A variable phase inverter was a type of power converter used by the Federation in at least the 24th century.
In 2365, Data and Geordi La Forge used one to help the USS Enterprise-D power up a version of the El-Baz shuttlecraft which came from six hours in their personal future. Inverters such as the one the two Starfleet officers were using had settings in terms of percentages. La Forge first adjusted the inverter "2% positive", then "2% negative" in order to coax the shuttle into working. According to the chief engineer, the positive setting should have been the correct one, but the shuttle only powered up after adjusting the power to "2% negative". Aside from the obvious implication that the two settings were the opposite of each other, it wasn't clear what the percentages were measuring. (TNG: "Time Squared")