Singly-fed electric machine
Singly-fed electric machines (i.e., electric motors or electric generators) belong to a category of electric machines that incorporate one multiphase winding set, which is independently excited, actively participates in the energy conversion process (i.e., is singly-fed), and determines the full electro-mechanical conversion power rating of the machine.
Singly-fed electric machines operate under either asynchronous (i.e., induction) or synchronous principles with a constant torque speed range up to synchronous speed .
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 Lorentz Relation.
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 Lorentz Relation. Common examples of an asynchronous singly-fed electric machine is 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.
Startup torque
Only induction motors produce useful startup torque,[citation needed] 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.
In the nomenclature promoted by one company, [1] electric machines are categorized as either singly-fed electric machines or doubly-fed electric machines. The rest of the industry does not use those definitions.
References
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