Does contactor coil current overload the driver output?
It can. A contactor coil draws a current above its holding current at the moment of pull-in, and this current may be high for an electronic output. Measure the coil current and compare it with the rated value of the output; if it exceeds it, put an interposing relay or a suitable semiconductor driver stage in between. Every switch-off without a suppression device across the coil hits the output stage with a high voltage.
There are two separate risks and both are overlooked.
The first is current. A contactor coil draws more current while pulling in than while holding. The rating of an electronic control output is most often given for small loads. Actually measure the coil current; if it does not stay below the output rating, do not drive it directly.
The second is the voltage generated at switch-off. The coil is an inductive load; when the current is interrupted, a reverse, high-amplitude voltage appears across it. This pulse wears out the output transistor or the relay contact in a short time.
Order of solutions:
- Connect a freewheeling diode or a suitable suppression device across the coil. Mount the diode as close to the coil as possible.
- If the current is too high, put an interposing relay in between: the electronic output drives the small relay, and that drives the contactor.
- Separate the coil supply from the control electronics supply, or at least shorten the shared return path; coil current creates voltage fluctuation on the common ground.
- Route the coil cables at least 15 cm away from signal and sensor cables.
After commissioning, watch that the control electronics do not restart while the contactor pulls in and drops out; if they do, the supply or ground sharing is faulty.
Full coverage of the topic: Installation, redundancy and field continuity