Relay Design and Construction:

The Relay Design and Construction is normally divided into the following stages:

• Selection of the operating characteristics.
• Selection of proper construction.
• Design of the contact movement from the point of view of utmost reliability.

The relay operating characteristic must match with the abnormal operating characteristic of the system. In other words it should clearly show the conditions for tripping under various abnormal operating conditions.

The most important considerations in the Relay Design and Construction are reliability, simplicity of construction and circuitry.

The Relay Design and Construction is divided into the following:

1. Contacts;

2. Bearings;

3. Electromechanical design; and

4. Terminations and housing.

1. Contacts: Contact performance is probably the most important item affecting reliability of the relay. Corrosion or dust deposition can cause nonoperation of relay, consequently the material and shape of the contacts are of considerable importance.

A good contact system design provides restricted contact resistance as well as reduced contact wear. The contact materials used are gold, gold alloys, platinum, palledium and silver. The selection of the contact material depends on a number of factors like the voltage per contact break, the current to break as well as the type of atmospheric pollution under which these contacts are operated. However there is no one ideal contact material which can be used universally under all conditions. There is also no method by which the suitability of contact material for use under any condition can be derived.

The following factors are to be considered for selecting a suitable contact material.

1. The nature of the current to be interrupted, i.e., direct or alternating.
2. Voltage at break and make operations.
3. The value of the currents to be interrupted.
4. Frequency of operation.
5. The actual speed of contact at make or break (this covers the arc duration and contact bounce).
6. The contact shape.
7. The contact closing force.

On the basis of practical experience the following are some of the rules recommended in the design of contact system of a relay:

• The contacts should be bounce proof to avoid arcing at the contacts thereby reducing the maintenance which ultimately results in increasing its life.
• Contact pressure is another very important factor to be kept in An increase in contact pressure leads to decrease in voltage drop or contact resistance.
• To promote accuracy and avoid sticking after long periods of inaction, the relay should be designed to maximum torque/friction ratio.
• The value of current that can be interrupted by a pair of contact in a.c. circuit is 2 to 8 times than that in a d.c. circuit.

In general domed shaped contacts or the cylindrical contacts at right angles give the best performance.

2. Bearing: The various arrangements in use are:

• Single ball bearings: Used for high sensitivity and low friction, a single ball bearing between two cup-shaped sapphire jewels is in use.
• Multi-ball bearings: Miniature types of less than 1.6 mm diameter are These provide low friction and greater resistance to shock and combine side-thrust and end-thrust in a single bearing.
• Pivot and jewel bearings: This is the most common type for precision relays, e.g. induction relays. Modern relays have spring-mounted jewels so that shocks are taken on a shoulder and not on the jewel.
• Knife edge bearings: These are used for hinged armature relays which normally operate many contacts.

3. Electromechanical Design: It consists of the design of the magnetic circuit and the mechanical fixtures of core, yoke and the armature.

The reluctance of the magnetic path is kept to a minimum by enlarging the pole face which makes the magnetic circuit more efficient.

D.C. electromagnets are usually less expensive and more efficient than a.c. electromagnets. But small a.c. electromagnets made from soft iron, low carbon steel core having a slot for mounting a shaded ring are quite Common.

The relay coil current is usually limited to 5A and the coil voltage to 220V, but still the insulation for the relay coil is designed to withstand at least 4 KV.

The relay coil is designed to carry about 15 times the normal current for one second. From the mechanical strength considerations it is desirable that the conductor diameter must not be less than 0.05 cm even if the above considerations are satisfied.

4. Terminations and Housing: The assembly of armature with magnet and the base is done with the help of a spring. The spring is insulated from the armature by moulded blocks. These moulded blocks provide dimensional stability and better appearance economically. Materials used for springs are stainless steel, nickel silver, phosphorous bronze and berillium copper, whereas for moulded blocks nylon is used. The fixed contacts are usually rivetted or spot-welded on to the terminal link.