Running DC Motors in Series

[Richard McClellan]

I recently finished building a three-wheeled robot using several left over parts from build season. It has two large driven wheels and a caster in the back. Each driven wheel is powered by two CIM motors, each going through a Dewalt gearboxes, and then both linked through sprocket and chain to the wheel output shaft.

In order to control the 4 CIMs, I did not have any spare 12V Victor 884's lying around, but I did have two 24V Victor 885's. The CIM motors from each side are wired in series off of the Victor 885. This setup has worked fairly well, and I haven't had any major issues with it, but I am wondering if there are any disadvantages to running them in series as opposed to each motor having its own motor controller, such as a loss in total power output.

In a standard DC motor, the brushes are always in contact with either 1 or 2 sets of windings. So, in the case of each motor powering one of its windings, the resistance is equal, and each motor should be getting 12V. The same is true if each motor is powering two of their windings. However, if one motor is powering one winding and the other is powering two windings, one motor will get 16V and the other will get 8V. With this setup, would the voltage provided to each motor actually change in this way, and could it be at all damaging to the motor or speed controller?