What is the advantage of CMM relay over normal Inv. O/C current relay?
Inv. O/C relay under protects at low current and over protects at high currents.
CMM: accounts for both +ve and –ve sequence currents i.e. Single phasing / unbalance supply conditions and gives three times more weightage for the –ve phase
sequence current heating than + ve sequence current heating. i.e. Net rotor heating = I1 2 + 3 I2 2 .
Therefore CMM relay protection characteristic is closely matched to motor heating characteristic. So it is better than thermal overload relay also.