Page 132 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

As shown in Fig 1a, three independent loops spaced about 120° apart are made to rotate in a magnetic field
with the assumption that the alternator shown is a rotating armature type. As shown in Fig 1a, the three loops
are electrically isolated from each other and the ends of the loops are connected to individual slip rings. As the
loops are rotating in a uniform magnetic field, they produce sine waves. In a practical alternator, these loops will
be replaced by a multi-turn winding element and distributed throughout the rotor slots but spaced apart at 120°
electrical degrees from each other. Further, in practice, there will not be six slip rings as shown in Fig 1a but will
have either four or three slip rings depending upon whether the three windings are connected in a star or delta
respectively.
We also know, as discussed earlier, that the rotating magnetic field type alternators are mostly used. In such
cases only two slip rings are required for exciting the field poles with DC supply. Fig 1b shows a stationary,
3-phase armature in which individual loops of each winding are replaced by coils spaced at 120 electrical degrees
apart. However, the rotating part having the magnetic poles is not shown.
Fig 1c shows the rotating armature type alternator in which the 3 coils of the three-phases are connected in star
which rotates in a two-pole magnetic field. According to Fig 1c, the coil `R’ moves under the influence of the `N’
pole cutting the flux at right angles, and produces the maximum induced voltage at position `Oo’ as shown in
the graph as per Faraday’s Laws of Electromagnetic induction. When the coil `R’ moves in a clockwise direction,
the emf induces falls to zero at 90 degrees, and then increases to -ve maximum under the influence of the south
pole at 180 degrees. Likewise the emf induced in the `R’ phase will become zero at 270 degrees and attain +ve
maximum at 360 degrees. In the same manner the emf produced by coils `Y’ and `B’ could be plotted on the same
graph. A study of the sine wave-forms produced by the three coils RYB shows that the voltage of coil `R’ leads
voltage of coil `Y’ by 120°, and the voltage of coil `Y’ leads voltage of coil `B’ by 120°.
Phase sequence: The phase sequence is the order in which the voltages follow one another, i.e. reach their
maximum value. The wave-form in Fig 1c shows that the voltage of coil R or phase R reaches its positive maximum
value first, earlier than the voltage of coil Y or phase `Y’, and after that the voltage of coil B or phase B reaches
its positive maximum value. Hence the phase sequence is said to the RYB.
If the rotation of the alternator shown in Fig 1c is changed from clockwise to anticlockwise direction, the phase
sequence will be changed as RBY. It is the most important factor for parallel connection of poly phase generators
and in poly phase windings. Further the direction of rotation of a 3-phase induction motor depends upon the phase
sequence of the 3-phase supply. If the phase sequence of the alternator is changed, all the 3-phase motors,
connected to that alternator, will run in the reverse direction though it may not affect lighting and heating loads.
The only difference in the construction of a single phase alternator and that of a 3-phase alternator lies in the main
winding. Otherwise both the types of alternators will have similar construction.
General testing of alternator: Alternaters are to be periodically checked for their general condition as they will
be in service continuously. This comes under preventive maintenance, and avoids unnecessary breakdowns or
damage to the machine. The usual checks that are to be carried out on an altenator are:
• continuity check of the windings
• insulation resistance value between windings
• insulation resistance value of the windings to the body

• checking the earth connection of the machine.
Continuity test: The continuity of the windings is checked by the following method as shown in Fig 2.
A test lamp is connected in series with one end to the neutral (star point) and the other end to one of the winding
terminals (R Y B). If the test lamp glows equally bright on all the terminals RYB then the continuity of the winding
is all right. In the same way, as shown in Fig 3, we can test the field leads F1 and F2 for field continuity.
Testing continuity with the test lamp only indicates the continuity in between two terminals but will not indicate
any short between the same windings. A more reliable test will be to use an ohmmeter to check the individual
resistances of the coils, and compare them to see that similar coils have the same resistance. The readings, when
recorded, will be useful for future reference also.

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CITS : Power - Electrician & Wireman - Lesson 76-85