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# inquiries about magnetostatic FEM simulation

## inquiries about magnetostatic FEM simulation

(OP)
Dear friends: my name is andres felipe zuñiga I am from Cali, Colombia. I am simulating shortcircuit forces in transformers and busbars using FEMM software. I am analyzing the results from a scientific paper but I have differences with my results.

I am simulating a three phase busbar disposition with magnetostatic module

the dimensions of the busbar are:
width 6mm
height 75mm
long 1000mm
separation among phase bars 7.12mm

current densities in t=0s
current density phase A=77.66A/mm^2
current density phase B=-38.83A/mm^2
current density phase C=-38.83A/mm^2

maximun lorentz force in t=0s
Phase A=-7524N
Phase B=6120N
PhaseC=1405N

current densities in t=3.333ms
current density phase A=38.83A/mm^2
current density phase B=38.83A/mm^2
current density phase C=-77.66A/mm^2

maximun lorentz force in t=3.333ms
Phase A=-1405N
Phase B=-6120N
PhaseC=7524N

results reported by article

maximun lorentz force:

Phase A=6578N appearing in t=0s
Phase B= 7051N appearing in t=3.333ms
Phase C=6578N appearing in t=3.333ms

The author in the article performed a time step simulation with f=50hz using Opera Software
I am performed a magnetostatic simulation and comparing the results by time instant using FEMM Software

in theory, the maximum shortcircuit forces are obtained in middle phase conductor (PhaseB), anybody can advice me about what is wrong in this simulation?

thanks a lot

### RE: inquiries about magnetostatic FEM simulation

I'm sure someone can help you if you actually ask a technical question, and you are not doing school work.

### RE: inquiries about magnetostatic FEM simulation

I do not understand how you can have any time factor in a magnetostatic solution. By definition it is steady state (time invariant).

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

### RE: inquiries about magnetostatic FEM simulation

#### Quote:

I do not understand how you can have any time factor in a magnetostatic solution. By definition it is steady state (time invariant).
Magnetostatic strictly speaking applies to dc. But for power frequency calculations of things like magnetic force, the magneto-quasi static approximation is almost always well justified. This simply considers the field as purely magnetic and neglects the electric field / capacitive effects and associated wave behavior. If current vs time profile is known (even though varying at sufficiently-low frequencies) then the instantaneous force at each time can be calculated with this (magneto-quasistatic) approach based on the instantaneous current at that time. Since the analysis at an instant in time is exactly equivalent to magnetostatic analysis of dc, it is often simply referred to as magnetostatic (drop the quasi).

I am not well versed in principles and standard assumptions for short circuit force analysis, but it strikes me that the actual inrush current waveforms for the three phases could be a challenge to predict. This is where the time-stepping simulation was probably required. But if you start with those currents specified (from a time stepping simulation), then you should be able to predict associated magnetic forces vs time from the currents vs time and from geometry without consideration of dynamics (magneto quasi static analysis).

OP - When you did your analysis, you should have set up current A in the opposite direction from B and C. (did you do that?)

=====================================
(2B)+(2B)' ?

### RE: inquiries about magnetostatic FEM simulation

(OP)

#### Quote (electricpete)

dear friend, yes i did it.

Your assumption is correct, always the sum of the instantaneous values of short circuit current is equal to zero. I have the functions that describe the short circuit current per phase, so with the magnetostatic simulation I run the cases in t=0 and t=3,333ms (where in theory the short circuit forces are maximums for each phase), Keeping the equilibrium in the sum of currents per phase in each case, but I Cant obtain the maximum force in the middle phase conductor.

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