A transient electric analysis determines the effects of time-dependent current or voltage excitation in electric devices. In this analysis, the time-varying electric and magnetic fields are uncoupled, and the electromagnetic field can be treated as quasistatic. Eddy currents are considered to be negligible, and the electric field is derived from the electric scalar potential. A transient electric analysis is assumed to be linear. Refer to Electromagnetics in the Mechanical APDL Theory Reference for more information.
You can use this analysis to determine the voltage, electric field, and electric current density distributions in an electric device as a function of time in response to time-dependent loading. The time scale of the loading is such that the capacitive effects and displacement current are considered to be important. It they are not important, you might be able to use a steady-state current conduction analysis instead.
The procedure for doing a transient quasistatic analysis consists of three main steps:
The next few topics discuss what you must do to perform these steps.
To build the model, you first specify a jobname and a title for your analysis as described in Steady-State Current Conduction Analysis. If you are using the GUI, set preferences for an electric analysis and then use the preprocessor (PREP7) to define the element types, the material properties, and the model geometry.
You can use the following types of elements in a transient electric analysis:
The default system of units is MKS. In the MKS system of units, free-space permittivity is set to 8.85e-12 Farads/meter. To specify your own system of units and free-space permittivity use one of the following:
To model resistive and capacitive effects, a transient electric analysis requires the specification of electrical resistivity and electric permittivity, respectively. Define electrical resistivity values as RSVX, RSVY, and RSVZ on the MP command. Define relative electric permittivity values as PERX, PERY, and PERZ on the MP command. These properties may be constant or temperature dependent.
In this step, you define the analysis type and options, apply loads to the model, specify load step options, and initiate the finite element solution. The next few topics explain how to perform the following tasks:
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.
If you want to restart a previous analysis (for example, to specify additional loads), issue the command ANTYPE,TRANSIENT,REST. You can restart an analysis only if you previously completed a transient analysis, and the files Jobname.emat, Jobname.esav, and Jobname.db from the previous run are available.
Next, you define which solution method and which solver you want to use. Transient electric analyses require the full solution method. To select a solution method, use one of the following:
You can use the sparse solver (default), the Jacobi Conjugate Gradient (JCG) solver, the Incomplete Cholesky Conjugate Gradient (ICCG) solver, or the Preconditioned Conjugate Gradient solver (PCG). To select an equation solver, use one of the following:
To specify the time at the end of a load step, use any of the following:
The integration time step is the time increment used in the time integration scheme. It determines the accuracy of your solution. The smaller the time step size, the higher the accuracy. The size of the first integration time step following any large step change in loading conditions is especially critical. You can reduce inaccuracies by reducing the integration time step size. You can specify it directly via the DELTIM command or indirectly via the NSUBST command.
When specifying multiple substeps within a load step, you need to indicate whether the loads are to be ramped or stepped. The KBC command is used for this purpose: KBC,0 indicates ramped loads (default), and KBC,1 indicates stepped loads.
To specify results data for the printed output file (Jobname.out), use one of the following:
You can also control the solution items sent to the results file (Jobname.rth). By default, the program writes only the last substep of each load step to the results file. If you want all substeps (that is, the solution at all time substeps) on the results file, use one of the following to specify a frequency or ALL or 1.
You can apply loads in a transient analysis either on the solid model (keypoints, lines, and areas) or on the finite element model (nodes and elements). You can specify current and voltage loads. The procedures and GUI paths you use to apply these loads are identical to those described in Steady-State Current Conduction Analysis.
You can also apply current and voltage loads using the independent current and voltage source options of CIRCU124. For more information, refer to Electric Circuit Analysis.
In this step, you initiate the solution for all load steps using one of the following:
The program writes results from a transient electric analysis to the results file, Jobname.rth. Results include the data listed below:
Primary data: Nodal DOF (VOLT).
Derived data:
Nodal electric field (EFX, EFY, EFZ, EFSUM).
Nodal conduction current densities (JCX, JCY, JCZ, JCSUM).
Element current densities (JSX, JSY, JSZ, JSSUM). This output item represents the total (that is, the sum of conduction and displacement current densities). It can be used as a source for a subsequent magnetic analysis.
Element conduction current densities (or total measurable current density) (JTX, JTY, JTZ, JTSUM).
Element Joule heat generation rate per unit volume (JHEAT).
Element stored electric energy (SENE).
Nodal reaction currents.
You can review analysis results in POST1, the general postprocessor, or in POST26, the time-history postprocessor. To access the general postprocessor, choose one of the following:
To access the time-history postprocessor, choose one of the following:
For a complete description of all postprocessing functions, see The General Postprocessor (POST1) and The Time-History Postprocessor (POST26) in the Basic Analysis Guide.
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 procedures for reviewing POST1 transient electric analysis results are identical to the procedures described in Steady-State Current Conduction Analysis.
To review results in POST26, the time-history postprocessor, the database must contain the same model for which the solution was calculated, and the Jobname.rth file (the results file) must be available. If the model is not in the database, restore it using one of the following:
The procedures for reviewing POST26 transient electric analysis results are identical to the procedures described in Harmonic Quasistatic Electric Analysis. Variable number 1 is reserved for time instead of frequency.