AC vs. DC Contactors
When people refer to DC or AC contactors they may be talking about either the control circuit or the circuit being controlled (i.e. being switched). On this page we will use AC or DC contactor to describe the type of power being switched, NOT the type of power energizing the coils.
Whereas the constantly changing voltage helps extinguish any arcs in an AC circuit (specifically when there is zero voltage), a DC contactor relies on other mechanisms to suppress arcing. The most important difference between an AC and DC contactor is that a DC contactor is designed to suppress electric arcing when the contacts open and close in a DC circuit. For example, voltages are much lower for DC than AC circuits, and the gaps between contacts are wider in a DC contactor.
An electric arc is simply the flow of an electric current through a gas. In electric circuits, arcing can occur anywhere a connection is made or broken. Thus, arcing occurs most frequently at switches where contacts repeatedly open (break) and close (make) a circuit.
Effects of Arcing
Extreme levels of heat
Electric arcs are actually examples of the less familiar but very common fourth state of matter called plasma (e.g. electric shocks, lightning, and neon lights). In order of increasing temperature (i.e. kinetic energy or heat) we have solids, liquids, gases, and plasma. Plasmas are essentially ionized matter where the electrons orbiting atoms are energized to such a degree that the outermost electrons are able to escape, and the surrounding air that is a natural insulator becomes conductive.
The level of kinetic (heat) energy carried by the ions in a plasma is on the order of thousands of degrees Celsius and get can several times hotter than the surface of the Sun. In short, electric arcs are very very hot and over time will damage your switch contacts as any amount of arcing will melt off or even vaporize tiny portions of the contact surfaces.
Cleans contact surface
One benefit is that arcs burn off the thin layers of oxidation and other contaminants that prevent current flow and build up on contact heads. In fact, contacts that are not switched for a period of days or weeks will oxidize faster than contacts that are regularly switched.
Arcing in an AC vs. DC circuit
Since arcing is the flow of current (i.e. electric charges) through a gas, arcs occur far more easily and with greater effect in DC circuits where the current is only flowing in one direction and has a steady voltage. In AC circuits where the polarity, voltage, and current are constantly changing and reversing, there is much less risk for damage from arcing.
In particular, the point where current changes direction is when there is zero voltage, and zero voltage means no current is flowing. Since current changes direction twice in every cycle and the U.S. and Europe use 60 and 50 Hz AC power respectively, zero voltage occurs 120 and 100 times every second. This means that arcs in AC circuits are self-extinguishing.
NOTE: When selecting contactors, you need to consider arc and surge suppressors separately. Many devices provide both arc and surge suppression (variable resistors, diodes, zener diodes, snubbers, etc.), but electric arcs and surges are not the same thing.
Arcing refers to electric current traveling through a gas, while surges refer to momentary spikes in voltage.
An arc will ignite and last as long as there is sufficient voltage and current in a circuit. In AC circuits, arcing still happens, but since zero voltage occurs so frequently any arcs are quickly extinguished, though they may reignite on their own if the voltage is too high.
Zero voltage does not occur in DC circuits unless the circuit is opened, so a good way to extinguish an arc is to simply have a larger gap between contacts. This is why having a fast switch time matters for high power applications (both AC and DC), because the faster a contact armature opens, the faster arcs will be extinguished.
Having instants of zero voltage in AC and not in DC is also why a DC contactor can be rated to switch, for example, 24V DC, but you could use the same contactor to switch 120V AC. Conversely, it would extremely dangerous if you tried to switch 120V DC with a 120V AC contactor.