A temporary component of current and voltage in an electric circuit which has been disturbed. In ordinary circuit problems, a stabilized condition of the circuit is assumed and steady-state values of current and voltage are sufficient. However, it often becomes important to know what occurs during the transition period following a circuit disturbance until the steady-state condition is reached. Transients occur only in circuits containing inductance or capacitance. In general, transients accompany any change in the amount or form of energy stored in the circuit.
The study of transient phenomena is very broad. The mathematical requirements become severe and go far beyond the borders of all known mathematics. Transient analysis often requires the use of calculating machines, models, and tests. Fourier and Laplace transforms have proven indispensable in the modern treatment of transients and these disciplines need be mastered by anyone going far in the study of transients. The analysis of lumped-parameter circuits is comparatively easy. An electric circuit or system under steady-state conditions of constant, or cyclic, applied voltages or currents is in a state of equilibrium. How ever, the circuit conditions of voltage, current, or frequency may change or be disturbed. Also, circuit elements may be switched in or out of the circuit. Any change of circuit condition or circuit elements causes a transient readjustment of voltages and currents from the initial state of equilibrium to the final state of equilibrium. In a sense the transient may be regarded as superimposed on the final steady state, so that Eq. (1) applies.
(Instantaneous condition) = (final condition) + (transient terms) [Eqn. 1]
Furthermore, since the instantaneous condition at the first instant of disturbance (time zero) must be the initial condition, it may be described by Eq. (2).
(Initial condition) = (final condition) + (transient terms at time zero) [Eqn. 2]
In many practical cases transients are one of three types:
1. Single-energy transients, in which only one form of energy storage (either electromagnetic or electrostatic) is present; the transient exhibits simple exponential decay from the initial to the final conditions.
2. Double-energy transients, in which both forms of energy storage are present; the transient is either aperiodic or a damped sinusoid.
3. Combination of 1 and 2.