Armature Reaction in DC Generator
Armature Reaction in DC Generator and dc generators. There are two windings in a dc generator and a dc motor:
- Field winding
- Armature Winding.
The purpose of field winding is to produce a magnetic field (called main flux) whereas the purpose of armature winding is to carry armature current.
Although the armature winding is not provided for the purpose of producing a magnetic field, still the current in the armature winding also produces a magnetic flux (called armature flux).
The armature flux distorts and weakens the main flux and create problems for the proper operation of the dc machines.
The action of armature flux on the main flux is called armature reaction in a dc generator.
The phenomenon of armature reaction in a dc generator is shown in the figure below. For the sake of clarity, we are taking only one pole.
When the generator is on no-load (Figure i), a small current is flowing through the armature and therefore flux produced in the armature is very small and it does not affect the main flux φ1 coming from the pole.
When the generator is loaded (Figure ii), high current start flowing through the armature conductors, thus a high flux φ2 is set up as shown in fig (ii).
By superimposing the fluxes φ1 and φ2 (Figure iii), we obtain the resulting flux φ3 as shown in fig (iii).
This is what happens to the flux under one pole under armature reaction in a dc generator.
From fig (iii) it is clear that flux density at the trailing pole tip (point B) is increased while at the leading pole tip (point A) it is decreased.
This unequal field distribution due to the armature reaction in dc generator produces the following two effects:
- The main flux is distorted.
- The main Flux is weakened.
The weakening of flux due to armature reaction in a dc generator also depends on the position of the brushes. For that, we need to understand the geometrical and magnetic neutral axes.
Geometrical and Magnetic Neutral Axes
The geometrical neutral axis and magnetic neutral axis should be clearly understood in order to get a clear idea of armature reaction in a dc generator.
The geometrical neutral axis (GNA) is the axis that bisects the angle between the center line of adjacent poles.
The magnetic neutral axis (MNA) is the axis drawn perpendicular to the mean direction of the flux passing through the center of the armature.
No e.m.f. is produced in the armature conductors along this axis because then they cut no flux. When no current is there in the armature conductors, the MNA coincides with GNA.
Explanation of Armature Reaction
The armature reaction in a dc generator is explained as below,
Consider no current in armature conductors, then MNA coincides with GNA.
Consider no current in armature conductors, then MNA coincides with GNA.
Now, when current start flowing through the armature conductors, due to the combined action of main flux and armature flux the MNA get shifted from GNA.
In case of a generator, the M.N.A. is shifted in the direction of rotation of the machine. In order to achieve sparkless commutation, the brushes should be moved along the new MNA.
Under such a condition, the armature reaction in a dc generator produces the following two effects:
- It demagnetizes or weakens the main flux.
- It cross-magnetizes or distorts the main flux.
Let us discuss these effects of armature reaction in a dc generator by considering a 2-pole generator(though the following remarks also hold good for a multipolar generator).
Fig (i) shows the flux due to main poles (main flux) when the armature conductors carry no current.
The flux across the air gap is uniform. The m.m.f. producing the main flux is represented in magnitude and direction by the vector OFm in fig (i). Note that OFm is perpendicular to GNA.
Fig (ii) shows the flux due to the current flowing in armature conductors of dc generator alone (main poles unexcited).
The armature conductors to the left of GNA. carry current “in” (×) and those to the right carry current “out” (•). The direction of magnetic lines of force can be found by corkscrew rule.
It is clear that armature flux is directed downward parallel to the brush axis. The m.m.f. producing the armature flux is represented in magnitude and direction by the vector OFA in fig (ii).
Fig (iii) shows the flux due to the main poles and that due to the current in armature conductors acting together. The resultant m.m.f. OF is the vector sum of OFm and OFA as shown in fig (iii).
Since MNA is always perpendicular to the resultant m.m.f., the MNA is shifted through an angle θ.
Note that MNA is shifted in the direction of rotation of the generator.
In order to achieve sparkless commutation, the brushes must lie along the MNA. Consequently, the brushes are shifted through an angle θ so as to lie along the new MNA as shown in Fig (iv).
Due to the brush shift, the m.m.f. FA of the armature is also rotated through the same angle θ. It is because some of the conductors which were earlier under N-pole now come under S-pole and vice-versa.
The result is that armature m.m.f. FA will no longer be vertically downward but will be rotated in the direction of rotation through an angle θ as shown in Fig (iv).
Now FA can be resolved into rectangular components Fc and Fd.
The component Fd is in direct opposition to the m.m.f. OFm due to main poles. It has a demagnetizing effect on the flux due to main poles. For this reason, it is called the demagnetizing or weakening component of armature reaction in dc machines.
The component Fc is at right angles to the m.m.f. OFm due to main poles. It distorts the main field. For this reason, it is called the cross magnetizing or distorting component of armature reaction in dc machines.
Characteristics of Series Wound DC Generators
In this post, we will learn the characteristics of a series wound dc generator. This article is the continuation of the dc generator characteristics.
The connection diagram of a series wound generator is shown in figure (i) below.
Since there is only one current (that which flows through the whole machine), the load current is the same as the exciting current.
The open circuit, internal and external characteristics of series wound dc generators are discussed here.
Open circuit characteristic
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 d.c. source as discussed in dc generator characteristics.
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 the armature reaction in dc generator, the flux in the machine will be less than the flux at no load.
Hence, e.m.f. E generated under load conditions will be less than the e.m.f. E0 generated under no load conditions.
Consequently, the internal characteristic curve lies below the O.C.C. curve; the difference between them representing the effect of armature reaction.
This curve also gives the relation between emf Eg and armature current Ia since Ia=If.
External or Load characteristic
Curve 3 shows the external characteristic of a series generator.
It gives the relation between terminal voltage and load current IL.
V = E – Ia (Ra + Rse )
Therefore, external characteristic curve will lie below internal characteristic curve by an .png)
amount equal to ohmic drop [i.e., Ia(Ra + Rse)] in the machine.
amount equal to ohmic drop [i.e., Ia(Ra + Rse)] in the machine.
This voltage drop for different values of load current may be represented by straight line OC.
The internal and external characteristics of a d.c. series generator can be plotted from one another as shown in Figure right.
Suppose we are given the internal characteristic of the 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 i.e. AB = Ohmic drop in the machine = OB(Ra + Rse)
Now raise a perpendicular from point B and mark a point b on this line such that ab = AB. Then point b will lie on the external characteristic of the generator.
Following a similar procedure, other points of the external characteristic can be located.
It is easy to see that we can also plot internal characteristic from the external characteristic. So external characteristic is what we obtain by deducting ohmic drop from internal characteristic.
Note:
From the external characteristic, it is observed that the terminal voltage first increases with the increase in load, reaches the maximum and finally decreases.
If load resistance is reduced sufficiently, the terminal voltage may fall to zero.
So if the series generator is operated on the initial straight line portion of the characteristic, it gives voltage approximately proportional to the load current.
If it is operated on the drooping portion of the characteristic, it gives approximately constant current irrespective of the external load circuit resistance.
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