The spindle inverters are what provides power and control to high frequency spindles (typically made by HSD, Columbo, and on older machines, Perske).

Porter cable, Kress and Dewalt spindles do not have inverters.

If you are unsure of what type of spindle you have, follow the guide on Finding your spindle

What follows are common inverters over the years, listed in order of recently employed by techno:

Altivar series inverter: (Used on Techno Routers since ~2008) Note that this display is showing rdy, indicating no problems, and that it is ready for use.

Teco (may also bear the name motortronics) Model FM100 inverter: (Used on Techno routers prior to 2008)