SOLUTION

Design Considerations of Electromagnetic Relay

The design of an electromagnetic relay is based on the construction of various components of that relay. The design considerations for the various components of an electromagnetic relay are,

Coils: The coils carry the currents and should be designed by considering the temperature rise associated with the coil. The temperature rise limits the current rating of the coil. Practically the coils are tested for the test called voltage withstand test in which 2500 volts are applied to the coil for one second. The continuous and short-time current ratings are specified with the relay coil. For the current operated relay, the standard continuous current ratings are 0.5 A, 1 A and.5 A while for the voltage-operated relay the standard voltage between the lines is 110 V.

Movable Assembly: The inertia of the movable assembly must be as low as possible for the fast operation of the relay. Thus the parts used are very light for the movable assembly. Similarly, for the fast operation, the distance of travel is also minimized.

Relay Contact: This is the most important part of the relay. The relay contacts must be robust as the contacts must perform make and break duty successfully. This is difficult because for making relay sensitive, the parts used in the moving system are light in weight. Hence seal in contact arrangement is used. This arrangement prevents the opening of relay contacts for the flow of trip current. The important design considerations of the relay contacts are,

• Contact resistance must be as low as possible.
• The contact pressure must be proper.
• The sparking must be as minimum as possible, preferably sparking free.
• The making and breaking capacity must be appropriate.
• The material should be corrosion-free.
• The material should have the self-cleaning action so that oxides formed can be readily breakdown.
• The dimensions of contact should not change due to the operation.
• The contacts must be free from bounce at the time of maximum torque condition.

The materials which are preferred for the relay contacts are silver due to low resistance and self-cleaning property, alloys of silver such as cadmium silver oxide for high current applications, gold~silver~platinum alloys for low current applications.

 Bearings: The pivot and jewel bearings are preferred for induction relays. The jewels absorb the shocks as spring mounted. The pin bearings and knife-edge bearings are used for armature relays.

Cases: The cases used are of the same width and depth The length depends on the type and application of the relay. The cases are usually flush-mounted. Nowadays cases of pressed steel cast aluminum or plastic are used. These are dust-proof cases.

Operation Indicator: This is a colored flag. When the relay operates, it is mechanically moved to indicate that the relay is operated. When the relay is reset the flag is reset manually. When the flag is operated the corresponding operation of the circuit breaker is assured.

Adjustment of Reset: The adjustment of reset is achieved using tapped auxiliary potential transformers and resistors. In some relays, adjusting spring tension or air gap length adjustment is used for adjustment of reset.