CHARACTERISTICS OF DC GENERATOR
Generally, the following three characteristics of DC generators are taken into consideration: (i) Open Circuit Characteristic (O.C.C.), (ii) Internal or Total Characteristic, and (iii) External Characteristic. These characteristics of DC generators are explained below.
1. Open Circuit Characteristic (O.C.C.) (E0/If)
Open circuit characteristic is also known as magnetic characteristic or no-load saturation characteristic. This characteristic shows the relation between generated emf at no load (E0) and the field current (If) at a given fixed speed. The O.C.C. curve is just the magnetization curve and it is practically similar for all types of generators. The data for the O.C.C. curve is obtained by operating the generator at no load and keeping a constant speed. The field current is gradually increased and the corresponding terminal voltage is recorded. The connection arrangement to obtain the O.C.C. curve is shown in the figure below. For shunt or series excited generators, the field winding is disconnected from the machine and connected across an external supply.
Now, from the emf equation of dc generator, we know that Eg = kɸ. Hence, the generated emf should be directly proportional to field flux (and hence, also directly proportional to the field current). However, even when the field current is zero, some amount of emf is generated (represented by OA in the figure below). This initially induced emf is due to the fact that there exists some residual magnetism in the field poles. Due to the residual magnetism, a small initial emf is induced in the armature. This initially induced emf aids the existing residual flux, hence, increasing the overall field flux. This consequently increases the induced emf. Thus, O.C.C. follows a straight line. However, as the flux density increases, the poles get saturated and the ɸ becomes practically constant. Thus, even if we increase the If further, ɸ remains constant and hence, Eg also remains constant. Hence, the O.C.C. curve looks like the B-H characteristic.
The above figure shows a typical no-load saturation curve or open circuit characteristics for all types of DC generators.
2. Internal Or Total Characteristic (E/Ia)
An internal characteristic curve shows the relation between the on-load generated emf (Eg) and the armature current (Ia). The on-load generated emf Eg is always less than E0 due to the armature reaction. Eg can be determined by subtracting the drop due to demagnetizing effect of armature reaction from no-load voltage E0. Therefore, the internal characteristic curve lies below the O.C.C. curve.
3. External Characteristic (V/IL)
An external characteristic curve shows the relation between terminal voltage (V) and the load current (IL). Terminal voltage V is less than the generated emf Eg due to voltage drop in the armature circuit. Therefore, the external characteristic curve lies below the internal characteristic curve. External characteristics are very important to determine the suitability of a generator for a given purpose. Therefore, this type of character is sometimes also called a performance characteristic or load characteristic.
Internal and external characteristic curves are shown below for each type of generator.
Characteristics Of Separately Excited DC Generator
If there is no armature reaction and armature voltage drop, the voltage will remain constant for any load current. Thus, the straight line AB in the above figure represents the no-load voltage vs. load current IL. Due to the demagnetizing effect of the armature reaction, the on-load generated emf is less than the no-load voltage. The curve AC represents the on-load generated emf Eg vs. load current IL i.e. internal characteristic (as Ia = IL for a separately excited dc generator). Also, the terminal voltage is lesser due to ohmic drop occurring in the armature and brushes. The curve AD represents the terminal voltage vs. load current i.e. external characteristic.
Characteristics Of DC Shunt Generator
To determine the internal and external load characteristics of a DC shunt generator the machine is allowed to build up its voltage before applying any external load. To build up the voltage of a shunt generator, the generator is driven at the rated speed by a prime mover. Initial voltage is induced due to residual magnetism in the field poles. The generator builds up its voltage as explained by the O.C.C. curve. When the generator has built up the voltage, it is gradually loaded with resistive load, and readings are taken at suitable intervals. The connection arrangement is shown in the figure below.
Unlike, separately excited DC generator, here, IL≠Ia. For a shunt generator, Ia=IL+If. Hence, the internal characteristic can be easily transmitted to Eg vs. IL by subtracting the correct value of If from Ia.
During normal running conditions, when the load resistance is decreased, the load current increases. But, as we go on decreasing the load resistance, the terminal voltage also falls. So, load resistance can be decreased up to a certain limit, after which the terminal voltage drastically decreases due to excessive armature reaction at a very high armature current and increased I2R losses. Hence, beyond this limit, any further decrease in load resistance results in decreasing load current. Consequently, the external characteristic curve turns back as shown by the dotted line in the above figure.
Characteristics Of DC Series Generator
The curve AB in the above figure is identical to the open circuit characteristic (O.C.C.) curve. This is because in DC series generators field winding is connected in series with armature and load. Hence, here load current is similar to the field current (i.e. IL=If). The curve OC and OD represent internal and external characteristics respectively. In a DC series generator, terminal voltage increases with the load current. This is because, as the load current increases, the field current also increases. However, beyond a certain limit, the terminal voltage starts decreasing with an increase in load. This is due to the excessive demagnetizing effects of the armature reaction.
Characteristics Of DC Compound Generator
The above figure shows the external characteristics of DC compound generators. If series winding amp-turns are adjusted so that, an increase in load current causes an increase in terminal voltage then the generator is called to be over-compounded. The external characteristic of the over-compounded generator is shown by curve AB in the above figure.
If series winding amp-turns are adjusted so that, the terminal voltage remains constant even if the load current is increased, then the generator is called to be flat compounded. The external characteristic of a flat compounded generator is shown by the curve AC.
If the series winding has a lesser number of turns than that would be required to be flat compounded, then the generator is called to be under compounded. The external characteristics of an under-compounded generator are shown by the curve AD.
The following are the three most important characteristics or curves of a dc generator:
1. No-load saturation Characteristic (E0/If):
It is also known as Magnetic Characteristic or Open-circuit Characteristic (O.C.C.). It shows the relation between the no-load generated MMF in the armature, E0, and the field or exciting current If at a given fixed speed.
It is just the magnetization curve for the material of the electromagnets. Its shape is practically the same for all generators whether separately excited or self-excited.
2. Internal or Total Characteristic (E/Ia):
It gives the relation between the MMF E actually induces in the armature (after allowing for the demagnetizing effect of armature reaction) and the armature current Ia. This characteristic is of interest mainly to the designer.
3. External Characteristic (V/I):
It is also referred to as a performance characteristic or sometimes a voltage-regulating curve. It gives the relation between that terminal voltage V and the load current I. This curve lies below the internal characteristic because it takes into account the voltage drop over the armature circuit resistance. The values of V are obtained by subtracting IaRa from the corresponding values of E. This characteristic is of great importance in judging the suitability of a generator for a particular purpose. It may be obtained in two ways
(i) By making simultaneous measurements with a suitable voltmeter and an ammeter on a loaded generator or
(ii) Graphically from the O.C.C.
Provided the armature and field resistances are known and also if the demagnetizing effect (under rated load conditions) or the armature reaction (from the short-circuit test) is known.
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