Core saturation

Figure 4-228. Magnetic flux-lines representation in a highly permeable iron alloy core saturated with an auxiliary magnetic field.

Description of the Experiment and Apparatus. The apparatus for this experiment is shown in Figure 8. At the left side of the figure, inside the dashed box, the procedure for core saturation is shown. Initially, the core tested is saturated with 1% acidified brine solution. The saturation of the core is achieved in the following manner. 

Figure 8-4 Signs of Core Saturation in the Push-Pull, with Voltage-Mode Control...

Effect of pressure reduction upon core saturation." Ibid., 1026. Boussinbsq, Joseph... 

Note A slight amount of core saturation may turn out to be acceptable on occasion, especially if it occurs... 

We see that the peak field in our application is within the design limits of the inductor, as expected, so we need not worry about core saturation. This is a basic qualification the inductor must pass before we can proceed with the rest of the analysis. 

The most basic question in design invariably is — what input voltage represents the worst-case point at which we need to start the design of the magnetics (from the viewpoint of core saturation) For the forward converter choke, this should be obvious — as for any buck converter, we need to set its current ripple ratio at around 0.4 at Vinmax- But coming to the transformer, we need some analysis before we can make a proper conclusion. 

Theoretically, there is no need for any air gap in a common mode choke, because the flux due to the line current is expected to cancel out completely. In practice, it doesn t fully, mainly due to slight differences in the individual winding arrangement (despite the equal number of turns). At a minimum, this causes the core to get dc-biased in one direction, and thereby cause an imbalance in the inductance it presents to the two lines. This would expectedly degrade the EMI performance, but in extreme cases, the core may even saturate. Note that core saturation in the filter is clearly not a catastrophic event (like the saturation of the main inductor/transformer of the converter can be), but since it is accompanied by severely worsening EMI-suppression efficacy, we need to prevent that too. Therefore, as in a forward converter transformer, a small air gap is usually present, even in a CM choke. 

So in DM chokes, the major concern is core saturation. But in CM chokes, underestimating the current will make it run excessively hot, due to higher copper losses. 

To put it in perspective, none of these issues have really ever been a show-stopper in any practical design scenario, nor have they allowed us to eventually reduce the size of the power supply. We note that size must ultimately dove-tail with reliability, because if we undersize the core for example, we will certainly cause core saturation and a fair amount of resulting silicon shrapnel in the lab We saw that last month. Now consider the equations ... 

Instead of relying completely on theory for the determination of mobility, most researchers also performed experimental measurements of the quantities of interest. Many of the first experiments on foam were performed with water and gas with the outlet at ambient pressure, and many were simply gas floods of packs or cores saturated with surfactant solution. Although for many such transient experiments, the published data were insufficient for the estimation of the steady-state mobilities required for the estimation of mobility-control effectiveness, this was not true for some of them. Calculated values of mobility and relative mobility were derived by Heller et al. (22), from the data published in six different papers (23—28). The values they found, given in terms of relative mobilities, ranged from 0.001 to 0.6 cP-1, or in terms of effective viscosities from 1000 down to 1.6 cP (1 to 0.0016 Pa-s). Not enough information was available to trace all of the relevant parameters that may have caused these differences. 

Procedure. The sequence of flow experiments for each oil started with the core saturated with the live oil at saturation pressure, 4.83 MPa. The pressure at the inlet end was always maintained at 4.83 MPa. A back-pressure regulator (BPR) was used to maintain constant pressure at the outlet end, and this pressure was reduced in steps of approximately 0.34 MPa, starting from the saturation pressure. The term pressure drawdown will be used in the following discussion to denote the pressure difference between the inlet end and the outlet end of the sand pack. The pressure drawdown reaches its maximum value when the pressure at the outlet end becomes equal to the atmospheric pressure. 

Permeability Reduction in Presence of Oil. Muecke (55) and Sarkar and Sharma (56) examined permeability reduction due to chemical fines migration in Berea core samples saturated with brine and at residual oil saturation. Liu and Civ an (49) modeled permeability impairment due to salinity shock with and without the presence of residual oil. In the first experiment, fresh water was injected into a Berea sandstone core saturated with 3 wt% sodium chloride. In the second experiment, the fresh water was injected into a similar core at residual oil saturation. Figure 4 shows that their model predicted the experimental data fairly... 

The fluids used for core saturation and injection were prepared as explained below. The properties of these fluids are shown in Table II. Note that all the viscosity measurements were made on a Brookfield viscosimeter at 6-RPM (6.57 s ) to preserve the consistency among the measured values,... 

The produced water (PW), which was used for core saturation, was a synthetic simulation of an actual... 

The untreated cores were saturated with brine, whereas the Dri-Film treated cores were saturated with Soltrol in a core saturator. The saturated cores were weighed, and with the help of the known density of the liquid the porosities were calculated. 

Operating voltage in excess of rating may cause core saturation and excessive stray losses, which could result in overheating and excessive noise levels (ANSI/IEEE, C57.94-1982 (R1987), C57.12.01-1989 (R1998)). 

The effective viscosity data from one of the six cores. Core A, are shown as a typical example (Fig. 15). When the polymer solution caused oil production, the water A/> determined at the original core saturation was no longer appropriate for calculating the effective viscosity. A correction step was used to adjust the water Ap to correspond to the new core saturation. The details of the correction method are discussed in the Appendix. The core saturation data, porosities, calculated water permeabilities, and pressure correction factors at each stage of the experiment are contained in Table 4. 

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