Electrical Contactors are a high voltage and current relay, designed to allow a battery pack to be electrically isolated from the system. Hence in the open state they need to electrically isolate and in the closed state they need to provide a robust low resistance electrical path. Degradation will primarily be an increase in contact resistance.
Introduction to Contactors
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, in the case of EV’s we use Normally Open (NO) devices to switch off the circuit in case of loss of actuation power supply or control.
We have previously looked at contactor faults and so this time wanted to look at degradation. We see some lifetime plots for contactors based on the switching current.
Durakool [1] show a typical estimated lifetime graph for a contactor. This shows lifetime versus switched current and for a range of DC voltages. Lifetime decreases as current and voltage increase.
Above 24V DC it is difficult for a contactor or a relay to switch, making or breaking, because the DC arc generated in air when the contacts open is only extinguished by distance between the contacts. Generalising, the higher the voltage, the bigger the gap required. (AC is different as it self-extinguishes as it goes through the zero-crossing point.)
If the contact surfaces are damaged or have a build up of deposits the resistance will increase.
High Resistance
It is normal to see resistance growth in electrical contactors over their lifetime. The key factors that result in resistance growth:
- Oxidation and Corrosion: Most metals, especially in air, easily form a non-conductive layer of oxides or other chemical films (e.g., sulfides, chlorides) on their surface. As the film thickness increases, the resistance rises rapidly. Silver contacts, for example, can slowly tarnish over time.
- Arcing and Electrical Wear: When contacts open or close under load, an electric arc can form between them. This arc erodes and ablates the contact surface, creating rough spots and generating highly resistive wear products that deposit on the contact area.
- Contamination: Dust, grease, dirt, oil residue, and other foreign debris can accumulate on the contact surfaces, acting as an insulating layer and disrupting the metal-to-metal connection.
- Mechanical Wear and Deformation: Repeated physical use causes the contact material to wear down or deform (stress relaxation), which can reduce the effective contact area or lower the contact pressure.
- Mechanical Vibration and Shock: This can directly result in mechanical contact surface damage or can cause the contactors to bounce (open slightly) and arc.
- Insufficient Contact Force: Contact resistance is inversely proportional to the contact force (pressure). Over time, or due to improper initial installation (e.g., under-torqued bolts), the pressure between the contacts can decrease, leading to a significant increase in resistance.
- Environmental Conditions: High temperature, humidity, and the presence of corrosive gases (e.g., hydrogen sulfide in industrial areas) accelerate the chemical degradation of contact surfaces.
As the resistance increases the Joule (I2R) heating of the contactors increases. That in turn means the contacts will heat up and higher temperatures accelerate any chemical degradation. Hence we have a positive feedback that can increase the rate of resistance growth.
The increased temperature of a contactor is sometimes used as an indicator for a part about to fail.
Resistance Increase at EOL
What would we expect the End of Life (EOL) resistance of a contactor to be?
In industrial contactor applications [3] they give a Contact resistance interpretation (400 A contactor, 12 kV class):
- New contacts: 50-150 µΩ (smooth surfaces, full contact area)
- Light wear (0-30% electrical life): 150-250 µΩ (minor pitting, still acceptable)
- Moderate wear (30-70% life): 250-400 µΩ (plan replacement within 12-24 months)
- Heavy wear (>70% life): 400-500 µΩ (replace within 3-6 months)
- Critical (>80% life): >500 µΩ (replace immediately, risk of welding or failure to interrupt)
Durakool [1] state that for their Electric Vehicle contactors: When the contacts are damaged by arcing, their contact resistance increases. We can say that if the contact resistance increases by 30% or more, then it is time to replace the contactor. One direct effect of the increase in resistance is an increased temperature on the contact terminals. So, an indirect way of measuring the contact resistance could be to monitor the temperature rise on the terminals at a known ambient temperature and fixed current.
This post has been built based on the support of and sponsorship from: Quarto Technical Services, Eatron Technologies and About:Energy
References
- HVDC Contactors and Failure Modes, Durakool (pdf version)
- Maintaining High Voltage Contactors: Best Practices and Common Failure Modes, Magtron
- Mechanical Life vs Electrical Life: Understanding Vacuum Contactor Endurance Ratings, Xbrele