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Language:enStatus: Standard
Term: time constant
Symbol: τ
Definition: for a quantity growing or decaying exponentially towards a constant value the duration of a time interval, at the end of which the absolute value of the difference between that constant value and the value of the variable quantity has decreased to e–1 times of the absolute value of the difference at the beginning of the interval, where e is the base of natural logarithms

Note 1 to entry: The time constant is the variable quantity τ appearing in the function F( t )=a+b e t τ MathType@MTEF@5@5@+=feaagCart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHXgaruavP1wzZbItLDhis9wBH5garmWu51MyVXgarqqtubsr4rNCHbGeaGqipG0dh9qqWrVepG0dbbL8F4rqqrVepeea0xe9LqFf0xc9q8qqaqFn0lXdHiVcFbIOFHK8Feea0dXdar=Jb9hs0dXdHuk9fr=xfr=xfrpeWZqaaiqaciWacmGadaGadeaabaGaaqaaaOqaaiaadAeadaqadaqaaiaadshaaiaawIcacaGLPaaacqGH9aqpcaWGHbGaey4kaSIaamOyaiabgwSixlaabwgadaahaaWcbeqaaiabgkHiTmaalaaabaGaamiDaaqaaiabes8a0baaaaaaaa@436C@ describing a time-dependent variable quantity.

Note 2 to entry: The time constant is the reciprocal of the damping coefficient δ (IEC 60027-6, Letter symbols to be used in electrical technology – Part 6: Control technology) of a damped oscillation, as well.

Note 3 to entry: In control technology the time constant usually is derived from the step response of a first-order lag element as the duration of the time interval, required to reach 63,2 % [i.e. (1 – e–1)-times] of the steady-state value of the step response.

Note 4 to entry: The time constant is the variable quantity τ appearing in the expression F( t )=a+b e t τ f( t ) MathType@MTEF@5@5@+=feaagCart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHXgaruavP1wzZbItLDhis9wBH5garmWu51MyVXgarqqtubsr4rNCHbGeaGqipG0dh9qqWrVepG0dbbL8F4rqqrVepeea0xe9LqFf0xc9q8qqaqFn0lXdHiVcFbIOFHK8Feea0dXdar=Jb9hs0dXdHuk9fr=xfr=xfrpeWZqaaiqaciWacmGadaGadeaabaGaaqaaaOqaaiaadAeadaqadaqaaiaadshaaiaawIcacaGLPaaacqGH9aqpcaWGHbGaey4kaSIaamOyaiabgwSixlaabwgadaahaaWcbeqaaiabgkHiTmaalaaabaGaamiDaaqaaiabes8a0baaaaGccqGHflY1caWGMbWaaeWaaeaacaWG0baacaGLOaGaayzkaaaaaa@492D@ of an exponentially damped oscillation, where f(t) is a sinusoidal function.

Note 5 to entry: This entry was numbered 351-24-24 in IEC 60050-351:2006.

Publication date:2013-11
Source:IEC 60050-111:1996,111-13-06, modified – The wording of the definition is slightly different and several notes to entry have been added to adapt to usage in control technology.
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