While this might be clear in your mind, to us not so much.

I think this is something on the rotor.

Not sure why just checking to see if the diodes meet their specifications isn't enough.

The general method is to bench test the component(s) (i.e. test when not rotating) - diodes, thyristors, etc. prior to attachment onto the moving assembly. Once installed and (correctly) wired up, the capability of the individual component is verified by having the rotating assembly operate as required - under all design points.

Note that design points may be a combination of any or all the following: speed, current, voltage, vibration, humidity, and/or temperature. In some cases it may also involve the chemistry of the surrounding medium (e.g. air).

A number of OEMs that do this kind of thing use a dedicated test stand (mandrel to hold the assembly, and an electric motor to drive it at the desired speed) and some mechanical contacts (brush to collector ring, for example) to extract the relevant electrical performance.

Converting energy to motion for more than half a century

This is a double post from "Electric motors & generators engineering"