Could someone please go into a little more depth on why there is arcing across the contacts. Why is it worse when breaking inductive loads? I know that the energy stored in the magnetic field of the motor coils is released when power is removed from the coils, but how does this cause arcing? Is it basically a small amount of energy stored in the magnetic field discharged rapidly as high power?
A common misconception is that "AC is crossing zero 120 times per second, so it stops flowing faster". While it's true that it stops flowing FASTER than DC in an equivalent air gap distance, that doesn't mean it is stopping fast, at least not at the molecular level where the contact material is being vaporized. Yes, the current crosses zero, but by then, the arc started and the air is ionized, so when the sine wave rises again it takes LESS current to re-ignite the arc, which keeps happening until there is sufficient air dielectric resistance to extinguish it. At voltages above about 12V there is ALWAYS arcing of some sort whenever you open a contact under load. With an inductive load, the arc is just sustained longer, so it does more damage. It's based on the "Inductive Time Constant" concept, that you cannot instantaneously change current flow in an inductor, it has a specific rate of change. In addition as you open a set of contacts in an AC circuit, the separation has a capacitive effect as current tries to continue flowing across the gap, which in effect increases the voltage across it. That, combined with the increased time it takes to make the inductive current stop flowing, increases the energy flow across the contacts, and as long as you have sufficient energy flow, the arc will be maintained.