We have a 32kW 400V WRM starter via 1 step Resistor. Once started a VFD is used to control Speed. There is excessive sparking on the sliprings when exceeding 90% rated speed.
If a VFD is not seeing an inductive load then the o/p voltage and current will not necessarily be sinusoidal. The sparking rather confirms that.
The start up resister is rather redundant and do not need to have any switchgear between VFD and motor. The exception to this is the local isolator.