What is a DC Generator : Construction and Its Working

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What is a DC Generator?

DC Generator is an Electrical Machine which converts Mechanical Energy into DC Electrical Energy. It mainly consists of armature, field coils which works as electromagnets in it. There are so many Parts of DC Generator which plays a significant role during its working. A DC Generator works on the principle of Faraday’s laws of electromagnetic induction. It provides DC Electrical Energy which is used in many applications like DC transmission lines, heavy DC Power Supplies etc. So, lets discuss about the working principle of DC Generator first and then we will move on to the construction of dc generator and will study further more topics and details of dc generator below:

DC Generator
DC Generator

Construction of DC Generator:

The construction of DC Generator consists of so many parts and each and every part has its own functions and benefits that we can get from it. The main parts of dc generators are Yoke, Field Poles, Field Winding, Armature and armature winding and commutator. The complete list of all the parts of DC Generator is as under:

Construction of DC Generator
Construction of DC Generator

Yoke/Main Cover:

Yoke is the main cover of the DC Generator and is generally made up of iron. It provides a mechanical support to the Field poles. It acts as a protecting cover for the whole machine. It also carries the magnetic flux produced by the Field poles. These yokes are made up of cast iron and for large machines cast steel or rolled steel is used.

Laminated Field Pole:

Laminated Field Poles are used as field magnets. These are used to spread out the flux in the air gap and it is also used to support the exciting coils. Pole cores are the solid piece and are made up of cast iron or cast steel. In modern age pole cores are made up of thin laminations of enameled steel. The thickness of lamination is from 1 milli meter to 0.25 milli meter. The laminated poles are more secured than the solid piece pole cores.

Field Windings:

Field Windings are the coils of a conductor wounded across the Field Poles/Pole Cores. When current is passed through these coils the poles reacts as an electromagnet and magnetic flux is produced in it.

Armature & Armature Core:

Armature core is a cylindrical or drum shaped and is built up of circular steel sheets or laminations. It is set to the shaft. The slots are punched on its outer side. In small machines or motors the armature slots are set directly to the shaft. These laminations are used for the cooling purposes for the armature and to reduce the eddy current losses and voltage drops. The circular stampings or slots are cut out in one piece.

Armature Winding:

The armature windings are in a wound form. These windings are insulated from each other and normally copper winding are generally used in it. These conductors or winding is placed in the armature slots which are lined with insulation.

Commutator:

The commutator provides a collection of currents from the armature winding or conductors. It converts the alternating current which is induced in the armature conductors into direct current (DC). It is a cylindrical shaped and have wedge shaped segments. These segments are insulated from each other. The number of segments are equal to the number of armature coils. Each segment is connected to the armature conductor through riser or strip. These segments have V-grooves to prevent from centrifugal forces.

Brush Gear:

Brush gears holds heavy carbon brushes and are used to transfer huge amount of excitation current from it to the terminal junction. Normally these are used in Highly Rated DC Generators. The brush gear is used to change its position according to the requirement either you want small output current or either you want maximum current and the most important use case of the brush gear is to adjust the carbon brushes in different direction to get maximum DC output from the DC Generator.

Capacitor:

Capacitor is a basic component of DC Generator which is used to remove some fluctuations and filters the DC output supply from its output terminals. It provides smoothness in output voltage and provides constant voltage.

Load Junction:

Load junction of a dc generator consists of the two output terminal wires negative and positive which are installed in the terminal box to provide the load current and are used to connect the load supply on it. This junction is very strong and the two wires are connected to the commutator from which they take DC in it and then provide it on its junction for the load connection.

Terminals:

In some modified DC Generators, the manufacturers provide an easy junction for the load and the manufacturers provide the two wires which are already connected to the load junction and these two wires are assembled and installed in the main cover or yoke of the DC Generator to connect directly to the load connection.

Suppressor Box:

A Suppressor box is used to reduce the irrelevant and unwanted noise of a high KV rated DC Generators. This box consists of various valves and some other filters like some holes, iron rings, resistive net etc. Which are used to reduce the noise from the generator.

Terminal Cover:

Terminal cover is a cap which protects the joints of the DC generator terminals and the load terminals and this cover is also used for safety measures to prevent from electric shock and fire.

Carbon Brush:

In DC generator, Carbon brushes are used to collect current from the commutator and then provide it to its main contacts. These brushes are commonly made up of carbon and graphite. These brushes are rectangular shaped. These brushes are mounted on the brush holders and the brush holder is mounted on a spindle. These brushes remain joined with the commutator by a spring. A flexible copper pigtail is mounted on the top of the carbon brushes which convey current from the brushes to the holder.

Ball Bearings:

Bearings are used to rotate shaft very easily. Usually ball bearings are frequently used in heavy machines due to its flexibility. Ball bearings work very efficiently in heavy machines. The balls and rollers of ball bearings are packed in hard oil.

Bearing Support Head:

Bearing support head looks like the shape of bearing and it holds the bearing in its case to provide support and alignment and it is fixed to straight the bearing and it provides the complete support in a required direction to provide uninterruptable function.

Shaft:

A Shaft of a DC Generator is a mechanical component. It consists of a mild steel and it provides torque and rotation. It is prototype and long and it is used for coupling in DC Generator to provide mechanical energy.

Shaft Spline:

Shaft spline is a small component installed on a shaft sometime it is fixed on a shaft and sometimes it comes in variable form. A shaft spline consists of teeth and in a circular form and is used for coupling/joining the other shaft to it for mechanical input for DC Generator.

Clamps:

Clamps are used to hold the load wires tightly and make the load terminals safe and clean.

Window Strap:

Window strap is basically a net made up of iron and it is connected to the yoke and it is used to provide air gap to the DC Generator for the purpose of cooling and it provides a better temperature for cooling.

Retaining Cap:

Retaining cap are made up of alloy of a non-magnetic steel and is tighten on the shaft to provide support to the shaft against centrifugal forces during rotation.

End Cover:

End cover is a part of yoke and it covers from the end of DC Generator with the help of bolts and it is used to protect the generator from its end.

Working Principle of DC Generator:

The working principle of dc generator is same as the faraday’s law of electromagnetic induction and a simple loop generator/ loop generator. The working principle of DC generator is when the armature starts moving through the mechanical energy applied on its shaft then the armature coils produce an EMF. This EMF links with the field poles Due to magnetic flux and the  current is generated in the field poles to provide DC on its output terminals/load terminals. For Further understanding let’s move on to the Faraday’s Laws of Electromagnetic Induction below:

Working Principle of DC Generator
Working Principle of DC Generator

Faraday’s Laws of Electromagnetic Induction:

Basically, faraday’s law of electromagnetic induction is further divided into two slightly different laws and dc generator works on the Faraday’s first law of electromagnetic induction. Both Faraday’s Laws of electromagnetic induction are described below:

Faraday’s First Law of Electromagnetic Induction:


Faraday’s first law of electromagnetic induction states that “When a conductor cuts a magnetic flux, an EMF is induced in that conductor.” A Simple loop generator/loop generator is based on this law of electromagnetic induction.

Faraday’s Second Law of Electromagnetic Induction:


Faraday’s second law of electromagnetic induction states that “The magnitude of generated EMF in a conductor(wire) is equal to the rate of change of flux linkage.”

The conversion of mechanical energy into electrical energy is based on the faraday’s first law of electromagnetic induction and this law is used in the construction of DC Generator. Without this law, DC Generator is not capable to convert mechanical energy to DC electrical energy.
For further explanation of the Working Principle of DC Generator, lets move on to the Simple form of DC Generator named as Single loop Generator. This generator works exactly on the Faraday’s First Law of Electromagnetic induction. This Loop Generator is described below in further more detail:

Single Loop Generator:

Loop Generator / Simple Loop Generator is a basic concept of the DC Generator. It is a conductor placed between the magnet bars. These magnetic bars may be of permanent magnet or electro magnet. The basic working of the simple loop generator is that when the two wires, conductors or coils are placed between the magnet bars and imagine the coil is rotating clockwise, the flux starts changing its value and as a result EMF is induced in it.

Construction:

A simple loop generator is basically consists of two conductors or coils; two magnet bars and two slip rings(which are insulated with each other and from central shaft) and a normal wire for connection as required. The rotating coil may be assumed as armature and the magnet bars field magnets.

Working Principle:

Assume that the coil or the two conductors are rotating in a clock-wise direction. The coil has a fixed position between the field and the flux linked with it starts changing its value. As a result, EMF is induced in it and it is proportional to the rate of change of flux linkage. When the coil reaches at 90 degree in the process of rotating, the coil slides don’t cut the flux and the flux move parallel between them. As a result, EMF is not induced in the coil. As the coil continues its rotation, the amount increases to its maximum and when the coil reaches again at 90 degrees then in this case maximum EMF is induced in the coil this is because of the maximum flux is produced.

The result is that when the coil rotates from 90 to 180 degrees, the flux increases gradually and the rate of change of flux linkage decreases.

When the coil rotates from 180 to 360 degrees the variation occurs in the magnitude of the EMF.at first EMF remains similar on the middle position EMF reaches to its maximum and at the end minimum EMF is produced.

Conclusion:

Remember that the current produced in the coils is alternating and we get this current rectified by the split rings. Hence it is cleared that the armature of the DC Generator also provides Alternating Voltages.

Types of DC Generator:

DC generators are classified into two different types which are described below:

Types of DC Generator
Types of DC Generator

Separately Excited DC Generators:

As described from its name that these DC Generators are those generators whose field magnets are excited/energized with a separate source of DC as shown in figure below:

Separately Excited DC Generators
Separately Excited DC Generators

Self-Excited DC Generators:

Self-excited dc generators are those generators whose field magnets are excited/energized by the dc generator’s own producing current. The flux is present in poles through residual magnetism so that a current is produced in it and it energizes the field magnets of dc generator. These self-excited generators are further divided into its three types which are described below:

Shunt Type DC Generators/Shunt Wound DC Generators

In these type of dc generators, the field winding is connected in parallel with the armature and this field winding is connected to the supply as shown in figure below:

Shunt Type DC Generators
Shunt Type DC Generators

Series Type DC Generator/Series Wound DC Generators:

In this type of dc generators, the field winding is connected in series with the armature. It consists of few turns of thick wire and carries full load current. These generators are also called as special purpose dc generators and are used as voltage boosters in many places.

Series Type DC Generator
Series Type DC Generator

Long Shunt Compound DC Generator:

Long shunt compound dc generators are those generators in which shunt field winding is connected in parallel connection with the armature and series field winding whereas the series field winding is connected in series with the armature of dc generator.

Long Shunt Compound DC Generator
Long Shunt Compound DC Generator

Losses in DC Generator

Every electrical machine which consume electric supply has many losses during its operation and so many electric power is wasted. In dc generator, there are so many kinds of losses occurred during its operation these losses are iron losses, hysteresis losses, eddy current losses, copper losses etc. These losses are mentioned below:


Iron Losses/Core Loss

These losses are occurred due to the iron core and magnetic flux produced in field poles and these losses are called iron losses/core losses. Iron loss consist of two losses which are described below:


Hysteresis Loss

This loss is occurred due to the changing magnetization of the armature core and this magnetization reverses the polarity which causes the opposite flux and this flux starts wasting.


Eddy Current Loss

Due to the rotation of the armature core it also cuts the magnetic flux and EMF is induced in the body of core. This current has low value and this current is known as eddy current and due to the wastage of eddy current so this is known as eddy current loss.


Copper Loss

Copper losses are not produced if we use standard conductor/conducting material in the winding but sometimes a material is not good for conduction and it takes so many power and current cannot flow easily through this conductor. So, these are called copper losses.


Mechanical Losses

These losses are friction losses at bearings and commutator and air friction loss in rotating armature.


Stray Losses

The addition of Magnetic losses and mechanical losses are known as stray losses.


Power Stages of DC Generator

There are many power stages of a DC Generator and these stages are shown below:

Power Stages of DC Generator
Power Stages of DC Generator

Applications of DC Generator

  • Separately excited dc generators are used as boosters and in the process of electroplating and as a regulator etc.

  • Compound dc generators are used as power supply of welding machines.

  • DC generators are also used to reduce the voltage drops in feeders.

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