Shading Ring

A shading coil is a single turn of conducting material (generally copper or aluminum) mounted in the face of the magnet assembly or armature. The alternating main magnetic flux induces currents in the shading coil and these currents set up auxiliary magnetic flux which is out of phase from the main flux. The auxiliary flux produces a magnetic pull out of phase from the pull due to the main flux and this keeps the armature sealed-in when the main flux falls to zero (which occurs 100 times per second with 50 cycles ac). Without the shading coil, the armature would tend to open each time the main flux goes through zero. Excessive noise, wear on the magnet faces, and heat would result.

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Partially correct i am afraid.

Shading coil's purpose is to give enough phase-shifted magnetic field to keep the contactor on when the main coil flux passes through zero, thereby avoiding chatter and mechanical destruction of the magnet, and the power contacts also. Here is an image of the main and shaded flux forces.

The shading coil does not help in reduction of contact bounce. Bounce reduction is achieved by different means by many designers...elastomer buffers on magnets, leaf springs on contacts and armatures, bell-crank design like this one ...

More recently, designers have resorted to using electronically controlled coils for contactors, some of which give just enough force to the magnet to close and seal, thereby practically eliminating bounce. Here is an image from a patent, which shows the tailored force curve.