Running speed control with FOC using the X-NUCLEO IHM08M, what causes the 0.1 Hz "step-wave" on top of the DC bus ?

[c1ron]

If I measure the DC supply voltage for my X-NUCLEO-IHM08M driver board - connected to a Teknic motor (i.e. Hudson brushless servomotor), running the ST FOC SDK with speed control, we see that there seems to be some "charge-discharge" low-frequency square-wave on top of the DC voltage. Please see the plot. The DC voltage is 30 V.

The speed reference is zero. The segment "Start motor" is an interval where the motor is in RUN state. Otherwise it is in IDLE state.

What causes this "disturbance" ?

Is this unusual ?