>It is highly unlikely that the engineers who designed the radio did not know how to calculate a simple series voltage drop.<
You would think so. But even the best engineers can get things wrong.
Agreed, but please consider that you are arriving at a conclusion based on one single data point of evidence. Unless and until we have confirmed observation by a goodly amount of other owners of the same rig reporting the same exact problem, you are basing that conclusion on a statistically insignificant sample.
You are also basing it on a fan that you have already explained to us that you opened up and replaced at least one component, cleaned and lubed bearings, possibly other things could have happened in disassembly/reassembly that can often go unnoticed, even among the best and most experienced of techs, it happens.
The manufacturers' data for running current and motor operating voltage is as I quoted. My example is actually well within the specification for running current.
This is another assumption on your part, in the sense that just because the motor circuit is drawing LESS than the published spec, it must be fine.
Suggest looking at inrush current draw at startup, but it is not likely to be able to find that in the rig's Service Manual specs.
I'd likely try comparing inrush to another known good or new fan of same basic type, or better yet, two or three samples. At the same time I'd also be timing the startup cycle comparisons in an effort to isolate a mean spinup time for such.
I mentioned earlier that perhaps one of the coils suffered internal shorts. It also could be a situation where one of the coils is OPEN or suffering from a cold connection, dry joint, etc.
In any case, I wouldn't blame the design as being at fault simply due to the fact that all observations to this point have been made with only the one user-modified fan unit in place. Given the total number of units same model and type out there in the world at this point, Occam points the finger at the fan first.