Principle of DC Generator
A machine that translates the applied mechanical energy to electrical energy is termed an electrical generator. A dc generator is an electrical machine that transforms the mechanical energy applied to its shaft into electrical energy with dc electrical quantities.
The principle of dc generator is that whenever magnetic flux lines are cut by a conductor or in other words when there is a change in the magnetic flux EMF is induced. If the conductor offers a closed path then the induced EMF will allow a flow of current through the conductor. Flemming’s Right-hand rule gives the direction of the induced emf in an electric generator.
Following are the chief components of a dc generator:
- Magnetic field.
- Conductor or a bunch of conductors is also called armatures.
- Conductors or armatures in motion concerning the magnetic field lines or, magnetic flux.
Diagram of DC Generator
The following figure shows a diagram of the dc generator.
Construction of a DC Generator
The construction of dc generator would include the five following chief parts. The dc generator and dc motor both have the same general construction.
Parts of DC Generator
1. Field System
The field system of dc generator consists of an even number of salient poles which are bolted inside the circular frame of the machine. The circular frame to which the poles are bolted is known as the Yoke of the machine. The purpose of the field system is to produce a uniform magnetic field line inside the dc generator within which the armature of the generator rotates. The field coils are mounted on the field poles and these coils carry the d.c. current where the coils are connected in such a way that the adjacent poles will have the opposite polarity.
2. Armature Core
In a dc generator, the armature core is attached to the shaft of the generator and it rotates between the field poles. The armature cores consist of slotted soft-iron lamination stacked together to form a cylindrical core. The laminations are about 0.4 mm to 0.6 mm thick. The reason behind laminating the core is to reduce the eddy current loss. Also, the laminations are individually coated with a thin layer of an insulation film such that they do not come in contact with each other electrically.
3. Armature Winding
Inside the slots of the armature core in a dc generator, the slots hold insulated conductors which are connected suitably to form windings and it is known as armature winding. During the energy conversion in a generator, the EMF is induced in this armature winding. The armature conductors are connected in series-parallel. They are connected in series in order to increase the voltage of the winding and they are connected in parallel in order to increase the current of the winding.
A commutator in a dc generator can be referred to as a mechanical rectifier that converts the alternating voltage induced in the armature winding to dc voltage across the brushes. The commutator is mounted on the shaft of the machine and is made up of copper segments that are insulated from each other by mica sheets. The commutator segments and the armature conductors are soldered together to form armature winding.
There two types of armature windings in a dc machine and they are lap winding and wave winding. The type of winding is based on the type of connection established between the armature conductor and commutator segments.
To form an electrical connection between the commutator segments of the machine and the stationary external electrical load, brushes are used. In a dc generator, brushes are used to carry current from the commutator segment to an external electrical load.
Brushes rest on the commutator segments and are made up of carbon. The brushes in the machines are replaced at times as they wear out on the account of friction.
Types of Armature Winding
There two types of armature windings in a dc machine and they are wave winding and lap winding.
1. Wave Winding
In a wave winding irrespective of the number of poles present the machine, there is the presence of two parallel paths.
If there are Z number of armature conductors then reach parallel path will have Z/2 conductors in series.
The total EMF generated will be the EMF generated per parallel path and the total armature current will be two times the current of a parallel path.
2. Lap Winding
According to the number of poles in a machine, there may be several parallel paths present. If Z and P be the number of armature conductors and number of poles in a dc machine then each parallel patch will have Z/P conductors in series.
The total EMF generated will be the EMF generated per parallel path and the total armature current will be P times the current of a parallel path.
Choice of Armature Winding
The number of armature coil that can fit in with the machine of a given size will depend on the number of segments that can be fit in with the commutator. In the case of multipolar machines where P>2, the number of coils in series present in each of the parallel paths is less in the lap winding than in the wave winding arrangement. Hence, lap winding has a greater current carrying capacity whereas wave winding has a larger terminal voltage.
Small DC Generator
The current carrying capacity in small dc generators or dc machines is not of critical consideration. Hence, to obtain a suitable voltage wave winding is implemented.
Large DC Generator
In large dc generators or dc machines, there are large numbers of armature conductors so, the suitable voltage can be easily obtained and current capacity becomes a critical issue. Hence, in the case of large machines lap winding is employed.
DC Generator Working
The working of a dc generator is established on Faraday’s Law of Electromagnetic Induction. Whenever magnetic flux lines are cut by a conductor or in other words when there is a change in the magnetic flux EMF is induced. The induced emf will cause a current to flow provided that the conductor offers a closed path.
Single Loop DC Generator
For understanding the working of dc generator we will consider a single loop dc generator. Here a single loop conductor ABCD is placed in between the opposite poles of a magnet. The conductor will cut the magnetic field lines as the conductors rotate from a vertical to a horizontal position. During this movement of the conductor, the conductor side AB and CD will cut the magnetic field lines (flux), and thereby EMF will be induced in the sides AB and CD.
Under this orientation of the conductor, EMF will be induced and current will circulate through the loop. The current direction can be decided with the help of Flemming’s Righthand Rule. According to this rule, when three fingers of the right hand, the thumb, index, and middle finger are held mutually perpendicular to each other. The thumb pointing to the direction of the motion of the conductor, the index finger pointing to the direction of magnetic field lines (flux), the middle finger will give the direction of the current.
In this case, the direction of the current will be from A to B and this current will circulate the loop.
After some angle of rotation of the conductors, the conductor will be oriented in this fashion. Here the direction of revolution is tangential to the direction of magnetic field lines (flux) hence there is no cutting of flux by the conductor and hence no emf is generated.
Again after a certain angle of rotation, the conductors will be oriented in this fashion. Here, the AB comes under the North pole and CD under the south pole, so the direction of the current, in this case, will be from B to A and it will circulate the loop.
From this, we can observe that, in every half cycle of rotation of the conductor, the direction of current in the conductor will be reversed. This reversal of current will produce an alternating EMF which is undesirable. Hence to rectify this alternating EMF to DC EMF we will use a mechanical rectifier called the commutator and this process of rectifying is known as commutation. Here a split ring commutation is used for commutation.
Here, in the first half of the revolution, the current follows a path AB-XY-CD and the carbon brush(1) is in contact with segment x. In the second half of the revolution of the conductor, the current direction is reversed which results in the reversal of segments x and y where segment y is in contact with the carbon brush(1). This ensures that under both revolutions the direction of current in the load is XY. Hence dc emf is generated by the dc generator and is fed to a static load as shown.
EMF Equation of DC Generator
To derive an expression for the total emf generated in a dc generator. Let,
Φ = flux/Pole in Weber(Wb)
Z = total number of armature conductors
P = number of poles in the machine
A = number of parallel paths (A=2 for wave winding and A=P for lap winding)
N = armature speed in RPM
Eg= EMF of the generator = EMF/parallel path
When the conductor undergoes one revolution, the flux cut by one conductor is
The total time required to complete one revolution is
The total EMF generated per conductor
For wave winding A=2 and for a lap winding A=P.
DC Generator Types
In a dc generator, electromagnets are employed for the production of the magnetic fields inside the machine rather than permanent magnets. Generators are classified according to the methods of the field excitation of these field magnets. The dc generator types are as follows:
1. Separately excited dc generator
2. Self-excited dc generator
It is further classified as:
- Series DC Generator
- Shunt DC Generator
- Compound DC Generator
Power Stage in a DC Generator
The power stage in a dc generator is represented by the figure below.
The iron and friction loss is equal to Y-X. And, Z-Y is equal to the copper losses in the machine.
- Mechanical Efficiency
- Electrical Efficiency
- Commercial or Overall Efficiency
Losses in a DC Generator
Following losses occurs in a dc generator.
1. Copper Losses
Copper losses occur in the dc generator due to the flow of currents in the winding. these losses can occur in armature winding and the field winding of the generator.
2. Iron Loss or Core Loss
These losses come into account in the armature of the dc generator when the armature rotates in a magnetic field. These losses are further classified as follows:
a. Hysteresis Loss
During every half cycle of the armature rotation, the armature is subject to magnetic reversal. Hence, hysteresis loss comes into account.
Where Bm = Maximum Flux density in the armature core
f = Frequency of magnetic reversal = NP/120 (where N is in RPM)
V = Volume of armature
η = Steinmetz hysteresis coefficient
b. Eddy Current Loss
The rotating armature of the dc generator in the magnetic field generates emf due to which eddy current circulates in the armature core. The loss accounted for by the flow of eddy current is eddy current loss.
Where Ke = Constant
Bm = Maximum Flux density in the armature core
f = Frequency of magnetic reversal
t = Thickness of lamination
V = Volume of the core
3. Mechanical Loss
These losses in a dc generator are due to the account of friction and windage. Friction may be due to the bearing friction, carbon brush friction. Windage friction is mainly due to the air friction of the rotating armature.
Iron loss and mechanical loss can be both combined and can be termed as a stray loss.
Armature Reaction in DC Generator
In a dc generator when EMF is induced in the armature winding and current starts to flow through the armature conductor, these armature conductor starts to produce its own magnetic flux, called armature flux. The armature flux will now distort the main flux produced by the field winding of the generator. The interference on the main flux by the flux produced by the armature is known as the armature reaction in dc generator.
The figure below illustrates the armature reaction in dc generator.
Under the no-load condition, a small amount of current flows through the armature conductor and it does not appreciably affect the main flux Φ1 produced by the poles. This is shown in figure (i).
Under the loaded condition, sufficient current starts to flow through the armature conductor of the dc generator thereby affecting the main flux Φ2 produced by the poles. This is shown in figure (ii).
Figure (iii) shows the resulting flux Φ3 obtained by superimposing Φ1 and Φ2.
By observing figure (iii) we can see that tip B of the pole has increased flux density than tip A. This clearly shows a distorted main flux due to the armature reaction.