MEP 1523 ELECTRICAL DRIVES Current ripple in unipolar and bipolar switching schemes

Current ripple – unavoidable in power electronic converter systems: Current ripple Undesirable because: Zero average – increase machine heating Ripple in torque – can be reflected in speed response Ratings of devices must consider ripple - higher rating of peak current

Current ripple H-bridge dc-dc converter with dc machine as load

Current ripple Approximate dc machine with RL load H-bridge dc-dc converter with dc machine as load

Current ripple Load is linear Principle of Superposition can be applied V dc V ave V dc V ave Unipolar Bipolar

Current ripple V dc V ave Unipolar

Current ripple V dc V ave Unipolar V ave v(t) = V ave + v ripple + i(t) = I ave + i ripple V dc  V ave

Current ripple - unipolar i(t) = I ave + i ripple = + i(t) I ave i ripple Switching frequency is high - Impedance of AC component dominated by L - Ripple is calculated based on v-i relation of L - L appear as short circuit in DC

Current ripple - unipolar i ripple

Current ripple - unipolar i ripple t V dc V dc -V ave T tri T Td AB ii +vL+vL vLvL i ripple

Current ripple - unipolar but V ave = d ab V dc, maximum when d ab = 0.5

It can be shown that for bipolar scheme, Max current ripple in bipolar scheme is four times that of unipolar scheme

Current ripple - bipolar V dc V ave Bipolar V ave v(t) = V ave + v ripple + i(t) = I ave + i ripple V dc  V ave -V dc

Current ripple - bipolar i ripple t 2V dc V dc -V ave T tri T tri d AB ii +vL+vL vLvL i ripple

Current ripple - bipolar but V ave = (2d ab  1)V dc, maximum when d ab = 0.5

Current ripple - bipolar Max ripple for bipolarMax ripple for unipolar