A-6
MIGII Digital Protection for Electrical Machines
GEK-113188A
A.5 THERMAL CURVE
APPENDIX A
A
A.5THERMAL CURVE
The time needed for an element to rise its temperature (from an initial temperature
θ
0
) the 63% of
θ
(
θ
is the temperature
difference between the initial and the final temperature) is called “Time Constant”, and it is represented by the letter
τ
.
Using an equation, it is the time it takes to reach an intermediate temperature
θ
i where:
If
θ
0
is the temperature origin, at a given time, the temperature can be written as:
Where:
θ
Temperature increase at a given time
θ
N
Rated temperature (temperature reached if I = IN)
IN
Protected element rated current
T
Time
τ
Time constant
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