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Transcript
Electrical Machines-I (PCEE 4203)
Lesson 13
CHARACTERISTICS OF SHUNT WOUND GENERATOR
Building up of voltage of a shunt generator at no load: The generator excites itself due to residual
magnetism and develops the voltage as shown in the figure 13.1. Line OP represents the field
resistance line. When the generator starts a small emf (OA) is induced due to residual magnetism that
causes a current (oa’) to flow in the field. When the field current is oa’, the generated emf is a’a”
which produces a field current ob’, which in turn produces a voltage b’b”. Thus it increases to D, and
magnetic characteristics is obtained. OB is the maximum voltage it generates as shown in the figure.
Final
voltage
P
B
D
OCC
C
E0
Field
resistance
line
b''
a'’
A
Residual
voltage
Final
exciting
current
a
O
a'
b'
If
Figure 13.1
LOAD CHARACTERISTICS
The terminal voltage at no load of a generator depends upon shunt field resistance and here in the
figure 13.2 it is shown by OM.As the generator is loaded, the terminal voltage falls due to the
following reasons
Internal
characteristic
EMF or Terminal voltage
M
O
External
characteristic
P
S
Q
R
Load current
Current under
short-circuit
condition
Figure 13.2
1. With the increase in armature current, armature voltage drop and contact brush drop increases.
P K Nayak, Assistant Professor, Department of Electrical Engineering, SIET, Dhenkanal.
1
Electrical Machines-I (PCEE 4203)
Lesson 13
2. As the load is increased, the current in armature conductors increases, so the effect of armature
reaction increases and, therefore, field is weakened. Hence the induced emf also decreases, and
terminal voltage falls.
3. Due to the above two factors terminal voltage decreases, this causes reduction in field current. In
turn the emf changes, therefore the terminal voltage of the generator to drop again.
As the voltage drops sharply, further decrease in load resistance causes decrease in current, so that the
characteristics turns back as shown in the figure 13.2 and short circuit condition occurs where voltage
is zero. But a small current is established due to a small voltage induced by the residual magnetism.
This is external characteristics. If the armature drop is added to the terminal voltage for all points and
plotted against armature current the new curve obtained will give relation between induced emf and
armature current, known as internal characteristics of the generator.
CRITICAL LOAD RESISTANCE
If the line OP is drawn from the origin, tangential to the internal characteristics, the gradient of this
line will give the minimum value of the external load resistance for which the generator will excite on
load. This resistance is known as critical load resistance. If the resistance of external load circuit is
less than this value, the generator will fail to build up its voltage.
COMPOUND GENERATOR CHARACTERISTICS
In a compound generator, both series and shunt excitation are combined as shown in Fig. 13.3. The
shunt winding can be connected either across the armature only (short-shunt connection S) or across
armature plus series field (long-shunt connection G). The compound generator can be cumulatively
compounded or differentially compounded generator. The latter is rarely used in practice. Therefore,
we shall discuss the characteristics of cumulatively compounded generator. It may be noted that
external characteristics of long and short shunt compound generators are almost identical.
Figure 13.3
Figure 13.4
P K Nayak, Assistant Professor, Department of Electrical Engineering, SIET, Dhenkanal.
2
Electrical Machines-I (PCEE 4203)
Lesson 13
EXTERNAL CHARACTERISTIC
Figure13.4 shows the external characteristics of a cumulatively compounded generator. The series
excitation aids the shunt excitation. The degree of
(i)
If series winding turns are so adjusted that with the increase in load current the terminal
voltage increases, it is called over-compounded generator. In such a case, as the load
current increases, the series field mmf. increases and tends to increase the flux and hence
the generated voltage. The increase in generated voltage is greater than the IaRa drop so
that instead of decreasing, the terminal voltage increases as shown by curve A in Figure
13.4.
(ii)
If series winding turns are so adjusted that with the increase in load current, the terminal
voltage substantially remains constant, it is called flat-compounded generator. The series
winding of such a machine has lesser number of turns than the one in over-compounded
machine and, therefore, does not increase the flux as much for a given load current.
Consequently, the full-load voltage is nearly equal to the no-load voltage as indicated by
curve B in Figure 13.4.
(iii)
If series field winding has lesser number of turns than for a flat compounded machine,
the terminal voltage falls with increase in load current as indicated by curve C in Figure
13.4. such a machine is called under-compounded generator.
P K Nayak, Assistant Professor, Department of Electrical Engineering, SIET, Dhenkanal.
3