I usually design circuits with the help of the excellent free simulator from Linear Technology, called LTSpice. But I have been mostly designing simple things like DC-DC converters and PWM drives with static resistive and/or inductive loads. But I have found a model for a DC motor and it may help with certain phenomena such as transients and unexpected waveforms. I found this on http://ecee.colorado.edu/~ecen2830/exp/exp2/index.html
Files can be found on http://www.enginuitysystems.com/pix/
Files can be found on http://www.enginuitysystems.com/pix/