Page 148 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Two important points to be noted are:
1 The magnitude of armature current varies with excitation. The current has large value both for low and high
values of excitation. In between, it has minimum value corresponding to a certain excitation. The variations of
current are known as ‘V’ curves because of their shape.
2 For the same input, armature current varies over a wide range and so causes the power factor also to vary
accordingly. When over excited, motor runs with leading power factor and with lagging power factor, when
under excited. In between the power factor is unity. The variation with power factor with excitation curve looks
like inverted ‘V’ curve (Fig 8). It would be noted that minimum armature current corresponds to unity power
factor.
Fig 6(c) represents the condition for over excited motor, i.e. when Eb>V. here the resultant voltage vector ER is
pulled anticlockwise and so as I. it is seen that now motor is drawing a leading current. It may also happen for
some value of excitation, that I may be in phase with V, i.e. power factor is unity. Fig 6(d). at that time the current
drawn by the motor would be minimum.
It is seen that an over-excited motor can be run with leading power factor. This property of the motor renders it
extremely useful for phase advancing (and so power factor correcting) purposes in the case of industrial loads
driven by induction motors and lighting and heating loads supplied through transformers. Both transformers and
induction motors draw lagging currents from the line. Especially on light loads, the power drawn by them has a
large reactive component and the power factor has a very low value. This reactive component, though essential
for operating the electric machinery, entails appreciable loss in many ways. By using synchronous motors in
conjunction with induction motors and transformers, the lagging reactive power required by the latter is supplied
locally by the leading reactive component taken by the former, thereby relieving the line and generators of much
of the reactive component. Hence, they now supply only the active component of the load current. When used
in this way, a synchronous motor is called a synchronous capacitor, because it draws, like a capacitor, leading
current from the line. Most synchronous capacitors are rated between 20 MVAR and 200 MVAR and many are
hydrogen-cooled.
V Curves
The V-curves of a synchronous motor show how armature current varies with its field current when motor input
is kept constant. These are obtained by plotting A.C. armature current against D.C. field current while motor
input is kept constant and are so called because of their shape (Fig 7). There is a family of such curves, each
corresponding to a definite power intake. In order to draw these curves experimentally, the motor is run from
constant voltage and constant-frequency bus-bars. Power input to motor is kept constant at a definite value. Next,
field current is increased in small steps and corresponding armature currents are noted. When plotted, we get
a V-curve for a particular constant motor input. Similar curves can be drawn by keeping motor input constant at
different values. A family of such curves is shown in Fig 7.

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