A lot of literature on bus bar design suggests that bus bars should be made to overlap or have equal area such as this.

The reason they mention is the total stray inductance falls due to increase in mutual inductance.

Bus bars represented by parallel conducting plates:

Bus Bars represented by parallel conducting plates

If top plate represents the top DC bus and bottom plate represents the bottom DC bus, the total inductance according to the paper is:

$$ L_{tot} = 2 \times (L_{self} - L_{mutual}) $$

Where \$ L_{tot} = Total inductance of bus bar, \$

\$ L_{self} = Self inductance of bus bar, \$ \$ L_{mutual} = Mutual inductance between top and bottom plates \$

But if currents are flowing in opposite directions are, should the formula not be:

$$ L_{tot} = 2 \times (L_{self} + L_{mutual}) $$ by right hand thumb rule - but then increasing overlap area and increasing mutual inductance would not make sense, yet that is how most bus bars for inverters are engineered.

Can someone please tell me what is the correct explanation of this?

  • \$\begingroup\$ To minimize inductance. \$\endgroup\$
    – winny
    Sep 14 at 7:33
  • \$\begingroup\$ I guess the equation already takes into account that currents flow in opposite directions, you can see it on depiction. \$\endgroup\$ Sep 14 at 9:47

The magnetic field lines surrounding the upper plate with positive current flowing into the page will be flowing from left to right above the conductor and right to left below that conductor as per the right-hand rule below: -

enter image description here

Image from here.

The field lines due to current travelling in the opposite direction due to the current in the lower plate will cancel out those upper-plate field lines outside the insulation layer. This means the inductance is reduced and reduces more as the distance between upper plate and lower plate get closer.

  • \$\begingroup\$ Th field lines due to lower plate will also go from right to left. The would not cancel rather will aid the flux in the insulation region will increase and hence mutual inductance would also increase. \$\endgroup\$ Sep 14 at 11:14
  • \$\begingroup\$ @SambeetPanigrahi oops a typo I meant to say outside the insulation layer and given that the overall external field is reduced the nett inductance also reduces. Yes they concentrate in the insulation layer. \$\endgroup\$
    – Andy aka
    Sep 14 at 11:28
  • \$\begingroup\$ As the two plates come closer the leakage inductance reduces and the mutual inductance tends to be the value of the inductance of each plate. \$\endgroup\$
    – Andy aka
    Sep 14 at 11:36

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