The variable frequency compressor does not stop after starting, but the speed will change. The non variable frequency compressor will stop after starting and the speed will not change.
The control panel of a variable frequency machine is different from that of a fixed frequency machine. Variable frequency machines generally have a percentage of the current load rate, while fixed frequency machines only have information on loading and unloading.
Is there a frequency converter in the electrical control box of the air compressor. Is the set pressure motor running continuously variable speed.