A transient electric circuit analysis analyzes an electric circuit subjected to time-varying source voltage or currents. The analysis determines the voltage and current distribution in an electric circuit as a function of time. Transient circuit analyses allow all circuit components.
To do this, you simply follow the procedure described in Building a Circuit. You will need to specify the following loads in the Circuit Builder as real constants:
Source loads for independent current and voltage sources.
Initial conditions, such as the initial current in inductors and the initial charge in capacitors.
Wave form loads, which can be sinusoidal, pulse, exponential, or piece-wise linear. (For details, see the CIRCU124 element description in the Element Reference.)
One you have built the circuit, you apply loads to and solve the model and then review the results. Other points to remember when doing a transient circuit analysis are:
Circuits in a transient analysis do not use an automatic time-stepping feature. However, you can use invoking automatic time-stepping to ramp the time step. (Doing this increase the time step by 3X each time point until the maximum time step is reached.)
During a transient solution, you can change real constants. Although this is accurate only when used together with the restart procedure, it is useful for simulating a switch with a resistor.
For processing transient results, the Mechanical APDL GUI has a special section for circuit elements. This enables you to process nodal voltages and currents and element-specific results.
You need to define the analysis type and options, apply loads, and initiate the finite element solution. To accomplish these tasks, follow the steps listed below.
To enter the SOLUTION processor, use either of the following:
To specify the analysis type, do either of the following:
In the GUI, choose menu path and choose a Transient analysis.
If this is a new analysis, issue the command ANTYPE,TRANSIENT,NEW.
You can restart (continue) a transient analysis from a previously completed transient run, and you can modify circuit component real constants. However, the Jobname.emat, Jobname.esav, and Jobname.db files must be available to restart an analysis.
To choose an equation solver, use either of the following:
Only the sparse solver is available for problems using the CIRCU124 element. Even if you choose a different solver, the sparse solver is used when CIRCU124 elements are present.
Normally, you specify source loads for circuits in the Circuit Builder, as element real constants. Besides the source loads, the only other "load" is a VOLT = 0 specification at the ground nodes. To specify VOLT = 0, use either of the following:
Should you need to modify the source loads, use one of the following:
To include transient effects in the analysis, you must turn time integration effects on or else a static solution will be performed. By default, time integration effects are on in a transient analysis, but you can turn them off to obtain a static solution.
To turn time integration effects on, use one of the following:
You can specify three general options: time, the integration time step, and automatic time stepping.
To specify the time at the end of a load step, use one of the following:
The integration time step is the time increment used in the time integration scheme. The time step size determines the accuracy of the solution: the smaller the value, the higher the accuracy. To specify time step size, use either of the following:
Use the DTIME field to specify initial time step size, the DTMIN field to specify minimum time step size, and DTMAX to specify maximum time step size.
Although in circuit analysis you cannot use automatic time stepping to increase or decrease the time step size automatically, you can use it to increase from an initial time step size to a final time step size. A factor of 3 is applied each time step to the time increment until the maximum time step size is reached.
To turn automatic time stepping on, use one of the following:
In cases where sharp changes exist in the source load wave form, you should break a transient analysis into several load steps. At the time when these sharp transitions take place, specify a new load step and re-specify the time step options if necessary to get a small initial time step at the transition point.
You can include any results data in the printed output file (Jobname.out). To use this option, choose either of the following:
By default, the printed output consists of just summary messages.
To control what data goes to the results file (Jobname.rth), choose either of the following:
Note: By default, the program writes only the last substep of each load step
to the results file. If you want all substeps included on the results
file, set the FREQ field to ALL or 1.
You must write the each load step to a load step file. Repeat the loading, load step options, and load step file writing operations until all load steps have been specified.
To write a load step file, use one of the following:
Use either the SAVE command or the SAVE_DB button on the Toolbar to save a backup copy of the database.
To start calculating the solution, use one of the following:
To leave the SOLUTION processor, use one of the following:
The program writes results from a transient circuit analysis to the results file, Jobname.rth. The results consist of two types of primary data: nodal voltages (VOLT) and nodal currents (CURR). The following derived data calculated for each element also are available:
Element voltage drop (VOLTAGE)
Element current (CURRENT)
Element control voltage (CONTROL VOLT)
Element control current (CONTROL CURR)
Element power (POWER)
Element applied load (SOURCE)
For more information about the derived results, see the Element Reference.
You can review analysis results in either POST1, the general postprocessor, or POST26, the time-history postprocessor. POST1 allows you to review results over the entire model at specific time points. POST26 allows you to review results at specific points in the model over the entire transient.
To use the time-history postprocessor, use either of the following:
To review results in POST26, the database must contain the same model for which the solution was calculated. Also, the results file (Jobname.rth) must be available.
To read the data from the results file into the database, use either of the following:
POST26 works with tables of result items, known as variables, versus time. Each variable is assigned a reference number, with variable number 1 reserved for time. The first thing you do, then, is define the variables using the commands or menu paths shown below.
To define variables for primary data:
To define variables for element data:
To define variables for reaction data:
Once you have defined the variables, you can graph them versus time or any other variable. To do so, use either of the following:
To list just the extreme variable values, use either of the following:
By reviewing the time-history results at strategic points throughout the model, you can identify the critical time points for further postprocessing in POST1.
To review results in POST1, the database must contain the same model for which the solution was calculated. Also, the results file (Jobname.rth) must be available.
The first step in using POST1 is to read in results at the desired time point.
To do so, use the TIME field on the
SET command or its equivalent menu path, . If you specify a
time value for which no results are available, the program does linear
interpolation to calculate the results at that time. The program uses the last
time point if the time specified is beyond the time span of the transient. (You
can also identify the results to be read by their load step and substep
numbers.)
Once the database contains the desired set of results, use one of the following to list the primary nodal data (voltages and currents):
To list the derived element results, use one of the following: