Attached Armature Type Relays

These have a coil or an electromagnet energized by a coil. The coil is energized by actuating/operating quantity i.e. V or I. The electromagnet is laminated. The plunger or an armature is moved/attracted by magnetic field produced by the operating quantity. Under normal working conditions the current through relay coil can not attract the armature or plunger because of spring tension (adjusted) when fault occurs I increases so armature or plunger gets attracted closing the trip contacts.

Operating principles: The electromagnetic force developed on moving element is proportional to square of the flux in air gap. If saturation is neglected force will be proportional to I².

F = K₁I² – K²

F = Net force

K₁ = Constant

K₂ = Restraining force including friction

I = Current in the relay coil

When relay is on the verge of operation.

K₁I₂ = K₂

I = √K₂/K₁ ………constant

Advantages:

1.    Relay responses to both A.C and D.C ( Hence Torque directly proportional to I² )

2.    Light moving parts, Small length to travel of armature or plunger in air gap. Therefore, relays are fast in operation.

3.    These relays are instantaneous but operating time varies with current (see characteristics). The operating time and resetting time can be adjusted by adjusting the air gap so relays can be made slow operating relay.

                                               

4.    The current time characteristic is inverse i.e. hyperbola.

5.    Ratio of reset to pickup. This can be as high as 90 to 95% for A.C. relays and 60 – 90% for DC relays.

6.    The relays are compact, robust and reliable.

7.    They do not have any directional features unless provided with polarized coil.

Applications:

1.    For over current protection

2.    Differential protection

3.    Auxiliary relays

4.    Definite time lag over current and earth fault protection.

Balanced Beam Relay

                       

It consists of a beam pivoted centrally. One armature is attached to other side. Current in operating coil gives operating force and current in restraining coil gives restraining force. The beam is given slight mechanical bias by means of a spring so that under normal working conditions the contacts are open.

The arrangement is similar to see saw in children park. When operating torque increase, beam ti8lts and thus closing the trip circuit contacts.

Operating Principle: T = K₁I₁² – K₂I₂²

Where                                     T = Net torque

                                        K₁, K₂ = Constant

                                                I = Current in operating coil

                                                I₂ = Current in restraining coil

When the relay is on the verge of operation, Net torque = 0

                                     K₁I₁²  =   K₂I₂²

                                    I₁/I₂ = √K₂/K₁ = Constant

Operating Characteristics:     

As  I₁/I₂ = √K₂/K_{1 =} constant

Operating characteristic is a straight line passing through origin as shown in Fig. When the operating current is less, the linearity of curve is not maintained, because of springs.

Limitations:

1.    Balanced beam relay is difficult to design for a wide range of currents because force is directly proportional to I₂.

2.    Relay is fast and instantaneous, it can have a time operation of one cycle.

3.    VA burden depends upon the application and is normally 0.2 to 0.6 VA for the current range of 0.1 – 0.6 Amp.