The present invention relates generally to tuning the flow of cooling air across converter and inverter heat sinks in a motor drive system. More specifically, present techniques relate to motor drive duct systems including heat sinks with separate, sequential heat sink fin sections disposed in a common cooling air path and having different geometries to optimize the flow of cooling air across and between fins of the separate heat sink fin sections. For example, the heat sink fin sections may have different fin lengths, fin heights, fin counts, fin pitch (e.g., distance between adjacent fins), and so forth. Each of these different geometric characteristics may be tuned to ensure that temperatures and temperature gradients across the heat sinks are maintained within acceptable ranges.