OVER LOAD INVERSE-TIME / INVERSE DEFINITE MINIMUM TIME LAG (I.D.M.T.) RELAY
The over load inverse time relay is shown in fig 26. It consists of an upper electromagnet that has been provided with two windings one primary and the other secondary. Primary is connected to a current transformer in the line which is under protection and is provided with eight tappings. These tappings are connected to a plug setting bridge by which the number of turns to be used can be adjusted in order to have the desired current setting. The second winding called secondary is energized by the induction effect and is wound over the central limb of the upper magnet as well as it is spread over the two limbs of the lower magnet. By this method, the leakage flux from the upper magnet entering the disc have been displaced in phase from the flux entering the disc from the lower magnet. The deflecting torque is produced on the disc in the fashion as already explained. The spindle of the disc carries a moving contact which bridges two fixed contacts after the disc has rotated through a certain angle which has been set before. Any setting for this angle is possible varying from 0 to 360°. The variation of this angle imparts to the relay, various time settings.
The speed of rotation of the disc is dependent upon the torque which in turn is dependent on the current setting, when the load current increases from this setting it will increase the speed of rotation of the disc resulting into decrease of operation time. Thus the time current characteristics of the relay observe inverse-Square law. The definite minimum time characteristics of the relay are obtained by the use of a saturated upper magnet. This ensures that there is no further increase in f1ux when the current has reached a certain value and any further increase of current will not affect the relay operation. This results in a flattened current time characteristic and the relay obtains its name as Inverse definite minimum time lag (I.D.M.T.) relay
The current time characteristics of the relay have been illustrated in Fig. 27. It represents the time required to close the trip contacts for different values of over current. Its horizontal scale is marked in terms of current-setting multipliers i.e. number of times the relay current is in excess of current setting