Priniciples of Electromechanical Relay Operation
3.8 Winding Circuit Design
There are four operating characteristics for electromagnetic realys, namely,
nonpick-up and pick-up on the operate stroke and hold and dropout on the
release stroke. Whether only one or a combination of these characteristics is
specified depends on circuit complexity and the relay's function. Definitions
for these characteristics, which were given in Chapter 1, shall be restated
here briefly. When a winding in energized:
1. Nonpick-up is the voltage (or current) at or below which (a) The armature
shall not move from its de-energized position, or (b) the normally closed
contacts shall not open and the normally open contact shall not close
2. Pick-up is the voltage (or current) at or below which: (a) The armature
shall assume its fully operated position (seated against the core), or (b)
normally closed contacts shall open and all normally open contact shall
close.
When the energizing voltage (or current) of an operated relay is reduced:
1. Hold is the voltage (or current) at or above which: (a) the armature shall
not move from its operated position, or (b) all normally open contacts shall
open and all normally closed contacts shall close.
2. Dropout is the voltage (or current) at or above which: (a) The armature
shall restore to its de-energized position, or (b) all normally open contacts
shall open and all normally contacts shall close.
For most applications only pick-up and drop out requirements are essential.
Special circuit conditions or other application considerations sometimes
necessitate specifying nonpick-up and hold requirements. These will be
discussed later. It is usually advisable to specify requirements in terms of
the armature position. In some designs, however, specified only in terms of
contact operation. Relays in which the armature hesitates while on the verge
of just closing or opening contacts or in which the contacts have barely
opened or closed could be in an extremely unstable state. A slight mechanical
disturbance or voltage fluctuation could cause intermittent opening or closing
of contacts. It is advisable, therefore, to specify requirements assuring
that the desired performance will be obtained, that is, that the contacts
will be fully operated. Specifying that the armature shall not move at a
particular energizing value and shall complete its stroke at another value
will provide this assurance. In some designs, such as certain sensitive
relays under marginal requirements, it may not be possible to guarantee that
the armature will seat on the pole piece fully, even though the contacts have
transferred properly.