U = 100;
ω = 314;
L =  .1;
proud = (U/(I*ω*L + R));
ParametricPlot[{-Im[proud], Re[proud]}, {R, 0, 1000}, 
      PlotPoints -> 500, PlotRange -> All, 
      AxesLabel -> {"-Im[i]", "Re[I]"}]