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Alternating Current (AC) comes in waves and the waves are called Hertz or Hz. Standard power that gets delivered to households is either 50hz or 60hz depending on the region of the world. This means that every second an electrical wave hits the high point 50 or 60 times a second.
https://www.fluke.com/en-us/learn/blog/electrical/what-is-frequency
A Variable Frequency Drive or VFD is a mechanism more broadly defined as a phase converter and is used to convert single phase power to three phase power which is sometimes needed for industrial machinery, such as the Spindle. The phase lines are combined and offset to get an overall increase in hz in the same amount of time.
https://www.georator.com/three-phase.html
The majority of households don't have access to Three Phase Power natively, because running Three Phase Power is cost prohibitive and generally unnecessary. Three Phase Power is used for industrial machinery and not for lights as an example.
GFCI or GFI are Ground Fault (Circuit) Interrupters are the same thing, and both terms are used interchangeably, and the difference is as it relates to including the circuit breaker or not. A GFI is part of the circuit that is the GFCI. These are commonly used below grade, where water may be present, and garages (NEC 210.8, [refer to your local code if the NEC doesn't apply]). They cost substantially more over standard outlets, and are available in both 15A and 20A, breakers as well as 220v configurations, the visual difference being the horizontal line present on the 20A.
A GFCI will look at the demand downstream (since the load is transferred through it) and when a VFD is plugged in and running the spindle, it will think that the circuit is being overloaded. That is because our 1.5kw motors demand what would be 12A from the VFD. Since it went from a low load to high load in a short amount of time it thinks someone is being electrocuted and kills the link to the power.
More information can be found here:
