Scope blue trace is FET drain voltage,
Scope Red Trace is Hall Effect output (after transistor)
Scope Green trace is coil current with current probe
Coil is made up of 16 turns (bi-filer) of AWG #25 wire (0.5mm, 0.02") wire diam, avg 18mm coil diam. Coil power supply, 20V, 20A C.C setting.
Sometimes the ball sheds just its red skin, other times it sheds is nickel/copper skin, and the unlucky ones turn to chum, poor little bastards! Sometimes it gives off a flash of light or sparks, I believe this is the friction when the ball disintegrates (explodes). I am putting the 3mm ball magnet in a nylon standoff that has been drilled to allow the ball to drop in, then another nylon screw to hold it in place (with some free play).
So the measured frequency is the ball's revolutions per second (RPS), multiply by 60 to get RPM.
63,480Hz (RPS) x 60 = 3,808,800 RPM
So one revolution takes: 63,480Hz = 15.75us (15.75 microseconds or 0.000,015,75 second)
Over a few months of tuning/testing, I have made many improvements on the system , I went from 100Krpm to 3.8Mrpm, I found the limit of my system is the tensile strength of the balls. 5.0mm balls only got me to 1.1Mrpm, so I switched to 3.0mm balls. I spent a total of $30 at this point in time.
Later I updated circuit/hardware/etc (more $) and tried 2.5mm balls, but disappointing they only got to ~2.2Mrpm, but they gave a good show, haha. Read the video description for a ton of additional info: