Page 126 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

MODULE 13 : Synchronous Machines

LESSON 76-85 : Alternator principle, EMF equation,

charateristics and voltage regulation parallel operation

Objectives

At the end of this lesson you shall be able to:
• explain the working principle of an alternator, construction and types of alternator
• state e.m.f. equavation of the alternator
• explain the load characteristics of an alternator and voltage regulation
• state the necessity and conditions of paralling of alternators
• explain the methods of paralling two 3 phase alternator.

Principle of an alternator: An alternator works on the same principle of electromagnetic induction as a DC
generator. That is, whenever a conductor moves in a magnetic field so as to cut the lines of force, an emf will be
induced in that conductor. Alternatively whenever there is relative motion between the field and the conductor,
then, the emf will be induced in the conductor. The amount of induced emf depends upon the rate of change of
cutting or linkage of flux.
In the case of DC generators, we have seen that the alternating current produced inside the rotating armature coils
has to be rectified to DC for the external circuit through the help of a commutator. But in the case of alternators,
the alternating current produced in the armature coils can be brought out to the external circuit with the help of
slip-rings. Alternatively the stationary conductors in the stator can produce alternating current when subjected to
the rotating magnetic field in an alternator.

Production of sine wave voltage by single loop alternator: Fig 2a shows a single loop alternator. As it rotates
in the magnetic field, the induced voltage in it varies in its direction and magnitude as follows.
To plot the magnitude and direction of the voltage induced in the wire loop of the AC generator in a graph, the
electrical degrees of displacement of the loop are kept in the `X’ axis as shown in Fig 1 through 30 electrical
degrees. As shown in Fig 2c, three divisions on the `X’ axis represent a quarter turn of the loop, and six divisions
a half turn. The magnitude of the induced voltage is kept in the `Y’ axis to a suitable scale.
The part above the X-axis represents the positive voltage, and the part below it the negative voltage as shown
in Fig 1.

The position of the loop at the time of starting is shown in Fig 2a and indicated in Fig 2c as `O’ position. At this
position, as the loop moves parallel to the main flux, the loop does not cut any lines of force, and hence, there will
be no voltage induced. This zero voltage is represented in the graph as the starting point of the curve as shown in
Fig 2c. The magnitude of the induced emf is given by the formula Eo = BLV Sin q

113