The Shading Coil

Author: E.E. Kimberly

If no provision were made to prevent it, the pulsating pull of an a-c magnet or solenoid would fall to zero twice every cycle and the armature would be pulled away slightly by its restoring spring, only to be pulled back again on the next rise of flux. Such action causes chattering of the armature. This chattering can be greatly reduced or even eliminated by surrounding a portion of the iron at the air-gap by a closed loop or ring of copper called a shading coil. A shading coil is shown in Fig. 7-13 (b). Because of the inductance of the coil, the flux through the shading coil does not pass through zero when the rest of the flux does. Therefore, there is always some flux in the gap to produce a closing force.