Electronic – Relay Contact Sticking Problem

My problem is that I don't understand what kind of load a contactor is (e.g. how much it is inductive, does it give current surges, or kickbacks as a DC relay, etc.)

The coil voltage is 230/240V rated and consumes 2.7 VA when activated (ignoring inrush). The holding current is therefore about 11.7mA RMS. It's mainly inductive and this is "indicated" by the inrush VA requirement - this is the magnetic core saturating causing this 9.2 VA. The 9.2 VA will last for less than 100 milli seconds (in my opinion) and you probably wouldn't even see this if you activated the coil right at the peak of a positive or negative voltage on the AC. Yes coil inrush is worst at zero-crossing!

The solenoid inductance can be calculated from the holding current of ~12mA. This implies an AC impedance of 230/0.012 = 19.17 kohms and, at 50 Hz, this suggests an inductance of 62 henries (this is needed to be a powerful solenoid).

Given that you have "almost no experience in designing circuits" I would use a control relay to activate the contactor and I'd put a snubber across the contactor coil. The snubber will prevent large arcs when the control relay open-circuits.

Contactor coil energy is 12mA (squared) x 62/2 = ~5 mJ and a 220nF 250V AC snubber capacitor would restrict back-emf from the coil to about a 200 volts kick-back. You'd probably get away with 100nF. A series resistor ought to be included and 100 ohms is going to be about the right ball-park (calculate power requirements though).

So, 100nF will restrict the kick-back to about 316 volts peak. The 100 ohms is really there to restrict the snubber current when the relay contact activates the contactor coil.

My guess is that all the 10nF or 100nF recommendations are for zero-crossing TRIAC applications, and that one has to be really careful to not apply that recommendation blindly to relay-based solutions.

So, I propose the following possible solutions:

1: RC snubber designed according to the calculations in How to calculate resistor and capacitor size for snubber circuits . This protects the relay from arcing and reduces EMI problems.

2: 100nF and 100 ohm RC-snubber in parallel with a MOV. The snubber doesn't protect the relay, but may reduce EMI-problems since such a snubber forms an RC low-pass filter with a 100kHz cutoff.

3: Just a MOV. May cause EMI-problems?

MOVs have a limited lifetime and must be sized appropriately to have enough longevity for the application. They may not be a suitable solution if the motor is to be turned off frequently. Placing the MOV across the relay protects the relay better, but means the MOV is always energized even when the motor is off. A short-circuit failure of the MOV will then start the motor.

The MOV may be changed to a bidirectional TVS diode. IEC 60950-1 (if it is applicable to the application) explicitly forbids TVS diodes for surge suppression. If the TVS is connected across the relay, it is basically connected the same way as a surge suppressor, and may thus not be allowed(?). IEC 60950-1 also states that MOV:s, if used, must be used together with "an interrupting means having an adequate breaking capacity", presumably a fuse.