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What is IDMT relay how it works.
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Answer Posted / venu
The over load inverse time relay . 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 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
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