Singly fed electric machine
Singly fed electric machines (electric motors or electric generators) belong to a category of electric machines that incorporate only one multiphase winding set that is independently excited and as a result, actively participates in the energy conversion process and determines the full electro-mechanical conversion power rating of the machine.
Asynchronous machines operate by a nominal slip between the stator active winding set and the rotating winding. The slip induces a current on the rotating winding according to Faraday's Law, which is always synchronized with the rotating magnetic field in the air-gap for useful torque production according to the Lorentz Relation. Without an independent electrical port to the rotor multiphase winding set, the rotor consumes real estate without actively participating in the energy conversion process (i.e., singly-fed).
In contrast, synchronous machines mechanically rotate a permanent magnet assembly or an electromagnet (with sliprings) assembly in synchronism with the current in the active winding set for useful torque production according to the Lorentz Relation. Common examples of an asynchronous singly fed electric machine are the squirrel cage induction motor or the wound-rotor induction motor. Common examples of the synchronous singly fed electric machine are the permanent magnet brushless DC motor and the field-excited synchronous motor. Like permanent magnets without a electrical power port, the DC electromagnet of the classical synchronous electric machine with slip-rings is a passive device and as a result, the rotor consumes real estate without actively participating in the energy conversion process (i.e., singly-fed). Although commonly the case, a synchronous electric machine with a DC field winding and slip-ring assembly should not be confused with the doubly fed electric machine.
Only induction motors produce useful startup torque, although very inefficiently. The most primitive synchronous electric machines (excited by a constant frequency at a constant voltage) do not produce useful startup torque and must incorporate other means for practical startup operation, such as electronic control or another induction motor in tandem. Electronic control of either induction or synchronous singly fed electric machines for variable speed, improved efficiency, and optimum performance is becoming common. The principal purpose of the squirrel-cage rotor winding or wound rotor winding of the induction motor or the rotating field winding of the field-excited synchronous electric machine is to develop a rotating magnetic field in the air gap. The electrical power contribution of these windings is dissipative and accordingly, these windings do not actively participate in the energy conversion process (i.e., they are "passive windings"). However, the field winding of the field-excited synchronous machine is commonly replaced with a permanent magnet assembly in many applications (i.e., brushless DC motor).
The wound induction machine has windings on both rotor and stator that exhibit electrical loss for a given power rating but only one winding set actively participates in the energy conversion process.
In contrast, the permanent magnet synchronous electric machine has one winding set, the active winding set, that exhibits electrical loss for a given power rating. As a result, the permanent magnet synchronous electric machine is more efficient than the induction electric machine for a given air-gap flux density. By far, the most commonly applied electric motor or generator belongs to the singly fed category.