EE6504 EM2 Notes
Anna University Regulation 2013 EEE EE6504 EM2 Notes, Electrical Machines 2 Lecture Handwritten Notes for all 5 units are provided below. Download link for EEE 5th SEM EE6504 Electrical Machines 2 Lecture Handwritten Notes are listed down for students to make perfect utilization and score maximum marks with our study materials.
ELECTRICAL MACHINES – II
UNIT I ALTERNATOR
Basic Principle
A.C. generators or Alternators (as they are usuall called) operate on the same fundamental principles of electro magnetic induction as D.C. generators. They also consist of an armature winding and a magnetic field. But there is one important difference between the two. Whereas in D.C. generators, the armature rotates And the field system is stationary, the arrangement in alternators is just the reverse of it. In their case, standard construction consists of armature winding mounted on a stationary element called stator and field windings on a rotating element called rotor. The details of construction had shown in fig. 1.1
1.1 CONSTRUCTIONAL DETAILS:
The stator consists of a cast-iron frame, which supports the armature core, having slots on its inner periphery for housing the armature conductors. The rotor is like a flywheel having alternate N and S poles fixed to its outer rim. The magnetic poles are excited (or magnetized) from direct current supplied by a D.C. source at 125 to 600 volts.
In most cases, necessary exciting (or magnetizing) current is obtained from a small D.C. shunt generator which is belted or mounted on the shaft of the alternator itself. Because the field magnets are rotating, this current is supplied through two slip rings. As the exciting voltage is relatively small, the slip-rings and brush gear are of light construction. Recently, brushless excitation systems have been developed in which a 3-phase A.C. exciter and a group of rectifiers supply D.C. to the alternator. Hence, brushes, slip-rings and commutator are eliminated.
When the rotor rotates, the stator conductors (being stationary) are cut by the magnetic flux, hence they have induced E.M.F. produced in them. Because the magnetic poles are alternately N and S, they induce an E.M.F. and hence current in armature conductors, which first flows in one direction and then in the other. Hence, an alternating E.M.F. is produced in the stator conductors (i) whose frequency depends on the number of N and S poles moving past a conductor in one second and (ii) whose direction is given by Fleming’s Right-hand rule.
1. Stator Frame
In D.C. machines, the outer frame (or yoke) serves to carry the magnetic flux but in alternators, it is not meant for that purpose. Here, it is used for holding the armature stampings and windings in position. Low speed large-diameter alternators have frames which because of ease of manufacture, are cast in sections. Ventilation is maintained with the help of holes cast in the frame itself.
The provision of radial ventilating Spaces in the stampings assists in cooling the machine. But, these days, instead of using castings, frames are generally fabricated from mild steel plates welded together in such a way as to form a frame having a box type section. In Fig. 1.2 is shown the section through the top of a typical stator.
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