Skip to main content

Characteristics of D.C. Generators

The d.c. generators have following characteristics in general,
1) Magnetization characteristics
2) Load characteristics
1.1 Magnetization Characteristics
       This characteristics is the graph of generated no load voltage E against the field current If, when speed of, generator is maintained constant. As it is plotted without load with open output terminals it is also called No load characteristics or Open circuit characteristics.
       Eo Vs If  is magnetization characteristics
       Where  Eo = No load induced e.m.f.
       But for generator,
       E = (ΦPNZ)/(60A)
...     E α   Φ                 with (PNZ) / (60A)  constant
...     E  α   If                    as   Φ  α   If
       Thus induced e.m.f. increases directly as If  increases. But after certain If  core gets saturated and flux also remain constant though If  increases. Hence after saturation, voltage also remains constatnt.
Note : Thus characteristics is linear till saturation and after that bends such that voltage remains constant though If  increases.
        The characteristics is shown in the Fig. 1.
Fig. 1  Magnetization characteristics
       Now the induced e.m.f. also varies with speed.
       Actually,             E   α   N Φ
       So if magnetization characteristics for various speeds are plotted we will get family of parallel characteristics as shown in the Fig. 2. For lower speeds, generated voltages are less so characteristics for lower speeds are below the characteristics for higher speeds.
Fig. 2
1.2 Load Characteristics
       These are further divided into two categories,
1) External characteristics
2) Internal characteristics
       The external characteristics is the graph of the terminal voltage Vt against load current IL.
       The internal characteristics is the graph of the generated induced e.m.f. against the armature current Ia.
Note : While plotting both the characteristics, the speed N of the generator is maintained constant.
Note : In most of the cases, the shunt field current is very small as compared with load current IL. Hence in practice, the internal characteristics shows the graph of induced e.m.f. E against load current IL, instead of Ia, neglecting Ish.
Labels:

Comments

Post a Comment

Comment Policy
We’re eager to see your comment. However, Please Keep in mind that all comments are moderated manually by our human reviewers according to our comment policy, and all the links are nofollow. Using Keywords in the name field area is forbidden. Let’s enjoy a personal and evocative conversation.

Popular posts from this blog

Transformer multiple choice questions part 1

Hello Engineer's Q.[1] A transformer transforms (a) frequency (b) voltage (c) current (d) voltage and current Ans : D Q.[2] Which of the following is not a basic element of a transformer ? (a) core (b) primary winding (c) secondary winding (d) mutual flux. Ans : D Q.[3] In an ideal transformer, (a) windings have no resistance (b) core has no losses (c) core has infinite permeability (d) all of the above. Ans : D Q.[4] The main purpose of using core in a transformer is to (a) decrease iron losses (b) prevent eddy current loss (c) eliminate magnetic hysteresis (d) decrease reluctance of the common magnetic circuit. Ans :D Q.[5] Transformer cores are laminated in order to (a) simplify its construction (b) minimize eddy current loss (c) reduce cost (d) reduce hysteresis loss. Ans : B Q.[6] A transformer having 1000 primary turns is connected to a 250-V a.c. supply. For a secondary voltage of 400 V, the number of secondary turns should be (a) 1600 (b) 250 (c) 400 (d) 1250 A...
7 comments
Read more

Armature Voltage Control Method or Rheostatic Control of dc motor

Speed Control of D.C. Shunt Motor (Part2)  2. Armature Voltage Control Method or Rheostatic Control        The speed is directly proportional to the voltage applied across the armature. As the supply voltage is normally constant, the voltage across the armature can be controlled by adding a variable resistance in series with the armature as shown in the Fig. 1. Fig. 1 Rheostat control of shunt motor        The field winding is excited by the normal voltage hence I sh is rated and constant in this method. Initially the reheostat position is minimum and rated voltage gets applied across the armature. So speed is also rated. For a given load, armature current is fixed. So when extra resistance is added in the armature circuit, I a remains same and there is voltage drop across the resistance added ( I a R). Hence voltage across the armature decreases, decreasing the speed below normal value. By varyi...
Post a Comment
Read more

Demagnetising and Cross Magnetizing Conductors

The conductors which are responsible for producing demagnetizing and distortion effects are shown in the Fig.1. Fig. 1        The brushes are lying along the new position of MNA which is at angle θ  from GNA. The conductors in the region AOC = BOD = 2θ  at the top and bottom of the armature are carrying current in such a direction as to send the flux in armature from right to left. Thus these conductors are in direct opposition to main field and called demagnetizing armature conductors.         The remaining armature conductors which are lying in the region AOD and BOC carry current in such a direction as to send the flux pointing vertically downwards i.e. at right angles to the main field flux. Hence these conductors are called cross magnetizing armature conductors which will cause distortion in main field flux.        These conductors are shown in the Fig. 2 Fig. 2  ...
Post a Comment
Read more