A relay is designed for low-power applications (I/O signals etc.), a contactor is designed for higher power (motors, heaters etc).
The difference lies in several points. To list a few:
1. Contact point materials - with high power applications the ability to withstand arcing at the contact points without destroying the surface are important. A very low contact resistance is not.
2. Extinguishing chambers – for the real high powers, there are chambers around the contact points designed to “blow” out the arching.
3. Number of contact point per pole – regular contactors usually use two contact points per pole, a relay usually have only one.
When choosing a contactor, it is important to know what load you are connecting. I don’t know American terminology, but I will try to explain (and use European terms). The ratings are usually printed on the contactor itself:
* With regular resistances (heaters), use the contactors thermal current rating Ith.
* With motors, you have several choices, depending on how you control the motor:
- AC3: Normal start/stop, switching on 5-6xIn, breaking 1xIn (In = nominal current)
- AC4: Heavy duty, start/stop, breaking of start current, reversing etc.
* AC15: Incandescent lighting
Those are just some oft the different ratings you can find on a contactor, typically defined as a power rating (kW or hp) at various voltages. You will usually find that AC3 ratings are in the range half of the thermal.
All this applies to AC only. Switching high power DC are particularly challenging, often involving special contactors and/or combining poles in series and parallel.
Regarding the use of “snubbers”: in control circuits, use those when controlling inductive loads like relays, contactors and solenoid valves. Use free wheeling diodes with DC, R/C with AC, varistors (voltage dependent resistors) can be used with both AC and DC. Connect in parallel with the load. Using snubbers is the best way to prolong the lift of your PLC output modules, as it significantly reduces the arcing over the internal relay contacts (or reduces the reverse voltage over the output transistor).
Hope this helps to clarify....
Anders