Please go through the construction of dc generator before reading this topic.
The mmf necessary to establish flux in the magnetic circuit of a dc generator can be obtained by means of
i. Permanent magnets
ii. Field coils excited from some external source and
iii. Field coils excited by the generator itself.
Generators are generally classified according to these methods of field excitation. On this basis, dc generators are divided into the following two classes:
Permanent magnet dc generators
Separately excited dc generators
Self-excited dc generators
In permanent magnet dc machines, a permanent magnet is used to establish flux in the magnetic circuit.
These generators are not found in industrial applications because of the low power generated from it. Such generators are employed only in small sizes like dynamos in motorcycles.
The behavior of a dc generator on load depends upon the method of field excitation adopted.
Separately Excited D.C. Generators
A dc generator whose field magnet winding is supplied from an independent external d.c. source (e.g., a battery etc.) is called a separately excited generator.
The figure shows the connections of a separately excited generator. The voltage output depends upon the speed of rotation of armature and the field current (Eg = φZNP/60 A). The greater the speed and field current, greater is the generated e.m.f.
It may be noted that separately excited d.c. generators are rarely used in practice. The d.c. generators are normally of self-excited type. Also, read characteristics of separately excited dc generator.
separately Excited DC Generator
Armature current, Ia = IL
Terminal voltage, V = Eg – Ia Ra
Electric power developed = EgIa
Power delivered to load = EgIa – I R = I E – I R = VIa
Self-Excited D.C. Generators
A d.c. generator whose field magnet winding is supplied current from the output of the generator itself is called a self-excited generator. There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature, namely;
i.Series generator
ii.Shunt generator
iii.Compound generator
DC Series generator
In a series wound generator, the field winding is connected in series with armature winding so that whole armature current flows through the field winding as well as the load.
The figure shows the connections of a series wound generator. Since the field winding carries the whole of load current, it has a few turns of thick wire having low resistance. Series generators are rarely used except for special purposes e.g., as boosters.
Armature current, Ia = Ise = IL = I(say)
Terminal voltage, V = EG – I(Ra + Rse)
Power developed in armature = EgIa
Power delivered to load
DC Shunt generator
In a shunt generator, the field winding is connected in parallel with the armature winding so that the terminal voltage of the generator is applied across it. The shunt field winding has many turns of fine wire having high resistance. Therefore, only a part of armature current flows through shunt field winding and the rest flows through the load. Also, read the characteristics of a shunt generator. The figure below shows the connections of a shunt-wound generator.
Shunt field current, Ish = V/Rsh
Armature current, Ia = IL + Ish
Terminal voltage, V = Eg – IaRa
Power developed in armature = EgIa
Power delivered to load = VIL
DC Compound generator
In a compound-wound generator, there are two sets of field windings on each pole – one is in series and the other in parallel with the armature.
A compound-wound generator may be:
Short Shunt in which only shunt field winding is in parallel with the armature winding.
Long Shunt in which shunt field winding is in parallel with both series field and armature winding.
Long shunt Compound Generator
Series field current, Ise = Ia = IL + Ish
Shunt field current, Ish = V/Rsh
Terminal voltage, V = Eg – Ia(Ra + Rse)
The power developed in armature = EgIa
Power delivered to load = VIL
Short Shunt Compound Generator
Series field current, Ise = IL
Shunt field current,
Terminal voltage, V = Eg – IaRa – IseRse
Power developed in armature = EgIa
Power delivered to load = VIL
In a compound generator, the major portion of excitation is usually supplied by the shunt field. The shunt field is slightly weaker and the series field is considerably weaker than those of the corresponding machine in which the entire excitation is produced by a single shunt or a single series winding.
Compound wound generators are of two types, known as cumulative wound and differential wound generators.
In cumulative wound generators the series field assists the shunt field, whereas, in differential wound generators, series field opposes the shunt field.
DC generator characteristics are the relations between excitation, terminal voltage and load exhibited graphically by means of curves. These characteristics of dc generators are very important in the design and operation of dc generators. This article describes the different characteristics of a dc generator in brief.
One question comes into our mind is that why the DC generator characteristics are plotted between excitation, terminal voltage, and load. Here is the answer. The speed of a dc machine operated as a generator is fixed by the prime mover to which the dc generator is coupled. The prime mover may be a turbine or diesel engine etc. The prime mover is equipped with a speed governor so that the speed of the generator is practically constant.
So the speed is practically constant for a generator, then the generator performance mainly deals with the relation between excitation, terminal voltage, and load. These relations can be best exhibited graphically by means of curves known as dc generator characteristics. These characteristics of a dc generator show the behavior of the generator under different load conditions.
DC Generator Characteristics
The three most important characteristics of a dc generator are given below
Magnetic or Open C– This curve shows the relation between the generated e.m.f. at no-load (E0) and the field current (If) at a constant speed. Internal or Total characteristic (E/Ia)– This curve shows the relation between the generated e.m.f. on load (E) and the armature current (Ia). External characteristic (V/IL) – This curve shows the relation between the terminal voltage (V) and load current (IL).
For a self-excited or separately excited DC generator, all the above three dc generator characteristics can be drawn. Of these characteristics, the shape of open circuit characteristic (OCC) is practically the same for all type dc generators.
Next, we are going to discuss the characteristics of all types of dc generators as given below:
Before going to these topics, we will discuss about the common characteristic of all generators, that is the open circuit characteristics or magnetic characteristics.
Open Circuit Characteristics
The Open Circuit Characteristic is one of the dc generator characteristics which is practically same for all dc generators. The OCC of even self-excited dc generator is obtained by running it as a separately excited generator. The procedure is explained below, The field winding of the dc generator (series or shunt) is disconnected from the machine and is separately excited from an external dc source as shown in Figure (ii).
The generator is run at a 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 (i).
Open Circuit Characteristics
The following points may be noted from OCC:
When the field current is zero, there is some generated emf 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.
It is because, in this range, the reluctance of iron is negligible as compared with that of the air gap. The air gap reluctance is a constant and hence linear relationship.
After point B on the curve, the reluctance of iron also comes into the picture.
It is because, at higher flux densities, μr for iron decreases and the reluctance of iron is no longer negligible. Consequently, the curve deviates from the linear relationship.
After point C on the curve, the magnetic saturation of poles begins and E0 tends to level off.
The Open Circuit Characteristic of even self-excited dc generator is obtained by running it as a separately excited generator.
Next: Read the characteristics of all types of dc generators given below:
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 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.
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.
(i) Generator is on no-load (ii) Generator on Load (iii) Superimposing Fluxes
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.
Geometric Neutral Axis (GNA) and Magnetic Neutral Axis (MNA)
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.
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).
(i)flux due to main poles (main flux) (ii) flux due to the current flowing in armature conductors alone (iii)flux due to the main poles and that due to the current in armature conductors acting together
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.
The dc generators and dc motors have the same general construction. In fact, when the machine is being assembled, the workmen usually do not know whether it is a dc generator or motor.
Any dc generator can be run as a dc motor and vice-versa
All dc machines have five principal components
Magnetic frame or Yoke
Pole Cores and Pole Shoes
Pole Coils or Field Coils
Armature core
Armature Winding
Commutator
Brushes and Bearings
The diagram given below represents the various parts of a DC machine.
1. Yoke (Magnetic Frame)
The outer frame or yoke serves a double purpose :
It provides mechanical support for the poles and acts as a protecting cover for the whole machine.
It carries the magnetic flux produced by the poles.
In small generators where cheapness rather than weight is the main consideration, yokes are made of cast iron. But for large machines usually cast steel or rolled steel is employed.
Yoke of DC Machine
The modern process of forming the yoke consists of rolling a steel slab around a cylindrical mandrel and then welding it at the bottom.
The feet and the terminal box etc. are welded to the frame afterward. Such yokes possess sufficient mechanical strength and have high permeability.
2. Pole Cores and Pole Shoes
The field magnets consist of pole cores and pole shoes. The pole shoes serve two purposes:
they spread out the flux in the air gap and also, being of larger cross-section, reduce the reluctance of the magnetic path
they support the exciting coils (or field coils)
Pole Cores and Pole Shoes
There are two main types of pole construction.
The pole core itself may be a solid piece made out of either cast iron or cast steel but the pole shoe is laminated and is fastened to the pole face by means of countersunk screws
In modern design, the complete pole cores and pole shoes are built of thin lamination of annealed steel which are riveted together under hydraulic pressure. The thickness of lamination varies from 1 mm to 0.25 mm.
3. Field system
The function of the field system is to produce a uniform magnetic field within which the armature rotates.
Field coils are mounted on the poles and carry the dc exciting current. The field coils are connected in such a way that adjacent poles have opposite polarity.
The m.m.f. developed by the field coils produces a magnetic flux that passes through the pole pieces, the air gap, the armature, and the frame.
Practical d.c. machines have air gaps ranging from 0.5 mm to 1.5 mm.
Field system
Since armature and field systems are composed of materials that have high permeability, most of the m.m.f. of field coils is required to set up flux in the air gap.
By reducing the length of the air gap, we can reduce the size of field coils (i.e. the number of turns).
4. Armature core and Lamination
The armature core is keyed to the machine shaft and rotates between the field poles.
It consists of slotted soft-iron lamination (about 0.4 to 0.6 mm thick) that are stacked to form a cylindrical core as shown in the figure.
Armature core and Lamination
The lamination are individually coated with a thin insulating film so that they do not come in electrical contact with each other.
The purpose of laminating the core is to reduce the eddy current loss. Thinner the lamination, greater is the resistance offered to the induced e.m.f., smaller the current and hence lesser the I²R loss in the core.
The lamination are slotted to accommodate and provide mechanical security to the armature winding and to give shorter air gap for the flux to cross between the pole face and the armature “teeth”.
5. Armature Winding
The slots of the armature core hold insulated conductors that are connected in a suitable manner. This is known as armature winding.
Armature Coil and Armature Core
This is the winding in which “working” e.m.f. is induced. The armature conductors are connected in series-parallel; the conductors being connected in series so as to increase the voltage and in parallel paths so as to increase the current.
The armature winding of a DC machine is a closed-circuit winding; the conductors being connected in a symmetrical manner forming a closed loop or series of closed loops.
Depending upon the manner in which the armature conductors are connected to the commutator segments, there are two types of the armature winding in a DC machine viz.,
(a) lap winding (b) wave winding.
6. Commutator
A commutator is a mechanical rectifier which converts the alternating voltage generated in the armature winding into a direct voltage across the brushes.
Commutator
The commutator is made of copper segments insulated from each other by mica sheets and mounted on the shaft of the machine.
The armature conductors are soldered to the commutator segments in a suitable manner to give rise to the armature winding.
Depending upon the manner in which the armature conductors are connected to the commutator segments, there are two types of the armature winding in a DC machine viz., Lap winding Wave winding. Great care is taken in building the commutator because any eccentricity will cause the brushes to bounce, producing unacceptable sparking.
The sparks may bum the brushes and overheat and carbonize the commutator.
7. Brushes
DC motors are of two types: one is brushed dc motor and the other one is brushless dc motor. Brushless dc motors are mainly used in high-speed applications such as multicopters (eg:- quadcopters).
The purpose of brushes in a dc generator is to ensure electrical connections between the rotating commutator and stationary external load circuit.
The brushes are made of carbon and rest on the commutator. The brush pressure is adjusted by means of adjustable springs.
Brushes
If the brush pressure is very large, the friction produces heating of the commutator and the brushes.
On the other hand, if it is too weak, the imperfect contact with the commutator may produce sparks. Multipole machines have as many brushes as they have poles. For example, a 4-pole machine has 4 brushes.
As we go round the commutator, the successive brushes have positive and negative polarities.
Brushes having the same polarity are connected together so that we have two terminals viz., the +ve terminal and the -ve terminal
The losses in a dc machine (generator or motor) may be divided into three classes viz (i) Copper losses (ii) Iron or core losses and (iii) Mechanical losses.
All these losses appear as heat and thus raise the temperature of the machine. They also lower the efficiency of the machine.
Copper losses
These losses occur due to currents in the various windings of the machine.
Armature copper loss = Ia2Ra
Shunt field copper loss = Ish2Rsh
Series field copper loss = Ise2Rse
Note. There is also brush contact loss due to brush contact resistance (i.e., resistance between the surface of brush and surface of commutator). This loss is generally included in armature copper loss.
Iron or Core losses
These losses occur in the armature of a d.c. machine and are due to the rotation of armature in th
Mechanical losses
These losses are due to friction and windage.
friction loss e.g., bearing friction, brush friction etc.
windage loss i.e., air friction of rotating armature.
These losses depend upon the speed of the machine. But for a given speed, they are practically constant.
Note.Iron losses and mechanical losses together are called stray losses.e magnetic field of the poles. They are of two types viz.,
hysteresis loss
eddy current loss.
(i) Hysteresis loss
Hysteresis loss occurs in the armature of the d.c. machine since any given part of the armature is subjected to magnetic field reversals as it passes under successive poles. Figure shows an armature rotating in two-pole machine. Consider a small piece ab of the
armature. When the piece ab is under N-pole, the magnetic lines pass from a to b. Half a revolution later, the same piece of iron is under S-pole and magnetic lines pass from b to a so that magnetism in the iron is reversed.
In order to reverse continuously the molecular magnets in the armature core, some amount of power has to be spent which is called hysteresis loss. It is given by Steinmetz formula. This formula is
Hysteresis loss Ph= η Bmax1.6 f V
Bmax = Maximum flux density in armature f = Frequency of magnetic reversals = NP/120 (where N is in r.p.m.) V = Volume of armature in m3 h = Steinmetz hysteresis co-efficient In order to reduce this loss in a d.c. machine, armature core is made of such materials which have a low value of Steinmetz hysteresis co-efficient e.g.,silicon steel.
(ii) Eddy current loss
In addition to the voltages induced in the armature conductors, there are also voltages induced in the armature core. These voltages produce circulating currents in the armature core as shown in Fig. These are called eddy currents and power loss due to their flow is called eddy current loss. The eddy current loss appears as heat which raises the temperature of the machine and lowers its efficiency.
If a continuous solid iron core is used, the resistance to eddy current path will be small due to large cross-sectional area of the core. Consequently, the magnitude of eddy current and hence eddy current loss will be large. The magnitude of eddy current can be reduced by making core resistance as high as practical. The core resistance can be greatly increased by constructing the core of thin, round iron sheets called lamination. The lamination are insulated from each other with a coating of varnish. The insulating coating has a high resistance, so very little current flows from one lamination to the other. Also, because each lamination is very thin, the resistance to current flowing through the width of a lamination is also quite large. Thus laminating a core increases the core resistance which decreases the eddy current and hence the eddy current loss.
Eddy current loss Pe = Ke Bmaxf2 t2 V
where Ke = Constant depending upon the electrical resistance of core and system of units used Bmax = Maximum flux density in Wb/m2 f = Frequency of magnetic reversals in Hz t = Thickness of lamination in m V = Volume of core inm³ It may be noted that eddy current loss depends upon the square of lamination thickness. For this reason, lamination thickness should be kept as small as possible.
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V = E – Ia (Ra + Rse )
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