5.3. Shunt Generator Characteristics
In a shunt generator the field circuit is connected directly across the armature. Appliances, motors, light bulbs, and other electrical devices connected in parallel across the generator terminals represent a load on the generator. As more devices are connected in parallel, the load on the generator increases; that is, the generator current increases. Because the generator current increases, the terminal voltage of the generator decreases. There are three factors that cause this decrease in voltage:
(i) Armature-circuit resistance (Ra),
(ii) Armature reaction, and
(iii) Reduction in field current.
- i. Armature-circuit resistance. The armature circuit of a generator, like every electrical circuit, contains resistance. This resistance includes the resistance of (i) the copper conductors of the armature winding, (ii) the commutator, (iii) contact resistance between brushes and commutator, and (iv) the brushes themselves. When no current flows through the armature, there is no IR drop in the armature and the voltage at the terminals is the same as the generated voltage. However, when there is current in the armature circuit, a voltage drop exists due to the armature resistance, and the terminal voltage is less than the generated voltage. The terminal voltage may be calculated from the following reaction:
V = Eg - Ia Ra
V = voltage at terminals of generator,
Eg= generated or induced voltage,
Ia= total armature current, and
Ra = armature-circuit resistance.
- ii. Armature reaction. When current flows in the armature conductors a flux surrounds these conductors. The direction of this armature flux is such that it reduces the flux from the field poles, resulting in both a reduced generated voltage and terminal voltage.
- iii. Reduction in the field current. The field circuit is connected across the terminals of the generators. When the terminal voltage of the generator becomes smaller because of the armature-resistance volt drop and armature reaction, the voltage across the field circuit also becomes smaller and therefore field current will be less. A reduction in the magnitude of field current also reduces the flux from the field poles, which in turn reduces the generated voltage and also the terminal voltage.
- See Figs. 34 and 35. The effect of the preceding three factors is shown in
Fig. 35, which shows external (load- voltage) characteristic of a shunt generator,
- As shown in the circuit of Fig. 34, the readings of the voltage across the
armature (and load), V are plotted as a function of load current, I. The voltage, V, is the same as Eg at no load (neglecting the IaRa and armature reaction drop produced by the field current). The effects of armature reaction, armature circuit voltage drop, and decrease in field current are all shown with progressive increase in load. Note that both the armature reaction and the IaRa drops are shown as dashed straight lines, representing theoretically linear voltage directly proportional to the increase in load current. The drop owing to decreased field current is a curved line, since it depends on the degree of saturation existing in the field at the value of load.
Fig 34. Shunt generator under load.
- Generally, the external load-voltage characteristic decreases with application of load only to a small extent up. to its rated load (current) value. Thus, the shunt generator is considered as having a fairly constant output voltage with application of load, and in – practice, is rarely operated beyond the rated load current value continuously for all any appreciable time.
Fig. 35.Shunt generator load characteristics.
- As shown in Fig. 35 further application of load causes the generator to reach a breakdownpoint beyond which further load causes it to ‘unbuild’ as it operates on the unsaturatedportion of its magnetisation curve. This unbuilding process continues until the terminal voltage is zero, at which point the load current is of such magnitude that the internal armature circuit voltage drop equals the e.m.f generated on the unsaturated or linear portion of its magnetisation curve.
- It may be noted that if the external load is decreased (an increase of external load resistances), the generator will tend to build up gradually along the dashed line shown in Fig. 35. Note that for any value of load current, the terminal or armature voltage is less (as the voltage increases) compared to the solid lines which yield a higher voltage (as the voltage decreases). This difference is due to hysteresis.
Effect of Varying Excitation. Fig. 36 shows the effect of varying excitation upon the
external characteristic of a shunt generator. With normal excitation the initial slope is small and heavy load current can be varied. With reduced excitation, the fall of voltage is more rapid and the maximum load current is reduced.
Fig. 36. Effect of varying excitation.
Voltage Regulation. The term ‘voltage regulation’ is used to indicate the degree of change in armature voltage produced by application of load. If there is little change from no-load to full load, the generator or voltage-supplying device is said to possess good voltage regulation. If the voltage changes appreciably with load, it is considered to have poor voltage regulation.
‘Voltage regulation’ is defined as the change in voltage from no-load to full load, expressed asa percentage of the rated terminal voltage (armature voltage at full load). ‘i.e. Per cent voltage regulation
Vfl = full load (rated) terminal voltage.
Internal or Total Characteristic. To determine internal characteristic from external characteristic the following procedure is adopted [see Fig. 37].
Steps : 1. From the given data, draw the external characteristic (I).
2. Draw the shunt field resistance line OL and armature resistance line
3. On the external characteristic take any point say F.
4. From point F draw vertical and horizontal lines intersecting X and Y axes respectively. Let these lines be FC and FA respectively.
5. Take point D on X-axis so that CD = AB representing the shunt field current, Ish.
Fig. 37. Determination of internal characteristic from external characteristic,
7.Take point G on line DH produced so that HG = DE (= laRa) representing the armature drop.
8.Following the above procedure take a number of points on external characteristic and find corresponding points lying on internal characteristic.
9.Draw a curve passing through these points which is the required internal characteristic.
External Characteristic and No-load Saturation Curve. The external characteristic of a shunt generator can be obtained directly from its no-load saturation curve as explained below. Following two cases will be considered:
A) When armature reaction is so small as to be negligible. This is more or less true for generators fitted with compoles.
(B) When armature reaction is not negligible.
(A) Armature Reaction Negligible:
Steps 1.From given data draw O.C.C. [see Fig. 38J,
2. Draw shunt field resistance line (say OS) meeting O.C.C. at any point (say A).
3. From point A draw horizontal line intersection Y-axis, say at point B.
Hence, OB is the maximum no-load or open circuit voltage.
4.Take any point (say L) on the O.C.C. and draw an ordinate, say, LMN intersecting field resistance line at M and X-axis at N. Now LN represents the generated e.m.f., MN represents the terminal voltage and LM represents voltage drop in armature.
5.From points Land M draw horizontal lines cutting vertical axis say at point D andiE
6.Draw armature resistance line OC.
Fig. 38.Determination of external and internal characteristics from O.C.C.
Similarly other points can be obtained and internal characteristic may be drawn through these points :
8. From point F draw vertical line intersecting produced line ME at any point G and X-axis say, at point T. Since FG = LM = CT, hence point G lies on the curve representing relation between armature current and terminal voltage.
9. Take TU = shunt field current (Ish) ON (scale being different. OU represents the load current corresponding to armature current represented by OT and terminal voltage OE.
10. From point U draw a vertical line intersecting line EG at H. Point H lies on the external characteristic.
Similarly other points may be obtained and curve may be drawn, which is the required external characteristic.
(B) Taking Armature Reaction into Account:
Here, in addition to considering the voltage drop in armature, voltage drop due to armature reaction is also taken into account.
Let IaRa= voltage drop in armature
Ish= increase in shunt field current to counteract the demagnetising effect.
Now if a right-angled triangle say lmn is drawn as that ln = voltage drop in armature, and mn=shunt field current. The triangle Imn is called as the drop reaction triangle.
In order to draw external and internal characteristic repeat the process as in A with following modifications in steps 4 and 9 respectively.
4.Take any point L on the O.C.C. and draw line LM parallel to the line lm of triangle mnand complete the triangle LMN. Now from the points Land M draw vertical lines cutting X-axis at points N’ and M’.Now LN’ represents the generated e.m.f MM’ represents the terminal voltage, LN represents the voltage drop in armature due to armature resistance, ON’ is the shunt field current to induce an e.m.f represented by LN’ and N’M’ is the increase in shunt field current to counteract the demagnctising effect.
9.Take TU = shunt field current ON’ (scale being different). OU represents the load current corresponding to the armature current represented by OT and terminal voltage MM’.
Voltage Control of Shunt Generators
- The terminal voltage of a shunt generator may be kept constant at all loads with the use of adjustable resistance, called a field rheostat, connected in series with the shunt-field circuit. By adjusting the resistance of the rheostat to suit the load on the machine, changes in terminal voltage with load may be prevented. When the load changes gradually, hand control of the rheostat may be used, although automatic control employing a voltage regulator is far more satisfactory.
- The terminal voltage may also be controlled automatically by the addition of a series-field winding. This method has the advantage of being automatic cheap, and generally satisfactory.