All electric machines are categorized as either Singly-Fed with one winding set that actively participates in the energy conversion process or Doubly-Fed. Although sometimes described as doubly-fed, the wound-rotor induction machine (slip-energy recovery) and the field-excited synchronous machine are singly-fed machines because only one winding set actively participates in the energy conversion process.
Features of doubly fed machines
Uniquely, doubly-fed electric machines can operate at constant torque to twice synchronous speed for a given frequency of excitation with each active winding set rated at half the total power of the machine (i.e., contiguous operation between sub-synchronous through super-synchronous speed range).
The sum of the power ratings of the multiphase winding sets determine the total electro-mechanical conversion power rating of the machine.
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