Contactors: The Basics

Contactors are electromechanical switches. They control the connection of battery Cells to the external electrical circuit. A contactor has an electromagnetic circuit. It simply works like a solenoid to push moving contact [1]. Therefore, contactors have two different electrical circuits inside: one is for actuation and the other one is to carry currents during the switch-on state.

There are two main types of contactors: Normally Open (NO) and Normally Closed (NC). A NO contactor does not allow current to flow when the actuation circuit is not powered. However, an NC contactor connects the external circuit when the contactor actuation coil is not energized. Therefore, we use EV contactors as NO to switch off the circuit in case of loss of actuation circuit supply.

Contactor Structure

A contactor has the following elements in its structure: an actuator coil, moving core, return spring, moving contact, at least two fixed contacts, and an arc chamber. The structure of a contactor is below:

parts of a mechanical contactor

When we actuate the contactor coil, the moving core starts to move towards the fixed contacts. Then, it stretches (or compresses) the return spring until the moving contact connects with the fixed contacts. This operation is quite fast, approximately 20ms for EV contactors. However, there is enough time to create arcs [2]. Therefore, there is an arc chamber to extinguish possible electrical arcs during switching. The contactor manufacturers make the arc chamber (or arc chute) with a dielectric medium such as low pressure hydrogen or nitrogen gas [3].

Boundary Conditions

Thermal and mechanical conditions are some of the constraints for contactor selection. Like fuses, contactors work together with other system elements to protect the electrical system. Therefore, the selection of neighboring components affects contactor sizing. Also, the following system specifications define contactor selection:

  • Minimum breaking current of the fuse*
  • Short circuit current of the system
  • System inductance

*In some cases, the short circuit current of the system is not high enough to melt the fuse element immediately. Especially in low temperatures, cells have higher internal resistance, and the low temperature slows down the melting process of the fuse. Therefore, contactor selection depends also on fuse selection.

Design Challenges

There are several design challenges around contactors that we will elaborate on:

  • How do I select and size my contactor?
  • How do I control my contactor?
  • What are the limits of my contactor for switching?


  1. Dong, Shuangbing. “Driving High-Voltage Contactors in EV and HEVs“, Texas Instruments, 2021. 04 Jan. 2022.
  2. Kroeker, Matthias, Hans-Joachim Faul, Roman Dietrich. “EVC 250 Main Contactor“, TE Connectivity, 2014. 04 Jan. 2022.
  3. Relay And Contactor Dielectric Materials“, Sensata Technologies, accessed 04 Jan. 2022.

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