Doubly-fed electric machines (i.e., electric motors or electric generators) belong to a category of electric machines that incorporate two multiphase winding sets of similar power rating that have independent means of excitation. As a result, doubly-fed electric machines are synchronous electric machines by nature but with both winding sets actively participating in the energy conversion process (i.e., doubly-fed or dual armature). 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.