Carel EVD / CDS-7: electronic expansion valves jam and drivers burn out
What happened
In a chiller, the electronic expansion valves repeatedly jammed and their EVD driver units failed, showing burn damage.
What was checked
- Compare the valve model and step count programmed in the controller with the valves actually installed on the unit.
- Check whether the controller is set to drive the valves to the zero position (calibration) at every start-up.
- After a failure, inspect the valves for mechanical sticking and the EVD drivers for heat damage.
Cause
The engineers found the controller was configured for CDS-9 valves while CDS-7 valves were actually installed. The programmed step count was too high, so the controller kept driving the valves beyond their travel; they jammed, and the stalled valves overloaded and burned the drivers.
What was done
The correct valve configuration (step count for the installed CDS-7 valves) was set in the controller parameters, and the jamming stopped — the problem was solved. In this case a dedicated UPS for the EVD drivers proved unnecessary, because the controller zeroes the valves at every start-up; note that CDS-7 valves are discontinued, so spares have to be sourced from remaining stock.
Safety
Before working on the valve drivers or controller wiring, isolate the chiller from the mains, wait for power-supply capacitors to discharge and confirm zero voltage with a meter; the refrigerant circuit remains pressurised.
Lesson: The valve configuration in the controller must match the valves actually installed — wrong step settings can mechanically jam the valves and take the drivers with them.
Covered in the course: Power and Cooling for Medical Equipment — Level 2: Fault-Finding →
Written from an anonymised case shared by service engineers. It is not a manufacturer instruction and grants no authorisation. Independent note; names are trademarks of their owners.