Balanced Beam Relay Engineering Study material

Balanced beam type relay can be said a variant of attraction armature type relay, but still these are treated as different types of relay as they are employed in different field of application. Balanced beam type relays were used in differential and distance protection schemes. The use of these relay becomes absolute as sophisticated induction disc type relay and induction cup type relays supersede them. The working principle of a Balance Beam Relay is quite simple. Here one beam is supported by one hinge.

The hinge supports the beam from somewhere in the middle of the beam. There are two forces acts on two ends of the beams, respectively. The direction of both of the forces is same. Not only direction, in normal working condition the torque produced by the forces in respect of the hinge, are also same. Due to these two same directional torques, the beam is held in horizontal position in normal working condition. One of these torques is restraining torque and other is operating torque.

 The restraining torque can be provided either by restraining coil or by restraining spring. This is a kind of attracted armature type relay. But balance beam relay is treated separately from their application point of view. When any fault occurred, the current through the operating coil, crosses its pick up value, and hence the mmf of operating coil increases and crosses its pick-up value. Due to this increased mmf, the coil attracts more strongly the beam end and hence, torque on respective end of the beam increases.

 As this torque is increased, the balance of the beam is being disturbed. Due to this unbalanced torque condition, the beam end associated with operating torque, moves downward, to close No contacts of the relay. Typical arrangements of both types of balance beam relay are illustrated below :

Now-a-days, balance beam relays become obsolete. In past days these relays were widely used in differential and impedance measurements. The uses of these relays are superseded by more sophisticated induction disc and cup type relays. The main drawbacks of balance beam relay, is poor reset / operate ratio, susceptibility to phase displacement between the two energizing and mal-operation on transients.