Or maybe not; the photos aren't good enough. From 72 to 84 V the frequency hardly moved - 326 Hz.
There is some kind of effect or limit being reached at 72 V or 326 Hz. Mind you, with 43 count in a 26" wheel that would be 35 mph.
Good measurements. You reached the magic 325 Hz limit. When you get close, I noticed the waveform gets squared off as the timing goes out of whack. If you had a few more volts, you could push it into the range where it just jitters.
Both fecther and Tiberius hit this magic figure of 325 or 326 on individual controllers.
Both have BAD spikes in the phase voltages.
So it shows that this is produced "digitally". most likely due to codes or program in the MCU.
For example, 16 bit register would have 65536 levels (0 - 65535) and it could not represent any level higher than the max level.
At 60V FULL throttle, it is 271 Hz.
Phase voltage becomes more square above 60V.
The frequency increases to 326 Hz until 72V.
The frequency remains almost constant at 326Hz until 84V and the phase voltage is "square".
It seems that the PSUs cut off the power if you try to exceed 84V ???
If you are using a real
battery above 72V or 84V, the supply current would be very high and damage the controller.
Did you note the supply current at 60V, 72V and 84V from your PSUs?
The squaring off of phase voltage at supply voltage above 60V is definitely NOT the proper design of 6-STEP controller.
Perhaps there will be no damaging effect if this controller is operating below 271Hz where the conduction period of the phase voltage remains 120 deg.