Generator characteristics gives the relation between terminal voltage and load current. It is of great importance in judging the suitability of a generator for a particular purpose. The characteristics of DC generator are explained below.
DC generator characteristics
The speed of a DC machine operating as a generator is fixed by the prime mover. For general-purpose operation, the prime mover is equipped with a speed governor so that the speed of the generator is practically constant.
In such a situation, the performance of the generator is mainly concerned with the relationship between the excitation, terminal voltage and the load.
These relationship can best be represented graphically through curves known as generator characteristics. These characteristics show at a glance the behavior of the generator under various load conditions.
The following are the three most important characteristics of a DC generator :
- Open Circuit Characteristic
- Internal or Total Characteristic
- External Characteristic
So let’s understand about them in detail.
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Open circuit characteristics (O.C.C.) (E0 /If )
This curve shows the relation between the generated e.m.f. at no-load (E0) and the field current (If ) at constant speed. It is also known as magnetic characteristic or no-load saturation curve. Its shape is practically the same for all generators whether separately or self-excited.
Internal or total characteristics? (E/Ia)
This curve shows the relation between the generated e.m.f. on load (E) and the armature current (Ia). The e.m.f. E will be less than E0 due to the effects of armature reaction. Therefore, this curve will lie below the open circuit characteristic.
External characteristics (V/IL)
This curve shows the relation between the terminal voltage V and load current (IL). The terminal voltage V will be less than E due to the voltage drop in the armature circuit. Therefore, this curve will lie below the internal characteristic.
Open circuit (or Magnetic) characteristics of DC generator
The O.C.C. for a DC generator is determined as follows. The field winding of a DC generator (series or shunt) is disconnected from the machine and is separately excited from an external DC source as show in Fig-2.
The generator is run at fixed speed (i.e. normal speed). The field current (If) is increased from zero in steps and the corresponding values of generated e.m.f. (E0) read off on a voltmeter connected across the armature terminals. On plotting the relation between E0 and If , we get the open circuit characteristic as shown in figure-1.
The following points may be noted from O.C.C. :
- When the field current is zero, there is some generated e.m.f. OA. This is due to the residual magnetism in the field poles.
- Over a fairly wide range of field current (up to point B in the curve), the curve is linear. This is because in this range, the reluctance of iron is negligible compared to that of the air gap. The air gap reluctance is constant and hence linear relationship.
- After point B on the curve, the reluctance of iron also comes into picture. This is because at higher flux densities, μr for iron decreases and reluctance of iron is no longer negligible. As a result, the curve deviates from linear relationship.
- After point C on the curve, magnetic saturation of the poles begins and E0 becomes flat.
It is important to note that Open Circuit Characteristic (O.C.C.) is the same for all types of DC generators.
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What are the characteristics of a separately excited DC generator?
The characteristics of a separately excited DC generator can be studied in two parts
- Open circuit (or magnetic) characteristics
- Load characteristics
1. Open circuit characteristics
These characteristics refer to the no-load conditions of the generator. The e.m.f. Eg generated in a generator is given by ;
For a given generator, Z, P and A are constants so that :
Eg ∝ Φ N ……eq-1
a. Eg − N characteristic. Suppose the field current If of a separately excited generator is kept constant and the speed N is varied. Then from eq-1 above,
Eg ∝ N (Since If is constant, Φ will be constant)
Therefore, for constant field current, Eg − N characteristic is a straight line passing through the origin. Figure-1 shows a family of Eg − N characteristics of a separately excited generator for different constant values of field current.
b. Eg − If characteristics. If the speed of the generator is kept constant and field current is varied, then,
Eg ∝ Φ (Because N is constant)
As If varies, Φ will vary according to the B – H curve of the magnetic circuit. Fig-2 shows a family of Eg − If characteristics. Note that when If is zero, the residual magnetism in poles will give rise to small initial e.m.f. as shown.
2. Load characteristics
The load characteristics of a separately excited DC generator can be divided into two parts such as internal characteristic and external characteristic.
a. Internal characteristics. It is the curve between emf actually induced in the armature (after taking armature reaction into account) and the load current IL.
Now, Eg ∝ Φ N
If lf and N are maintained constant, it may appear at first as if Eg will be constant. However, even if If remains constant, the effective value of flux per pole will be reduced due to armature reaction. Hence Eg − IL curve will droop as shown in figure below.
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What is the critical field resistance of a shunt generator?
The voltage build up in a shunt generator depends on the field circuit resistance. If the field circuit resistance is R1 (line OA), then the generator will create a voltage OM as shown in figure above. If the field circuit resistance is increased to R2 (line OB), then the generator will create a voltage OL, which will be slightly less than OM.
As the field circuit resistance increases, the slope of resistance line also increases. When the field resistance line becomes tangent (line OC) to O.C.C., the generator would just excite.
If the field circuit resistance increases beyond this point (say line OD), the generator will fail to excite. The field circuit resistance represented by line OC (tangent to O.C.C.) is called critical field resistance RC for the shunt generator.
The maximum field circuit resistance (for a given speed) with which the shunt generator would just excite is known as its critical field resistance.
It should be noted that shunt generator will build up voltage only if field circuit resistance is less than the critical field resistance.
What are characteristics of series generator?
Fig-1 shows the connections of a series wound generator. Since there is only one current (that which flows through the entire machine), the load current is the same as the exciting current.
1. O.C.C.
Curve 1 shows the open circuit characteristic (O.C.C.) of a series generator. It can be obtained experimentally by disconnecting the field winding from the machine and exciting it from a separate DC source.
2. Internal characteristic.
Curve 2 shows the total or internal characteristic of a series generator. It gives the relation between the generated e.m.f. E on load and armature current.
Due to armature reaction the flux in the machine will be less than the flux at no-load. Therefore, emf E generated under load conditions will be less than the emf E0 generated under no-load conditions.
As a result, internal characteristic curve lies below the O.C.C. curve, the difference between them representing the effect of armature reaction [See Fig-2].
3. External characteristic.
Curve 3 shows the external characteristic of a series generator. This gives the relationship between terminal voltage V and load current IL.
V = E − Ia (Ra + Rse)
Therefore, external characteristic curve will lie below internal characteristic curve by an amount equal to ohmic drop [i.e., Ia (Ra + Rse)] in the machine as shown in Fig-2.
The internal and external characteristics of a DC series generator can be plotted from one another (see the figure above). Suppose we are given an internal characteristic of a generator. Let the line OC represent the resistance of the whole machine i.e., Ra + Rse. If the load current is OB, drop in the machine is AB that is,
AB = ohmic drop in the machine
= OB (Ra + Rse)
Now take a perpendicular from point B and mark a point b on this line such that ab = AB. Then point b will lie on external characteristic of the generator. Following a similar process, other points of external characteristic can be located.
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Characteristics of DC shunt generator
Fig-1 shows the connections of a shunt wound generator. The armature current Ia splits into two parts; a small fraction Ish flowing through shunt field winding while the major part IL goes to the external load.
1. O.C.C.
The O.C.C. of a shunt generator is similar in shape to that of a series generator as shown in Fig-2. Line OA represents the shunt field circuit resistance.
When the generator is run at normal speed, it will create a voltage OM. At no-load, the terminal voltage of the generator will be constant (= OM) which is represented by the horizontal dotted line MC.
2. Internal characteristic.
When the generator is loaded, flux per pole is decreases due to armature reaction. Therefore, e.m.f. E generated on load is less than the e.m.f. generated at no-load. As a result, the internal characteristic (E/Ia) drops down slightly as shown in Fig-2.
3. External characteristic.
Curve 2 shows the external characteristic of the shunt generator. This gives the relationship between terminal voltage V and load current IL.
V= E − Ia Ra
= E − (IL + Lsh) Ra
Therefore, external characteristic curve will lie below the internal characteristic curve by an amount equal to drop in the armature circuit [that is (IL + Lsh) Ra] as shown in Fig-2.
What are the characteristics of a compound generator?
In a compound generator, both series and shunt excitation are combined as shown in figure-1. The shunt winding can be connected either to the armature only (short-shunt connection S) or to the armature plus series field (long-shunt connection G).
Compound generators can be cumulatively compounded or differentially compounded generator. The latter is rarely used in practice. Therefore, we will discuss the characteristics of cumulatively compounded generator. It may be noted that external characteristics of long and short shunt compounded generators are almost identical.
External characteristic
Fig-2 shows the external characteristics of a cumulatively compounded generator. Series excitation assists in shunt excitation. The degree of compounding depends on the increase in series excitation with increase in load current.
1. If the 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 m.m.f. increases and tends to increase the flux and hence the generated voltage. The increase in generated voltage is greater than the Ia Ra drop so that instead of decreasing, the terminal voltage increases as shown by curve A in Fig-2.
2. If the series winding turns are so adjusted that with an increase in load current, the terminal voltage remains largely constant, it is called flat-compounded generator.
The series winding of such a machine has a smaller number of turns than an over-compounded machine and, therefore, does not increase the flux as much for a given load current. As a result, the full-load voltage is nearly equal to the no-load voltage as indicated by curve B in Fig-2.
3. 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 Fig-2. Such a machine is called under-compounded generator.
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