What is a DC Generator : Construction and Its Working

The initial electromagnetic generator (Faraday disk) was invented by British scientist namely Michael Faraday in the year 1831. A DC generator is an electrical device used for generating electrical energy. The main function of this device is to change mechanical energy into electrical energy. There are several types of mechanical energy sources available such as hand cranks, internal combustion engines, water turbines, gas and steam turbines. The generator provides power to all the electrical power grids. The reverse function of the generator can be done by an electric motor. The main function of the motor is to convert electrical energy to mechanical. Motors, as well as generators, have similar features. This article discusses an overview of DC generators.

What is a DC Generator?

A DC generator or direct current generator is one kind of electrical machine, and the main function of this machine is to convert mechanical energy into DC (direct current) electricity. The energy alteration process uses the principle of energetically induced electromotive force. The dc generator diagram is shown below.

When a conductor slashes magnetic flux, then energetically induced electromotive force will be generated in it based on the Electromagnetic Induction principle of Faraday’s Laws. This electromotive force can cause a flow of current when the conductor circuit is not opened.

Construction

A DC generator is also used as a DC motor without changing its construction. Therefore, a DC motor otherwise a DC generator can be generally called a DC machine. The construction of a 4-pole DC generator is shown below. This generator comprises of several parts like yoke, poles & pole shoes, field winding, an armature core, armature winding, commutator & brushes. But the two essential parts of this device are the stator as well as the rotor.

Stator

The stator is an essential part of the DC generator, and the main function of this is to provide the magnetic fields where the coils spin. This includes stable magnets, where two of them are with reverse poles facing. These magnets are located to fit in the region of the rotor.

Rotor or Armature Core

Rotor or armature core is the second essential part of the DC generator, and it includes slotted iron laminations with slots that are stacked to shape a cylindrical armature core. Generally, these laminations are offered to decrease the loss because of the eddy current.

Armature Windings

The armature core slots are mainly used for holding the armature windings. These are in a closed circuit winding form, and it is connected in series to parallel for enhancing the sum of produced current.

Yoke

The external structure of the DC generator is Yoke, and it is made with cast iron otherwise steel. It gives the necessary mechanical power for carrying the magnetic-flux given through the poles.

Poles

These are mainly used to hold the field windings. Usually, these windings are wound on the poles, & they are connected in series otherwise parallel by the armature windings. In addition, the poles will give joint toward the yoke with the welding method otherwise by using screws.

Pole Shoe

The pole shoe is mainly utilized for spreading the magnetic flux as well as to avoid the field coil from falling.

Commutator

The working of the commutator is like a rectifier for changing AC voltage to the DC voltage within the armature winding to across the brushes. It is designed with a copper segment, and each copper segment is protected from each other with the help of mica sheets. It is located on the shaft of the machine.

DC Generator Commutator Function

The main function of the commutator in the dc generator is to change the AC to DC. It acts like a reversing switch and its role in the generator is discussed below.

The emf which is induced within the armature coil of the generator is alternating. So, the flow of current within the armature coil can also be alternating current. This current can be reversed through the commutator at the accurate moment once the armature coil crosses the magnetic unbiased axis. Thus, the load attains a DC or uni-directional current.

The commutator guarantees that the flow of current from the generator will flow forever in a single direction. The brushes will make high-quality electrical connections among the generator & the load by moving on the commutator.

Brushes

The electrical connections can be ensured between the commutator as well as the exterior load circuit with the help of brushes.

Working Principle

The working principle of the DC generator is based on Faraday’s laws of electromagnetic induction. When a conductor is located in an unstable magnetic field, an electromotive force gets induced within the conductor. The induced e.m.f magnitude can be measured from the equation of the electromotive force of a generator.

If the conductor is present with a closed lane, the current which is induced will flow in the lane. In this generator, field coils will generate an electromagnetic field as well as the armature conductors are turned into the field. Therefore, an electromagnetically induced electromotive force (e.m.f) will be generated within the armature conductors. The path of induced current will be provided by Fleming’s right-hand rule.

DC Generator E.M.F Equation

The emf equation of dc generator according to Faraday’s Laws of Electromagnetic Induction is Eg= PØZN/60 A

Where Φ is

flux or pole within Webber

‘Z’ is a total no.of armature conductor

‘P’ is a number of poles in a generator

‘A’ is a number of parallel lanes within the armature

‘N’ is the rotation of armature in r.p.m (revolutions per minute)

‘E’ is the induced e.m.f in any parallel lane within the armature

‘Eg’ is the generated e.m.f in any one of the parallel lanes

‘N/60’ is the number of turns per second

Time for one turn will be dt = 60/N sec

Types of DC Generator

The classification of DC generators can be done in two most important categories namely separately excited as well as self-excited.

Separately Excited

In separately excited type, the field coils are strengthened from an autonomous exterior DC source.

Self Excited

In the self-excited type, the field coils are strengthened from the generated current with the generator. The generation of the first electromotive force will occur because of its outstanding magnetism within field poles.

The produced electromotive force will cause a fraction of current to supply in the field coils, therefore which will increase the field flux as well as electromotive force generation. Further, these types of dc generators can be classified into three types namely series wound, shunt-wound, and compound wound.

• In a series wound, both the field winding & armature winding are connected in series with each other.
• In shunt-wound, both the field winding & armature winding are connected in parallel with each other.
• The compound winding is the blend of series winding & shunt winding.

The Efficiency of DC Generator

DC generators are very reliable with efficiency ratings of 85-95%

Consider the output of a generator is VI

The input of a generator is VI + Losses

Input = VI + I2aRa + Wc

If the shunt field current is insignificant, then Ia = I (approximately)

After that, n = VI/ (VI+Ia2Ra+wc) = 1/(1+Ira/V+wc/VI)

For highest efficiency d/dt (Ira/V+wc/VI) = 0 otherwise I2ra = wc

Therefore efficiency is highest once variable loss is equivalent to the constant loss

The load current equivalent to the highest efficiency is I2ra = wc otherwise I = √wc/ra

Losses in DC Generator

There are different kinds of machines available in the market where the total input energy cannot be changed into output due to the loss in the input energy. So different losses can occur in this type of generator.

Copper Loss

In armature copper loss (Ia2Ra), where the armature current is ‘Ia’ & the armature resistance is ‘Ra’. For generators like shunt-wound, the field copper loss is equivalent to Ish2Rsh which is almost stable. For generators like a series wound, the field copper loss is equivalent to Ise2 Rse which is also almost stable. For generators like compound-wound, the filed copper loss is similar to Icomp2 Rcomp which is also nearly stable. In full load losses, copper losses occur 20-30% because of the brush contact.

Core or Iron or Magnetic Loss

The classification of core losses can be done into two types like hysteresis and eddy current

Hysteresis Loss

This loss mainly occurs because of the reversal of the armature core. Every part of the rotor core passed below the two poles like north & south alternately & achieves S & N polarity correspondingly. Whenever the core supplies below one set of poles, then the core will finish one series of frequency reversal. Please refer to this link to know more about What is Hysteresis Loss: Factors & Its Applications

Eddy Current Loss

The armature core slashes the magnetic flux throughout its revolution & e.m.f can be induced within the outside of the core, based on the electromagnetic induction laws, this emf is extremely tiny, however, it sets up a large current in the surface of the core. This huge current is known as eddy current whereas the loss is called the eddy current loss.

Core losses are stable for compound & shunt generators because their field currents are nearly stable. This loss mainly occurs 20 % to 30 % in full-load losses.

Mechanical Loss

Mechanical loss can be defined as the rotating armature’s air friction or windage losses Friction loss mainly occurs 10 % to 20 % of full load losses at bearings & commutator.

Stray Loss

Stray losses mainly occur by combining the losses like core as well as mechanical. These losses are also called rotational losses.

Difference between AC and DC Generator

Before we can discuss the difference between AC & DC generator, we have to know the concept of generators. Generally, generators are classified into two types like AC and DC. The main function of these generators is to change the power from mechanical to electrical. An AC generator generates an alternating current whereas the DC generator generates direct power.

Both generators use Faraday’s law to generate electrical power. This law tells that once a conductor shifts within a magnetic field then it slashes magnetic lines of force to stimulate an EMF or electromagnetic force within the conductor. This induced emf’s magnitude mainly depends on the magnetic line force connection through the conductor. Once the circuit of the conductor is closed then the emf can cause flow of current. The main parts of a dc generator are the magnetic field & conductors that move within the magnetic field.

The main differences between AC & DC generators are one of the most important electrical topics. These differences can assist students to study about this topic but before that, one should know about the AC generators as well as dc generators in every detail so that differences are very simple to understand. Please refer to this link to know more about The Difference between AC and DC Generator.

Characteristics

The characteristic of the DC generator can be defined as the graphical representation among the two separate quantities. This graph will show the steady-state characteristics which explain the main relationship between the terminal voltage, loads & excitation through this graph. The most essential characteristics of this generator are discussed below.

Magnetization Characteristics

The magnetization characteristics provide the difference of producing voltage otherwise no-load voltage through field current at a stable speed. This kind of characteristic is also known as an open circuit otherwise no-load characteristic.

Internal Characteristics

The dc generator’s internal characteristics can be plotted between the load current as well as generated voltage.

External or Load Characteristics

The load or external type characteristics provide the main relationships among the load current as well as terminal voltage at a stable speed.

Advantages

The advantages of a dc generator include the following.

• DC generators generate large output.
• The terminal load of these generators is high.
• The designing of dc generators are very simple
• These are used to generate uneven output power.
• These are extremely consistent with 85-95%.of efficiency ratings
• They give a reliable output.
• They are lightweight as well as compact.

Disadvantages

The disadvantages of a dc generator include the following.

• DC generator cannot be used with a transformer
• The efficiency of this generator is low due to many losses like copper, mechanical, eddy, etc.
• A voltage drop can occur over long distances
• It uses a split ring commutator so it will complicate the machine design
• Expensive
• High maintenance
• The sparks will be generated while generating energy
• More energy will be lost while transmission

Applications of DC Generators

The applications of different types of DC generators include the following.

• The separately excited type DC generator is used for boosting as well as electroplating. It is used for power and lighting purpose using a field regulator
• The self-excited DC generator or shunt DC generator is used for power as well as ordinary lighting using the regulator. It can be used for battery lighting.
• The series DC generator is used in arc lamps for lighting, stable current generator, and booster.
• A compound DC generator is used to provide the power supply for DC welding machines.
• Level compound DC generator is used to provide a power supply for hostels, lodges, offices, etc.
• Over compound, DC generator is used to reimburse the voltage drop within Feeders.

Thus, this is all about the DC generator. From the above information finally, we can conclude that the main advantages of DC generators include simple construction & design, the parallel operation is easy, and system stability problems are less not like the alternators. Here is a question for you, what are the disadvantages of DC generators?