CURRENT (OR VOLTAGE) - BALANCE RELAYS Two basically different types of current-balance relay are used. Based on the production of actuating torque, one may be called the “overcurrent” type and the other the “directional” type. OVERCURRENT TYPE The overcurrent type of current-balance relay has one overcurrent element arranged to produce torque in opposition to another overcurrent element, both elements acting on the same moving structure. Figure 9 shows schematically an electromagnetic-attraction “”balanced-beam” type of structure. Another commonly used structure is an induction-type relay having two overcurrent elements acting in opposition on a rotor. If we neglect the negative-torque effect of the control spring, the torque equation of either type is: T = K1I12 – K2I22 When the relay is on the verge of operating, the net torque is zero, and: K1I12 = K2I22 Therefore, the operating characteristic is
—
√
I1 —= I2
K2 ––– = constant K1
Fig. 9. A balanced-beam type of current-balance relay.
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PROTECTIVE RELAY TYPES