Priniciples of Electromechanical Relay Operation
3.25 Magnetically Polarized Relays
The more common magnetically polarized relay uses the effect of a permanent
magnet introduced into the magnetic circuit or circuits. The permanent magnet
supplies flux to either of two permeable paths than can be completed by an
armature. To transfer the armature and its associated contacts from one
position to the other requires energizing current through the electromagnetic
coil using the correct polarity. This principle is illustrated by the two
magnetic circuit configurations in Fig. 3.12. Energization of the coil by a
current pulse of proper polarity reverses the flux in the iron structure of
the relay; in the other, flux in the armature is reversed. The number of
ampere turns provided by the coil must be sufficient to overcome the
reluctance of the open pole gap (see dotted line flux path). One advantage of
this type of relay is the capability of latching operation where the contacts
remain in the last position without continued application of electromagnetic
power. It also augments the amount of armature force available with a given
relay size and input power. Polarized relays are usually small, have faster
operate and release times, and are more resistant to shock and vibration
forces.
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