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==Construction and operation==
A variable-frequency transformer is a [[doubly fed electric machine]] resembling a vertical shaft [[hydroelectric]] generator with a [[three-phase power|three-phase]] wound [[Rotor (electric)|rotor]], connected by [[slip ring]]s to one external power circuit. A [[direct-current]] torque motor is mounted on the same shaft. Changing the direction of torque applied to the shaft changes the direction of power flow; with no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and [[stator]].
The variable-frequency transformer behaves as a continuously adjustable [[quadrature booster|phase-shifting transformer]]. It allows control of the power flow between two networks. Unlike power electronics solutions such as back-to-back [[high-voltage direct current|HVDC]], the variable frequency transformer does not demand harmonic filters and reactive power compensation. Limitations of the concept are the [[current-carrying capacity]] of the slip rings for the rotor winding.
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