In the module, containing six lessons (12-17), the study of circuits, consisting of the
linear elements – resistance, inductance and capacitance, fed from single-phase ac supply,
has been presented. In this module, which may also be termed as an extension of the
previous one, containing three lessons (18-20), the solution of currents in the balanced
circuits, fed from three-phase ac supply, along with the measurement of power, will be
In this (first) lesson of this module, the generation of three-phase balanced voltages is
taken up first. Then, the two types of connections (star and delta), normally used for the
above supply, followed by line and phase quantities (voltages and currents) for the
connections, in both supply and load sides (both being balanced), are described.
: Three-phase balanced voltage, star- and delta-connections, balanced load.
After going through this lesson, the students will be able to answer the following
How to generate three-phase balanced voltages?
What are the two types of connections (star and delta) normally used for three-phase
What are meant by the terms – line and phase quantities (voltages and currents), for
the two types of connections in both supply and load sides (both being balanced)?
Generation of Three-phase Balanced Voltages
In the first lesson (No. 12) of the previous module, the generation of single-phase
voltage, using a multi-turn coil placed inside a magnet, was described. It may be noted
that, the scheme shown was a schematic one, whereas in a machine, the windings are
distributed in number of slots. Same would be the case with a normal three-phase
generator. Three windings, with equal no. of turns in each one, are used, so as to obtain
equal voltage in magnitude in all three phases. Also to obtain a balanced three-phase
voltage, the windings are to be placed at an electrical angle of
with each other, such
that the voltages in each phase are also at an angle of
with each other, which will be
described in the next section. The schematic diagram with multi-turn coils, as was shown
earlier in Fig. 12.1 for a single-phase one, placed at angle of
with each other, in a
2-pole configuration, is shown in Fig. 18.1a. The waveforms in each of the three
windings (R, Y & B), are also shown in Fig. 18.1b. The windings are in the stator, with
the poles shown in the rotor, which is rotating at a synchronous speed of
rpm), to obtain a frequency of
being no. of poles [
(see lesson no. 12).
Version 2 EE IIT, Kharagpur