The solenoid in a vacuum relay is simply an electromagnet. There are
several factors which can cause undersirable dynamic characteristics of any
electromagnet, notably (1) heating of the solenoid, (2) friction, and (3)
coercive force of the material used in the armature. I'll address each one
of these factors singularly, however, any or all may apply to a specific
electromagnet at any given time. [Sticking contacts and their causes is a
different matter and is not addressed here].
1. Heating effects: Heating of the solenoid and armature may be attibuted
to the applied current or the operating ambient, or both. Heating has a
two-fold effect on the electromagnet. (1) Heating causes an increase in
the solenoid resistance which results in a reduction of the ampere-turns
when a fixed voltage is used to activate the solenoid. (2) Heating also
reduces the permeability of the iron-core armature, causing a reduction in
the induced flux. Both these effects result in a reduction of the force
produced to move the armature and when they are cumulative the net effect
can become substantial.
2. Friction effects: Friction associated with the movment of the armature
may cause variable effects on the operation of the electromagnet. At the
start of travel of the armature the initial force is minimum while the
static forces are maximum. In a similar manner, when the solenoid current
is removed, the spring which is used to cause a restoring force to be
applied to the armature, must overcome the static friction forces. In
either case, the armature may fail to close fully (solenoid current
applied) or may fail to open fully (solenoid current removed).
3. Coercive force: Coercive force of the magnetic material used for the
plunger may cause sufficient "residual" magnetism to remain which causes
sticking when the solenoid current is removed. This effect is minimized in
the design of the armature by the use of "soft" magnetic materials and by
including a small air gap at the end-of-travel of the energized position of
the armature. The effects of coercive force generally can be overcome by
reversing the applied field. Some electromagnet/solenoid applications are
specifically designed to apply the field current in one direction (to close
the armature) and (momentarily) in the reverse direction to open the
armature.
73, Dave, K1FK
Fort Kent, ME
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