The contactor is the component that physically connects and disconnects the vehicle from the supply. It is also the component whose failure mode is most consequential, because a contactor that welds closed leaves a circuit live that the charger believes it has isolated.
Selecting it the way one would select a general purpose switching device produces a charger that works and then does not, on a timescale of years rather than months.
The duty is unusual in two ways
It carries near-rated current continuously for hours, which is a thermal problem. And it accumulates a high cycle count over a product life measured in years of daily use, which is a mechanical and electrical wear problem.
Devices selected for one of those duties are not automatically suitable for the other, and charging demands both simultaneously.
Making and breaking under load
In normal operation the control pilot sequence means the contactor closes before significant current flows and opens after the vehicle has stopped drawing. That is gentle duty and it is what most cycles look like.
The cycles that damage it are the abnormal ones: an emergency disconnect, a protection trip, a vehicle that stops drawing abruptly, a supply interruption. Those break current under load, and each one contributes disproportionately to contact erosion.
Welding is the failure mode to design against
Contacts weld when they are made or broken under sufficient current and the resulting arc melts material that then solidifies as a bridge. Once welded, the contactor cannot open, and the charger's command to isolate has no effect.
This is why feedback matters. A design that assumes the contactor followed its command cannot detect this condition, and will continue to present a live circuit as isolated.
Selection factors that differ from general switching
- Continuous current rating at the maximum ambient the enclosure will actually reach, not at laboratory conditions.
- Electrical endurance at the relevant load type, rather than mechanical cycle count, which is usually a much larger and less relevant number.
- Coil power, because a contactor held closed for hours dissipates continuously and contributes to enclosure temperature.
- Contact configuration, including whether an auxiliary contact is available for state feedback.
- Behaviour under a coil supply dip, which should not produce contact chatter.
Coil dissipation is an enclosure problem
The coil holding the contactor closed consumes power for the entire session, and that heat goes into the enclosure alongside heat from the conductors and the electronics. In a sealed outdoor unit in a hot climate, that contribution is not negligible.
Economiser circuits that reduce holding power after pull-in exist for this reason, and whether one is used is a thermal design decision rather than a component preference.
Feedback and diagnostics
An auxiliary contact or a voltage sense on the output lets firmware confirm the contactor actually did what it was told. This is what makes welded contactor detection possible and it needs to be designed in rather than added later.
It also enables a useful diagnostic: a contactor whose response time is lengthening over its life is a maintenance signal available long before it fails outright, provided anyone is recording it.
Why this is a poor cost-down target
The contactor is a visible line on a bill of materials and an obvious candidate for substitution. It is also a safety path component whose failure mode is a live circuit, and whose weaknesses appear after eighteen months rather than in acceptance testing.
A substitution here is a certification and qualification exercise including endurance testing, not a procurement decision, and the saving rarely survives that arithmetic.