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The time dependent inductance variation is an essential ingredient in the design of magneto-cumulative generator. In most theoretical works, the inductance is modeled by a simple exponential form as a function of time. It is widely accepted that it is proper approximation for the time dependent inductance due to the constant expansion velocity of the liner during the explosion process. In this simple model, however, the magnetic field compression is not correctly treated. Since the inductance is related with the magnetic energy, which is determined by the magnetic flux density, the apposite consideration of the compression of the magnetic field is required. In order to include the effect of the compression of the magnetic field properly, we perform the finite element method calculation for the time dependent inductances in MCG during the explosion process. We find that the time dependent inductance can not be modeled with simple exponential model. Especially, the time derivative of the inductance shows more pronounces difference with the simple exponential model. In order to confirm the calculation results, we design mimic-explosion experiments in order to measure the inductances during the explosion process. It is revealed that the measurement data is well matched with the calculated one.