are electrically operated switches employed for control and switching of loads, contactors are better suited to handle 10A or above, while relays are happiest operating below 10 amps.
Contactor vs. Relay – the Differences
The Institute of Electrical and Electronics Engineers (IEEE) defines a relay as: “A device by means of which contacts in one circuit are operated by a change in conditions in the same circuit, or in one or more associated circuits.”
Meanwhile, the Institute defines a contactor as: “A device for repeatedly establishing and interrupting an electric circuit under normal conditions.”
Drilling down into it a bit farther, Electrical Classroom notes the following contrasts between the two:
|Relatively smaller in size||Larger when 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 call for 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’s also important to consider the function you’re asking the component to serve 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 fitted with auxiliary contacts to give them the capability of operating in a normally closed condition. In this configuration, switching can occur regardless of whether the contactor’s coils are energized or de-energized.
The Bottom Line…
As we mentioned above, the primary situation in which you’ll want to use a contactor is in any high-powered circuit in which an overload condition is likely to occur. This is particularly true if subsequently leaving the circuit in an energized state will create a hazardous environment for people and/or equipment.
On the other hand, in situations in which you only need the capability of switching low power, a contactor is likely to be overkill. 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.