Despite the two terms often being used interchangeably, relays and contactors are different devices. A relay is a device used to control the contacts of an electrical circuit due to a change of parameters or conditions in the same circuit or another associated circuit. Contactors, adversely, are devices used to interrupt or establish connections in an electrical circuit repeatedly under different conditions. As both devices perform the task of switching a circuit, their definitions alone are not really enough to differentiate the two. In this blog, we will take a deeper look at the differences between relays and contactors.
The first difference between contactors and relays is in their load capacities. Relays are generally capable of carrying loads of 10A or less, while a contactor would be used for loads in excess of 10A. The second difference is in the device’s contact standards. Contactors are designed to operate almost exclusively with normally open contacts. This differs from relays, which can be normally open and/or normally closed, depending on the desired function. This means when a contactor is de-energized, there is no connection, while a relay could still be connected while de-energized.
Third, contactors are often fitted with auxiliary contacts which can be normally open or normally closed. These contactors are used to perform additional functions related to the control of the contactor. For example, the contactor could transmit power to the motor while the auxiliary contact is used to turn on a light indicating the motor is operating properly. Another difference is found in the safety features of the two devices. Because contactors frequently carry higher loads, they often consist of added safety features such as spring-loaded contacts to ensure the circuit is broken when de-energized. This is critical because, in high load situations, contacts can weld themselves together, creating the dangerous situation of a circuit being energized while it is supposed to be off. Spring-loaded contacts reduce the chances of this while ensuring all circuits are broken simultaneously. Spring-loaded contacts are uncommon in relays, as they are typically used for lower power applications.
The final two differences between contactors and relays are the added safety features for arc suppression and overload protection. Magnetic arc suppression works by extending the path an arc would have to travel. If the distance is too long for the energy to overcome, the arc is suppressed. Because relays are not designed for high loads, arcing is less of a concern and therefore arc suppression is not generally needed in relays. Finally, contactors are commonly connected to overloads that will interrupt the circuit when it exceeds a set threshold for a predetermined amount of time (usually 10-30 seconds). This is to help protect the equipment beyond the contactor from current damage. Once again, overloads are uncommon in relays.
As contactors and relays are different devices, they are used in different applications. Contactors are designed for and used in three-phase applications, while a relay is more suited to single phase applications. Additionally, a contactor joins two poles together, without a common circuit between them, while a relay has a common contact that connects to a neutral position. Lastly, contactors are rated for up to 1000 volts, while relays are typically only rated to 250 volts. Whether you need a contactor or relay, ensure you are getting it from a trusted source.
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