Priniciples of Electromechanical Relay Operation


3.7 Design Analysis

The armature type electromagnetic relay is a complex electromechanical device in which electrical energy is converted, through linkages, into mechanical motion that actuates electrical contacts. Its performance and reliability, therefore, depend upon the interactions of many design parameters related to applications and performance criteria. It is essential, then, that the suitability of a design be evaluated in terms of the factors that may affect its performance under specified environments, winding circuit conditions, mechanical life requirements, and contact loads.

The initial phase in the evaluation and application of any relay should be an analysis of its design and study of the controls exercised to assure consistent performance. To a large extent, design criteria depend upon the type of application, the economic considerations, and the consequences of failure. A design that is adequate for most applications might not be suitable for use in critical circuits requiring close controls and a high degree of stability. In such cases, performances studies of limited quantities of relays cannot always be relied upon as the sole means of establishing the suitability of a design. Whether a design is basically sound, employs quality materials, and is a product of good workmanship and controlled techniques must also be determined. Relay design analysis is an art in involving good engineering judgment. This judgment must be based, to a large extent, on experience and an appreciation of cost factors related to performance requirements and the consequences of failure. It is not possible to present a comprehensive treatment of the subject in this handbook. The following discussion, however, outlines a few design factors that warrant consideration in specifying relays for critical applications.

1.Friction. Friction can significantly affect performance. Plated armature bearings that tend to rust often produce excessive friction. Excessive slide or wipe between actuating or actuated members also may cause variable friction.

2. Finishes. Some finishes intended for corrosion protection are susceptible to a whisker-like growth that can cause voltage breakdown between plated parts and other metallic members of a relay having small clearances. Wear-resistant finishes on pole faces and backstop surfaces are often necessary on relays requiring a long mechanical life or high degree of electrical characteristic stability.

3. Contact Adjustments. Contact force and contact overtravel may be affected significantly be environment, wear, and contact margins. It is essential, therefore, that the minimum adjustments provide sufficient margin to assure reliable contact performance for the required life and applicable environments. The design and contact adjusting techniques should assure also that the full force of movable contact springs is exerted against the fixed contacts. With buffer-actuated contact springs it is essential that there be a gap between the actuator and movable spring when the armature is fully released. If a buffer gap is not provided, appreciable contact follow is required to assure adequate contact force. For relays with armature card lift-off actuation, clearance is necessary between the card and movable springs of closed contacts in both the energized and de-energized positions. In many instances (such as normally open contacts of permissive make actuated designs), contact force cannot be measured directly, however, the overtravel or follow through distance after contact make can be measured instead. To determine the type of measurement required to assure reliable contact one needs to study the actuating system.

4. Manufacturing Cleanliness. Relay parts-particularly contacts, bearing, and polefaces-should free form particulate contamination, and contacts should be free from organic films when controlling low energy circuits.

5. Contact Materials. The contact materials employed should be suitable for contact load, environment, and other performance requirements. Wherever possible, materials that tend to stick should be avoided.

6. Insulating Materials. Insulating materials within contact chambers, or adjacent to contacts of open relays, should emit a minimum amount of vapors that might impair contact performance. Design employing inorganic materials, or in which the contacts are isolated from organic materials, offer greater assurance of reliable contact performance. Insulating materials should be A) free form corrosion promoting impurities, B) dimensionally stable to minimize adjustment changes with fluctuation in temperature and humidity, C) free from a tendency to shed particular to shed particles that may contaminate contacts or become entrapped in bearings or air gaps, and D) suitable for the environmental temperature ranges.

7. Soldering Fluxes. Virtually all liquid or paste soldering fluxes and chemical strippers for enameled wire are highly corrosive. Unless the design lends itself to thorough washing and neutralizing processes, the use of the fluxes and strippers should be avoided. Sealing techniques for hermetically sealed relays should insure that flux or vapors are not entrapped within the contact chamber.