All About Contactors

Written by
The MurCal Blogster
Published on
October 21, 2022 12:42:31 PM PDT October 21, 2022 12:42:31 PM PDTst, October 21, 2022 12:42:31 PM PDT


High-voltage DC contactors were developed by the US military for its related military equipment. Eventually, the civilian market demanded high-voltage and high-current switching of contacts, especially for high-voltage DC relays used in new energy vehicles. Research and development on relays have since been undertaken by major relay manufacturers.

Contactors v. Relay: Which is Right for Me? 

It is not uncommon in the industry for the terms to be used nearly interchangeably because the differences are not always clear. We thought we'd address that.

According to the Institute of Electrical and Electronics Engineers:

Relay – “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”

Contactor – “A device for repeatedly establishing and interrupting an electric circuit under normal conditions”

1. Load Capacity

Relays are generally classified as carrying loads of 10A or less, while a contactor would be used for loads greater than 10A.

2. Open/Closed Contact Standards

Almost all contacts are designed to operate with normally open contacts. In contrast, relays can and often are both normally open and normally closed, depending on their function. Therefore, a contactor has no connection when de-energized. 

3. Auxiliary Contacts

Contactors often come with auxiliary contacts, which can either be NO or NC. These contacts perform additional functions related to the contactor's control. A contactor transmits power to the motor, while an auxiliary contact is in the control circuit of the starter and turns on a pilot light to indicate that the motor is running.

4. Safety Features (Spring-Loaded Contacts)

Typically, contactors carry high loads, so they have additional safety features like spring-loaded contacts to break the circuit when de-energized. Because contacts can weld together in high load situations, this is important. Since relays are typically for lower power, spring-loaded contacts are much less common.

5. Safety Features (Arc Suppression)

ARC suppression is another safety feature commonly included in contactors due to their high loads. Magnetic arc suppression works by extending the path an arc would have to travel. If this distance is extended further than the energy can overcome, the arc is suppressed. Since relays aren’t designed for high loads, arcing is less of a concern and arc suppression is much less common on relays.

6. Safety Features (Overloads)

Lastly, contactors are commonly connected to overloads that will interrupt the circuit if the current exceeds a set threshold for a selected time period, usually 10-30seconds. This is to help protect the equipment downstream of the contactor from damage due to current. Overloads are much less common on relays.

Contactor vs Relay Applications

Contactors are built for and used in 3-phase applications where a relay is used in single phase ones. A contactor joins 2 poles together, without a common circuit between them, while a relay has a common contact that connects to a neutral position. 

Also, contactors are commonly rated for up to 1000V, while relays are usually rated to only 250V.

When to Use a Relay:10A or less current - Up to 250VAC - 1 phase

When to Use a Contactor: 9A or more current - Up to 1000VAC - 1 or 3 phase

If you are considering using an item, always consult the specifications and speak with a licensed electrician. The information provided here is for informational purposes only.

How Does a DC Contactor Work?

There are three essential components of a contactor:

--The coil/electromagnet: The contactor consists of two irons, one is fixed, and the other one is movable. The insulated copper coil is located on the fixed time core. 


--Contacts: Six main contacts for power connection. Three are fixed, and the other three are able to move with the movable lion core. They are made from pure copper, and the contact points are made from special alloy to withstand the high starting current and temperature. It is located between the coil and the movable core auxiliary contacts. It could normally be open or close, and it is not designed to carry high loads the same as the main contacts.

--The frame or enclosure.

When applying the control voltage on the coil leads a one and a two, a magnetic field will be generated, attracting the movable lion core towards the fixed core, so the movable contacts which are fixed to the movable core, will move as well towards the fixed contacts to make a contactor and allow the power lines to run to the load. If we interrupt the control voltage, supplying the coil, the magnetic field will disappear, and the fourth spring will push the movable parts back again to their original position that will interrupt the power going to the load.



Here at MurCal we carry DC Contactors by Trombetta & Egis Mobile Electric


Trombetta designs and manufactures the world’s leading industrial DC contactors for motors. They range from 10 A to 1000 A, and up to 48 VDC. These contactors are designed for demanding environments including high shock vibration applications and high temperatures of up to 85 degrees C. 

We can customize Trombetta’s standard DC contactor designs to meet your exact specifications, including unique coil operation and mounting bracket configuration. Our customers use Trombetta DC contactors as components in new systems or as OEM replacement parts in existing systems.

See the selection here --> DC Contactors | MurCal


Egis Mobile Electric’s DC Contactors feature include “Ignition Protection”, safe for installation in closed compartments with potential explosive gases. Their “High Electrical Switching Endurance” delivers 6,000 cycles at 250A / 450V, 100,000 cycles at 250A / 14V. They also have “High Interrupt Current Capability” which enables it to safely open in a locked starter situation (up to 1,500 A @ 14 Vdc). Bullet-proof Construction means sealed devices are ideal for high temp materials for engine compartment applications, and also features stainless self-locking hardware. 4 Year Industry Leading Warranty rounds up this high-end product line.

See the Selection Here -->Contactors, 100A/250A | MurCal


Contact Us With Questions!


Phone: 661-272-4700

Email: sales@MurCal.Com

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