Q. State and briefly explain the various phenomena useful for electromechanical energy conversion in rotating machines.
Sol. In the rotating machines there are mainly two type of torque developed are important which are:
(1) Electromagnetic (or interaction) torque
(2) Reluctance torque
1. Electromagnetic torque: Illustrate a silent - pole stator with 2 poles and a cylindrical rotor with on conductor. When stator coils are energized, stator magnetic flux is set up and its path, with no current in the door conductor. If rotor conductor carries a current indicated by, say dot, then the magnetic flux picture is as depicted, with no current in the stator coil.
When stator coils and rotor conductor, both carry currents, then the flux produced by the rotor current interact with the stator - produced flux, giving the resultant magnetic flux distribution. Since the magnetic flux lines behave like stretched rubber bands, the rotor conductor experiences a force in the upward direction. The clockwise torque developed due tot the interaction of stator and rotor magnetic fields, is called interaction or electromagnetic torque.
Consider now one current - carrying coil on the rotor. The direction of current in the coil is indicated by dot under stator North Pole and by cross under stator south pole. The rotor current produce rotor flux and this creates two poles on the rotor. The stator S pole attracts rotor N pole and repels rotor S pole, resulting in clockwise torque. Similarly stator N pole attracts rotor S ploe and repels rotor N poles, resulting again in clockwise torque. The total torque developed in this manner, is called interaction or electromagnetic torque.
The physical understanding of interaction torque can further be highlighted by referring one permanent magnet free to rotate, is placed in the field of a stationary magnet. The tendency of the field to align themselves in the same direction is called interaction torque. The angle b/w the stator - field axis and rotor - field axis is called the torque angle δ. Note that the torque angle δ is 90o. The magnitude of electromagnetic or interaction torque in all rotating machines is given by
Te ? (Stator field strength) (Rotor field strength) sin δ
2. Reluctance Torque or Alignment Torque: Reluctance torque is a torque experienced by a ferromagnetic object placed in an external magnetic field, which causes the object to line up with the external magnetic field. This torque occurs because the external magnetic field induces an internal magnetic field in the object, and a torque is produced b/w the two fields twisting the object around to line up with the external magnetic field. Thus, a torque is exerted on the object so that it tries to position itself to give minimum reluctance for the magnetic flux. Reluctance torque is also called the alignment torque or saliency torque.
A reluctance motor depends on reluctance torque for its operation.