There you go, the controller should dynamically adjust it's on-time to maximize efficiency in the system. The iron and copper in the armature are acting like the inductor in a switching power supply. Those laminations are typically very inefficient at high frequencies, which can result in heating and loss. At the same time, you don't want the field to reach anywhere near steady state current, so the switching frequency can't be too low either.
I've seen some power supply circuits that measure the dI/dt to figure out the switching point.
I guess this idea should be a new thread.