July 12, 2020
Why are we interested in the DC generator? is it because it is an electrical device that converts mechanical energy into electrical energy ??!! … I don’t think so, DC generator is used everywhere:
In factories that need a large amount of current to produce aluminum, chlorine, and similar industrial materials.
Also in locomotives and ships which are driven by diesel-electric motors.
As well as electric razors, remote control cars, electric car windows, Flat-screen TVs, and other unlimited uses.
Honestly, that’s very good, but not satisfied enough, DC generator must have other privileges and that’s realized by:
The simple and compact design
The high reliability.
The high efficiency, which reaches 85% and sometimes 95%.
The Lighter weight,
And the lowest DC voltage ripples which are less than 5mV.
Before anything, we should explain that the DC generator is also known as DC dynamo, as the dynamo uses electromagnetism to produce direct current electric power.
Working principle of DC generator:
Let’s start our interesting exploration with the principle of Faraday’s law of electromagnetic induction, which is our base. Because the DC generator depends on this principle to produce DC power.
Faraday’s law states that when we put a conductor in a varying magnetic field an induced EMF will generate. This EMF will equal the rate of change of flux linkages.
Remember; there would be a relative space or relative time variation between the magnetic field and the conductor to generate this EMF.
This law directs us to focus on the most important elements of the DC generator which are:
And the motion of the conductor in the magnetic field.
And to get a better idea of the construction of the DC generator we will imagine that we have a simple loop DC generator with only one turn coil and one pair of a magnet.
Simple/Single loop DC generator
Here, they are static and need a motion; the conductor or the magnetic field must move, and we guess that there is a mechanical input caused by a rotation of the coil.
In this case, the current and the voltage have the same direction because the current causes this voltage.
Firstly, we will try to rotate the coil in the clockwise direction with the help of a prime-mover. Then, the coil will move to the right angle position.
Also, it will cut the flux lines. According to Faraday’s law, an EMF induced in the coil sides and the flux linked to the coil will be maximized because the coil parallels the magnetic field direction.
Fleming’s right-hand rule
The strength of the field on both sides of the coil isn’t the same, so it will be a change in flux sequentially. Since we have a closed-loop, a current will flow in the coil.
However, this current will lead us to an important rule Fleming’s right-hand rule which leads us to the direction of the current.
In this case and according to Fleming’s right-hand rule; the current flow from A to B, C then D.
If we again rotate the coil to the next right angle position, the coil will be perpendicular to the direction of the magnetic field, and there is no change in magnetic field strength, as a result, the current will be zero.
Again; when we rotate the coil to the next right position, the coil will be parallel to the magnetic field direction and a maximum EMF will be induced, but with a reversed current.
Artlessly; it’s the working principle of DC generator, but we notice that the current has a reversed position and it isn’t desirable in DC generator, and we solve that by commutator to convert this current to DC.
Hence the DC generator produced the required amount of EMF.
Working of DC generator with the commutator
When we use the commutator, we deal with an open loop. In the first case, the current passes through the coil and brushes segments.
Secondly, the current passes reversal in the coil and also in brushes segments but it has the same direction in the load resistance, and the current is unidirectional.
Construction of DC generator:
Components of DC generator
DC generator like all other machines consists of:
A stator: which is the stationary part, contains inside:
Yoke (magnetic frame).
Pole cores and pole shoes.
Brushes and brush holders.
The rotor: it’s the rotating part, in this machine; it’s the commutator which contains inside:
Those are the parts of DC generators.
Types of Dc generator:
Permanent-magnet DC generator.
Separately-excited DC generators.
And self-excited DC generators.
Each type has its special characteristics, applications, advantages, and also triples. And we will also explain intensively.
Advantages of DC generator:
As we say above, we depend on the DC generator in many fields because:
It has a simple design and construction.
It’s ideal for running big motors and big appliances which require direct current to provide power.
It reduces fluctuations described for some steady-state applications by smoothing the output voltage by the regular arrangement of coils around the armature.
Disadvantages of DC generator:
Unfortunately, like all other machines, it has some disadvantages
Dc generator can’t be applied to a transformer.
There would be a voltage drop over long distances.
DC generator has low efficiency because there are copper losses, eddy current losses, hysteresis losses, and mechanical losses.
Applications of Dc generators
Nowadays cheap and economical rectifiers and filters grant a new life to DC generators and give them great importance in all fields around us.
It also leads to diversity so there are many types of DC generators that we illustrated and which has many applications.
Before we talk separately about applications of each type of Dc generators; we find them in electrolytic processes, welding processes, variable-speed motor drives.
Also, in factories which need a large amount of current to produce aluminum, chlorine, and similar industrial materials.
Again, in locomotives and ships which are driven by diesel-electric motors. As well as electric razors, remote control cars, electric car windows, Flat-screen TVs, and other unlimited uses.
Applications of separately excited Dc generators:
From the name, separate needs an external source required to supply the needed field magnet winding current which may be:
Or another DC source
And that makes us think in cost; Separately excited DC generators need another external source.
It would be more expensive than the other types, so separately excited Dc generators are used where self-excited ones are unsatisfactory, so they are used where it’s important to have quick and response control.
Ward Leonard method of DC machine
In Ward-Leonard systems of speed control functions because of the availability of lower voltages.
For testing purposes in laboratories, which need a wide range of voltage output
As a supply source of DC motors because the property to operate in a stable condition with any field excitation.
Where the characteristics of quickness and responsiveness are important during work.
As an excitation source for large alternators in power generation station.
And they are also used as auxiliary and emergency power supplies.
Applications of the shunt wound DC generators:
Shunt-wound generators may be self-excited or separately excited depending upon the DC source used for starting it and it’s dropping voltage characteristic.
And as well as it has constant terminal voltage from no load to full load so those generators are used where a constant voltage is required.
Below are some uses
For the efficient work of the electrical appliances as the regulator helps to maintain the workable voltage.
In elector plating.
For general lighting purposes.
To charge batteries because they give a constant output voltage.
To provide excitation for alternators.
And also for small power supply.
Applications of series wound DC generators:
As the name explains; these generators carry a constant amount of current for a constant load, but it makes the series generator work with limited applications as a constant power source.
Applications of Series wound Dc generators :
In Dc locomotives for regenerative braking to provide field excitation current.
With distribution networks as a booster that helps to increase the voltage.
In series arc lighting and heavy power supply.
In Offices, hotels, homes, schools and other applications supplied by flat compound generators
For the arc welding purpose.
Regenerative braking applications of series wound generators
Applications of compound DC generators:
As we explained earlier, the compound generator is a combination of both series and shunt DC generator and it may be connected in
Long shunt compound generator
Short shunt compound generator
So it gives compound generators wide usage in:
Small distance operations such as supplying hotels, offices, homes, and lodges.
Regulating the voltage for a broad variety of load range
Adjusting the characteristics of the generator to compensate for the drop in resistance and accordingly provide a constant voltage supply.
Arc welding to regulate the voltage drop and give constant current.
Feeders to compensate for the voltage drop.
Lighting, power supply purposes, and heavy power services because of their constant voltage property.
Elevator motors applications of compound Dc generators
And also to supply power for:
Electrified railroad motors
Many industries as industrial motors
Driving a motor.
The efficiency of the DC generator
The efficiency of the DC generator… At normal conditions; when we hear efficiency, our mind automatically thinks in output and input. Of course, DC generators convert mechanical power into electrical power.
The efficiency of the DC generator depends on input mechanical power and output electrical power.
But in order to understand efficiency well, we must know losses which fortunately are less compared to Dc motor
Losses in DC generator are
Constant losses of Dc generator which are:
Core or iron losses listed in hysteresis losses and eddy current losses
And mechanical losses which are windage losses, friction losses (brush friction losses) and bearing friction losses
Variable losses of Dc generator are:
Copper losses listed in armature copper losses, field copper losses, and losses caused by brush contact resistance.
Stray losses which are copper stray load losses and core stray losses.
And that explained by the power flow diagram of a Dc generator
Power flow diagram of a DC generator
After we listed all types of losses we need to understand why and when they happened surely to be more aware of efficiency.
Core or iron losses:
This occurs in the armature core as an inevitable result of the rotation of the armature core in the magnetic flux, and the Popup losses:
These losses happen when the armature passes under the north and the south poles which lead to the reverse of magnetization of the armature core, and this loses depends on the iron volume, flux density maximum value, and the frequency.
Eddy current losses:
These losses are a result of the flow of eddy currents which are sets of currents produced in the armature core when it cuts the magnetic flux.
We laminate stack and revet the armature core to reduce these losses as much as possible. When we laminated the armature core the magnitude of eddy current reduced, and as a result, eddy current losses reduced.
From the name, these losses occur in the armature windings due to the brush contact resistance.
These losses are a result of varying the load which produces a current pass through the armature winding resistance and raise the temperature.
And mathematically: armature copper losses=Ia^2 Ra
Field copper losses:
Also from the name is a result of the flow of current in the field windings.
They expressed as field copper losses=If^2 Rf
Brush contact drop:
brush; when we hear this word we deduce that they are losses produced when contact happens between the brush and the commutator and of course they are constant with the load.
Stray load losses:
In contrast, they are losses varying with the load and it’s difficult to account so we assume that they are 1% of the full load, and they contain:
Copper stray load losses:
they are a result of skin effect affected the conductor and also due to eddy currents passing through the conductor.
Core stray load losses:
these losses depend on the flux density, good; but how! When the load current passes through the armature conductor, the flux density distorted in the teeth and the core.
That results from a net increase in the core losses especially in the teeth and simply that is stray load losses.
Let’s be more definite with equations:
The main equation we all know is:
P (mechanical input) = P (Elec) + losses.
And the generating efficiency will be:
ƞ=P (out) /P (in)= P(Elec)/[P(Elec)+losses].
The constant losses can be expressed as Pi
The variable losses can also express as Pcu
So; ƞ= Pout/ (Pelec +Pcu+Pi)
To be more accurate, the total resistance of the armature circuit including the resistance of the brush contact, the resistance of the series winding, and the resistance of inter-pole winding and compensating winding will be R
The output current= I
The shunt field current= Ish
The armature current= Ia = I+Ish
The terminal voltage= V
The total copper losses in the armature circuit= Ia^2 Rat.
The variable losses= Pcu
The constant losses= Pi
So the input power; Pin= Pout+Pcu+Pi
Maximum efficiency of dc generator
Without debate our total concentration is at maximum efficiency and the conditions for maximum efficiency:
We suppose that:
The output power= VI
And all losses are constant expect the armature copper losses so;
Losses= (I+Ish)^2 Ra +Pc = I^2 Ra+Pc
We neglect Ish because it’s little if we compare with the load current I
If you remember; to have the maximum we should differentiate so:
dƞ/(dI )=0=((VI+I^2 Ra+Pc)V-VI(V+2IRa))/(VI+I^2 Ra+Pc)^2
According to this equation, the efficiency will be maximized when variable losses= constant losses
This equation tells us that efficiency is proportional to the load current when the load current increases the efficiency increases as it reaches its maximum value when the load current is as in the previous equation.
And it can be clear by the efficiency curve drawn between the efficiency and the load current:
Construction of DC generator
Previously we talked about the working principle of DC generator, advantages, and disadvantages of DC generator. Let’s start.
Now we will recognize in detail construction of Dc generator
We will deal with a 4 pole DC generator and it consists of a stator and a rotor.
The stator of DC generator:
It’s the main fixed part of the generator, the stator is responsible for supplying the magnetic fields during coil rotation, and it mainly consists of:
DC generator Yoke (magnetic frame):
The yoke is the outer frame of the machine used to support and protect the internal parts. It’s made of permeability material processing sufficient mechanical strength and it also carries the magnetic field flux produced by field winding.
In small generators, we take care to make the yoke of cheap and heavier cast iron.
But in a large generator, we fabricate yoke of light cast steel or rolled steel.
Pole cores and pole shoes:
Pole cores are the most important of the field magnet, they have this great importance as they help:
Establishing the required magnetic flux how! when the current pass through the field winding placed on this pole, the core becomes electromagnetic. Hence, it establishes the magnetic flux and this flux can be varied by varying the current through field winding.
We fabricate them of cast steel or iron laminations to reduce the eddy current losses.
We connect these poles to yoke with bolts or welding.
In the other hand, the pole shoes are very important too;
Reduce the reluctance of the magnetic path by Increasing the cross-sectional area of the magnetic circuit.
Support the field winding.
Field coil wound around a pole shoe Pole core and pole shoe
DC generator field windings:
Generally, we fabricate field winding of low resistive materials copper or aluminum, we can connect this field windings to the armature in series or shunt.
When we connect the windings in series, we use less numbered turns with large cross-section conductors, instead of when they are shunted they would be in large turns with a less cross-section to withstand the supply voltage.
When the current pass through the winding coils, the pole core becomes electromagnetism, then produce the necessary magnetic flux. We take care to connect the field coils in a way that the poles are made of positive polarity.
Brushes and brush holders:
Brushes are important to us because we use them to ensure electrical connections between the rotating commutator and external load circuit because they collect current from commutator segments.
They have a rectangular block-shaped usually made of carbon or graphite which are conducting materials.
The brushes are housed in the brush holders which are secured to the front end housing with clamps.
Brushes also an added gain from its cheap coast, ability to minimize the sparking, and facilitate the collection of current from rotating commutator to stationary terminals.
But unfortunately, they require high maintenance, and they reduce the terminal voltage due to the brush contact drop.
Commutator brush types
End covers (housing):
We fabricate end covers from cast iron or cast steel attached to the ends of the mainframe.
The rotor :
Of course; it has no difference than other machines but here in DC generator differ according to the type permanent, separately excited or self-excited which lead us to look more in types of DC generator, but in all, it consists of:
DC generator Armature core:
Armature core has slots to accommodate the armature winding, and it is important as:
it helps to house the conductors in the slots.
It provides a low reactance path to the magnetic flux.
It’s also a cylindrical rotating part made of high permeability silicon steel stamping or lamination to reduce eddy current losses and minimize the hysteresis loss.
The armature windings are the heart of the generator in which the conversion of mechanical power to electrical power takes place.
Usually, we make Armature winding of aluminum to reduce the cost of the machine.
We can connect in one of the following:
In lap winding, we connect the conductors in a way that the number of parallel paths equals the number of poles to make each path in final have armature conductors/pole connected in series.
Here we use a number of brushes equal to the number of parallel paths, half of these brushes are positive and the others are negative.
In wave winding, we connect the conductors in a way that we have two parallel paths irrespective of the number of poles.
And finally, there will be two parallel paths (armature conductors/2) conductors in series. And here the number of brushes used equal two (number of parallel paths).
Of course, there are more and more about the lap and wave winding but it isn’t our interest now.
Fairly we consider it the rotor, and it is the most important in DC generator as:
It connects between the rotating armature conductors and stationary external circuits by the brushes.
It converts the alternating current produced in armature conductors to a unidirectional current in the external load circuit.
Sequentially it also converts the alternating torque to continuous torque in the armature in motor action.
And you should know that commutator is a mechanical rectifier made of wedge-shaped high conductivity hard drawn copper bars rotates with the armature.
We also use the commutator to transfer the current from the wire coil to the brushes and it keeps the current at the brush positive.
Commonly, we use the shaft to transfer mechanical power to the generator and turns the coil through the magnetic field.
And it is mild steel, cast iron or cast steel with a maximum bearing strength. And we support this shaft between two bearings. The armature core, commutator, and cooling fans are housed on the shaft.
When we spoke previously about motors, we gave a wide background about bearings. But now we will add that: we use ball bearings in a small machine and roller bearing in a heavy DC generator, and both are fitted in the end housing.
We also take care to use high carbon steel in bearing construction because the carbon is a very hard material.