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.
A number of issues.
In the first circuit, of course, the capacitor should be
simulate this circuit – Schematic created using CircuitLab
Just as importantly, don't get hung up on the value just yet. Its' value is determined by switching speed of the switch, and the allowable droop in voltage when there is no source connected.
Using a mechanical contactor for switching a DC current is generally a bit tricky. The problem is that, unlike AC, when you try to break a current flow an arc will develop between the contacts, and without the voltage reversal inherent in AC, the arc can persist and damage the contacts. DC contactors do work around this, but they tend to be expensive. (Actually, contactors in general tend to be expensive, but you probably already know this.) Solid-state DC relays are probably your best bet if you want to go the contactor route. Digikey has some 160 amp units. For $150 +.
Going MOSFET is probably your best choice, and for this application you need a p-type high-side unit configured like so
simulate this circuit
And you're in luck. The MOSFET I've shown is available from Digikey for $25.
M2 is almost any n-type with a voltage rating greater than 50 volts.
I show the input drive as 0/5 volts. If you must use a lower logic level (like 3.3) that's certainly possible, but you must remember to get "logic-level" MOSFETs, since otherwise they may require 4 volts to turn on fully.
This circuit ought to be easily capable of 1 usec switching. That means that you will have to learn the fine art of protecting against inductive surges, but that's a story for another time.
You'll also note that this circuit does not require a separate "contactor" supply. If, for some reason (like you can get one for free) you decide to go the contactor route, note that you don't need a separate supply for it. You just use the battery you're switching to drive it.
You can use two of these circuits to create a double-throw effect, but you have to make sure that there is a delay between releasing one before activating the other. The delay should be on the order of a microsecond or so, but check your actual circuit operation first.
Best Answer
Wikipedia's Contactor article explains it pretty well.
Further down the same article ...
Magnetic suppression and arc dividers are typically utilized when switching multi-horsepower motors. Magnetic suppression is accomplished by forcing the arc to follow the longer field lines of a fixed magnet placed in close proximity to the contacts. The longer path is specifically designed to force an arc length that can’t be sustained by the availableinductive energies. Figure 3 shows a schematic representation of magnetic arc suppression. Source: Automation Direct, Electrical Arcs - Part 1 of 2 part series.
The article linked above is well worth a read.
Your questions:
Look carefully at the application and contact rating - particularly for motor or inductive loads. If you are satisfied that either will suffice you can choose based on some other criteria such as cost.
Generally not. While doing this does reduce the long term heating of the individual contacts due to steady current running through them it is a problem during switching due to timing differences. Even wiring contacts of the same relay in parallel is risky as they never are perfectly aligned and the first one to make and last one to break carry the full switching action.