IEVref: | 351-45-32 | ID: | |

Language: | en | Status: Standard | |

Term: | time constant | ||

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Symbol: | τ
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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 logarithmsNote 1 to entry: The time constant is the variable quantity Note 2 to entry: The time constant is the reciprocal of the damping coefficient 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 Note 4 to entry: The time constant is the variable quantity 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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Note 1 to entry: The time constant is the variable quantity *τ* appearing in the function $F\left(t\right)=a+b\cdot {\text{e}}^{-\frac{t}{\tau}}$ 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 $F\left(t\right)=a+b\cdot {\text{e}}^{-\frac{t}{\tau}}\cdot f\left(t\right)$ 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.