Features of Electrical rotating machines
All the rotating electrical machines, used for generation purposes, electric drives or for control systems, have many common essential features from the construction point of view. For example, every rotating electrical machine must possess,
(1) stator (stationary member)
(2) rotor (rotating member)
(3) air-gap separating the stator and rotor and
(4) shaft, bearing, foundation etc. In addition to it, every electrical machine usually has
(a) exciting or field winding, which produces the working flux and
(b) armature woinding in which the working e.m.f. is induced by the working flux.
The current in a winding that varies as the machine is loaded is called load current. The current that produces only a working magnetic flux and does not vary with the load on the machine is called magnetizing current, exciting current or field current. The winding on the machine that carries only load current is called armature winding. The winding that handles only exciting current is called field winding. Current in the field winding is always dc. A winding which handles both the exciting current and load current is called the primary winding of that device. The primary winding is usually the power-input winding. The power-output winding for such machines is called the secondary winding.
The armature winding handles all the power that is being converted or transformed. The rating of armature winding is equal to the power rating of the machine. The field winding power rating is about to 1/2 to 2% of the rated power of the machine. The power input to dc field winding is dissipated as I²R loss in the field winding (once the required field current is established).
The armature windings of both the D.C. and A.C. machines have to deal with alternating current only-this is the reason why the armature structures of all rotating machines are laminated in order to reduce the eddy current losses. Further, almost all the rotating maichines have even number of alternate N and S poles (called hetropolar structure). If power is fed to or taken from the rotor it is obvious that sliding contacts are essential. All types of large rotating machines are provided with radial and axial ventilating ducts for cooling purposes.
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