Priniciples of Electromechanical Relay Operation
3.16 Shaded-Pole Relays
Shaded pole AC relays are generally constructed like simple DC electromagnetic
relays with a portion of the core pole face separated from the rest of the
pole face and enclosed in a loop of copper. This loop produces a lag in the
timing of the ac magnetic flux in one portion of the pole face with respect to
that in the unshaded portion. While the current in the coil passes through
zero twice each cycle, the flux in the armature gap remains at a high enough
level to hold the armature operated. If the minimum pull exceeds the
mechanical forces (spring and gravity) tending to restore the armature to its
de-energized position, there will be little, or no buzzing, of the armature or
contact chatter.
The current drawn by a shaded pole relay is determined by the ac impedance of
the coil at the power line frequency. This impedance depends upon the coil
constants, the characteristics of the magnetic circuit, and the armature
position. For example, the impedance of a relay may be twice as large with
the armature operated (seated) as with it de-energized. It is customary to
express the sensitivity or actual power required of such an ac
relay in volt-amperes, but the degree to which the iron is magnetized is
better measured by E^2/Ro the ratio of the square of the applied
voltage to the coil resistance for a full coil. The degree of magnetization determines the
inductance and impedance for any particular frequency at various applied
voltages. From the applied voltage and impedance, the actual volt-amperes can
be determined. Some relays will not remain energized or may chatter badly if
the coil current is reduced to half of the rated pick-up value. Many ac
relays are intended to be energized at the rated voltage value even though
there may be considerable margin between pick-up voltage and rated voltage.
This is sometimes referred to as on-off operation, or per the IEC notation of
an "all or nothing" relay.