Priniciples of Electromechanical Relay Operation


3.4 Power-Force-Stroke Relationship DC Electromagnets

The most common form of actuator or motor system for electromagnetic relays consist of an energizing coil and a permeable iron circuit. It has both a fixed portion (open loop) and a movable member, called the armature, that completes the magnetic circuit by closing the air gap. This armature must be hinged, pivoted, spring-mounted, or somehow free to move within certain constraints so that its motion can do useful work, namely, causing the contacts of the controlled circuit to perform a switching function. In addition, it must normally store some energy in a spring (or springs) for the return stroke and for holding selected contacts closed when the relay coil is in the de-energized condition. The factors of the greatest importance over the life of the relay are the following:

(1) To provide, at a minimum reasonable expenditure of power, adequate magnetic pull to assure reliable closing of contacts.
(2) To provide sufficient separation between open contacts.
(3) To provide the desired operate and release time characteristics.
Contact Actuating Systems. Actuation of contacts in electromagnetic relays is accomplished by four methods:
(1) Direct armature clapper actuation (simplest construction)
(2) Multiple spring flexure (as in low power relays)
(3) Permissive make actuation (typical of wire spring relays)
(4) Direct solenoid actuation (typical of power contactors)
Only the first three systems will be analyzed here in as much as solenoid actuation is readily understood and generally not applicable to relays switching less than 25 amperes.

Direct Armature Clapper Operation. One of the simplest contact arrangement employed in relays is the Break Before Make or transfer (form C) contact combination employing a clapper type armature-(see Fig.3.1)
It uses a flat or helical coil (armature return spring) to provide the return or restoring force on the armature in the de-energized position. The break (or normally closed) contact is normally closed under action of the return spring. The sequence of spring forces to be overcome by the armature in transferring contacts to the fully operated position is graphed in Fig.3.2(broken curve 1,2,3,4).


The moving contact spring attached to the armature is held against the break contact with a force created by the return spring that also causes flexure in the movable contact spring. When the coil is energized, the armature will immediately begin to move in proportion to the energizing power. Between points 1 and 2, the flexed armature spring is being relieved, but the force to be overcome by the armature is also determined by the spring rate (deflection per unit loading) of the return spring. At point 2, the movable contact spring is completely relieved, and the normally closed (break) contacts open. The armature continues to overcome the spring rate of the return spring until the normally open (make) contact is met at point 3. Load on this contact is built up at the combined spring rates until the armature bottoms on the core at point 4.


Flexure Operation. In low power relays, the basic spring arrangement are typically those illustrated in Fig.3.3. In this so-called flexure operation, a return spring is tensioned to hold the armature in the de-energized position. The moving contact spring is independently tensioned against the normally closed (break) contact. A slight separation, X1, is provided between the insulated actuator card or buffer and the moving contact spring to assure that the full force of the moving contact spring is exerted against the stationary break contact. When the relay is energized, the armature, through the actuator, lifts the moving spring contact off the break contact and pushes is toward the stationary, normally open (make) contact. After the contacts touch, further armature travel builds up contact force until the armature seats on the core or against the armature stop. The force developed at the contact interface is a function of the flexure of the moving contact spring and the stiffness of the stationary contact spring system.


In Fig. 3.4, at point 2, the moving contact spring is picked up by the armature buffer, and between points 2 and 3, the normally closed contact preload is overcome. Between points 3 and 4, the contact gap, X2, is closed and normally open contact is met at point 4. Pressure is built up on the Make contacts between points 4 and 5. Armature overtravel, or contact wear allowance, ends at point 6, where the armature is seated against the pole face.
It should be evident from this force curve that contact pressures are the result of spring deflections caused by overtravel or built-in preload (the adjustment process). Therefore, unless spring preloading is expressed in the relay design, contact force will be reduced proportionally during relay life as a result of mechanical deformation, electrical erosion of contacts, and armature bearing wear.


Fig. 3.5 shows a contact construction in which initial tension or preloaded is built into the spring. With such an arrangement, any contact lift-off at all requires a minimum force equal to the initial tension. Furthermore, the contact force does not diminish appreciably during the wear-out process. On the other hand, the force to be overcome by the armature during the pickup stroke is a step function. This makes it possible for the pickup power measured at the start of the armature travel to be less than that required to complete the stroke after the engagement of the contacts. In this case, such a relay is not applied in circuits where the full specified pickup power is abruptly applied to the coil as in "all or nothing" relays.


Permissive Make Actuation. This form of contact transfer is illustrated Fig. 3.6. In this case, two moving springs and one stationary contact, from a contact set or a transfer. The moving spring (B) for the break contact is tensed against the stationary contact, and a slight separation is provided between it and the insulating actuator to assure that the full force of the pretensioned moving contact is exerted against the stationary contact. As the armature moves toward the core, the moving contact is lifted off the stationary contact.


The moving Make contact spring (A) is tensed against the insulating actuator with a force equal to the desired make contact force. As the armature moves toward the core, the actuating insulator follows. After a prescribed travel, it leaves the moving spring as the moving contact comes to rest on the stationary contact. The armature continues to move toward the core. This provides separation between the actuating member and moving spring and assures that the full pretensioned force of the moving spring is exerted against the stationary contact.
The moving springs of the system usually are extremely compliant. Card wear and erosion of the contacts have virtually no effect on the contact force until the erosion or wear has progressed to the point that the movable contacts rest on the card and their force is not exerted against the fixed contacts. This effect is independent of spring compliance. Balance springs are required with permissive make actuation to overcome the force of the moving Make contacts is restoring the armature to its de-energized position.
Permissive make actuation can provide somewhat longer life than flexure actuation. Contacts, however, are more susceptible to contact chatter under the severe vibration and shock conditions that are encountered in many applications. Another advantage of permissive make actuation is that any tendency of an individual contact to stick or weld is overcome, to some extent, by the forces of the other contact springs acting on the system. In the case of the flexure system, only the restoring force of each individual moving spring is available to overcome any sticking tendency of an individual, normally open, contact pair.