Browsing: DC Generator

Applications of DC Generators

DC generators are rarely used. There are various applications of dc generators that are given below:

Separately excited generators

Separately excited generator are required, separate supply to excite its field. Due to this, the use is restricted to some special applications such as electroplating, electrorefining of materials, etc.

DC shunt generators

These generators are commonly used in battery charging and ordinary lighting purposes.

DC series generators

These are commonly  used as boosters on DC feeders, as a constant current generators for welding generators and arc lamps.

Cumulatively compound generators

These are used for domestic lightning purposes and to transmit energy over long distances.

Differential compound generators

The use of differential compound generators is very rare and it is used for special applications like electric arc welding.

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DC Generator

A dc generator is an electrical machine that takes mechanical energy and converts it into electrical energy. A generator does not produce electrical energy by magic, it simply converts mechanical energy into electrical energy. This mechanical energy rotates the armature of a generator and thus electrical energy is produced in the armature conductor. For producing electricity from a generator, the following three requirements are essential:

Also Read :- Working Principle of DC Generator

  • Conductors
  • Magnetic field, and
  • Mechanical energy

Conductors

Insulated conductors are placed in the armature slots. These conductors may be copper conductors or aluminium conductors.

Magnetic Field

The magnetic field is produced by the use of either a permanent magnet or an electromagnet. However, in all generators the electromagnet is mostly used. The reason is that the magnetic field can be easily controlled. These field poles are provided on the pole shoe of the generator.

Mechanical Energy

For the rotation of the conductor, mechanical energy is obtained from the prime mover (diesel engine, steam turbine, water turbine, etc). The mechanical energy given to the generator rotates the armature conductors which cuts the magnetic lines of force, and emf is induced.

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Working Principle of DC Generator

A DC generator works on the principle of Faraday’s law of electromagnetic induction which says whenever a conductor is moved in the magnetic field or flux, an emf is generated in the conductors and the magnitude of the induced emf is directly proportional to the rate of change of flux linkage.

The emf generated in the conductor is of alternating nature and the generator which gives out electrical energy in the form of alternating current is called a synchronous generator or alternator.

The generator, that gives out electrical energy in the form of direct current is called a dc generator. The essential difference between the alternator and dc generator is that is an alternator slip rings are used to collect the supply, while in the dc generator, the commutator is used for this purpose. This commutator converts the alternating current induced in the conductors into direct current for the external circuit.

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Types of DC Generators

DC generators are generally classified according to the methods of their field excitation. Based upon the method of excitation, dc generators can be divided into;

  1. Separately excited DC generator
  2. Self-excited DC generator 
  1. Separately excited DC generator

A DC generator in which current is supplied to the field winding from an independent external DC source (e.g. a battery) is called separately excited DC generator.  The flux produced by the poles depends upon the field current. 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.

separately excited generator

Important Relation;

Here, Ia = IL = I

Ia = armature current

IL = Line current

Ra = armature resistance

Vb = brush drop per brush

Terminal Voltage, V  = Eg – IaRa

Terminal voltage, V  = Eg – IaRa – 2vb

Power developed = Eg Ia

Power output = VIL = VIa

  1. Self – excited generators

Separately – excited generators are those whose field magnets are energized by the current produced by the generators themselves. Due to residual magnetism, there is always present some flux is the poles. When the armature is rotated, some e.m.f. and hence some induced current is produced which is partly or fully passed through the coils, thereby strengthening the residual  There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature, namely;

  1. Series wound DC generators
  2. Shunt wound DC generators
  3. Compound wound DC generators
  1. Series wound DC generators

In series wound DC generator, the field winding is connected in series with the armature winding forming a series circuit. Therefore, full line current IL or armature current Ia flows through it. As they carry full load current, they consists of relatively few turns of thick wire or strip.

Important Relation;

Here, Ia = IL = Ise

Ia = armature current

IL = Line current

Ra = armature resistance

Rse = series resistance

Vb = brush drop per brush

Terminal Voltage, V  = Eg – IaRa – IseRse

Or

Terminal Voltage, V  = Eg – Ia (  Ra – Rse)- 2vb

Power developed = Eg Ia

Power output = VIL = VIa

2. Shunt wound DC generators

in a shunt wound DC generators, the field winding is connected across the armature winding forming a parallel or shunt circuit. Therefore, full terminal voltage is applied across them. As they carry very small load current, they consists of many turns of fine wire.

Important Relation;

Here, Ia = IL = Ish

Ia = armature current

IL = Line current

Ra = armature resistance

Rsh = series resistance

Vb = brush drop per brush

Ish = shunt current

Ish = V/Rsh

Ia = IL + Ish

Terminal Voltage, V  = Eg – IaRa

Or

Terminal Voltage, V  = Eg – Ia Ra– 2vb

Power developed = Eg Ia

Power output = VIL = VIa

3. Compound wound DC generators

In compound wound DC generator, there are two sets of field windings on each pole. One of them (having many turns of fine wire ) is connected across the armature and the other winding (having few turns of thick wires ) is connected in series with the armature winding. A compound wound DC generator may be; Long Shunt and Short Shunt.

Long shunt in which the shunt field winding is connected in parallel with combination of both armature and series field winding.

Short shunt in which the shunt field winding is connected in parallel with only armature winding.

Compound Short Shunt Generator

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EMF Equation of DC Generator

An electrical generator is an electrical device that converts mechanical energy (or power) into electrical energy (or power). The working principle of dc generator is based on dynamically induced emf. According to the Faraday’s law of electromagnetic induction whenever a conductor cuts by the magnetic flux or field, dynamically induced e.m.f. produced in it.

Let,

Φ = flux per pole in Weber (Wb)

Z = total number of armature conductors

N = speed of armature in r.p.m.

P = number of poles

A = number of parallel paths in armature winding

Eg = generated emf in volts

Flux cut by one conductor in one revolution = Wb

Time taken to complete one revolution, dt = 60/N

The number of conductors conducted in series in each parallel path = Z/A

Average induced e.m.f. across each parallel path or across armature terminals,

For Wave Winding

Number of parallel paths, A = 2

For Lap Winding

Number of parallel paths, A = Number of poles

Conclusion

Thus, we conclude that the induced e.m.f. is directly proportional flux per pole (Φ) and speed (N).

 

 

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Construction of DC Machines

DC machine consists of two main parts. One of them is armature and other is field system. The field system is stationary part of the dc machine and armature is rotating part. In this article we will discuss about the construction of DC machines

YOKE

It is a outer body of the machine. It is cylindrical in shape. It serves the following purpose in machine:

  • It supports inner part of the machines e.g. poles are fixed on it.
  • It provides mechanical protection to the inner parts of the machine.
  • It provides low reluctance part for the magnetic flux.

Cast Iron yokes are made for small machines and for large machine. It is made up of fabricated steel.

Construction of DC Machine

POLES

Pole consists of two main parts.

  • Pole Core
  • Pole shoe

POLE CORE

The Pole Core is made of thin cast steel or wrought iron laminations which are riveted together. The pole core is circular in section and filed coil is wound over it.

POLE SHOES

Pole Shoes are also made of cast steel or wrought iron laminations and it screwed to the pole face. Pole shoes have larger cross section area. The poles sever following purposes.

  • It supports the filed coils.
  • It spreads out the magnetic flux in the air gap.

FIELD OR EXCITING COILS

Fields coils are made of enameled copper. The coils are wound on the former then placed around the pole core as the field coils of all the poles are connected in series in, such as way that when current flows through them, the adjacent poles attain opposite polarity. When direct current passed through the field coils, the required magnetic flux produced.

ARMATURE

The armature is the rotating part of the dc machine. The conversion of energy takes place in the armature. The armature core is of laminated silicon steel. Laminations are used to reduce the eddy current losses and silicon steel is used to reduced the hysteresis losses. Armature is cylindrical in shape and keyed to the rotating shaft. At the outer periphery slots are cut, which accommodate the armature winding.

ARMATURE WINDING

Enameled copper is used for the construction of armature winding. The armature winding is housed in armature slots, which is suitably connected. The armature winding is heart of the dc machine.

On the basis of connections, there are two types of armature winding.

  • Lap Winding
  • Wave Winding

COMMUTATOR

It is the most important part of the DC machine. It is just a reversing switch. Commutator connects the rotating armature conductors to the stationary external circuit through brushes.

In generating action, it converts alternating voltage into direct voltage and in motoring action it converts unidirectional torque into alternating torque.

The commutator is a form of rotating switch placed between armature and external circuit and so arranged it will reverse the connections with the external circuit at the instant of each reversal of current in the armature.

The commutator is of cylindrical shape and is made up of wedge shape hard drawn copper segments. The segments are insulated from each other by a thin sheet of mica.

BRUSHES

The Brushes are pressed upon the commutator and make the connecting link between the armature winding and external circuit. Carbon is used for the construction of Brushes because it is conducting material and good lubricating material.

BRUSH ROCKER

It holds the spindles of the brush holders. It is fitted on to the stationary frame of the machine with nut and bolts.

END HOUSING

End housings are attached to the ends of the main frame and support bearings. The front housing supports the bearings and the brush assemblies whereas the rear housing usually support the bearing only.

BEARINGS

The function of Bearings is to reduce friction between the rotating and stationary parts of the machine. Usually ball or roller bearing are used.

SHAFT

The Shaft is made of mild steel. The shaft is used to transfer mechanical poser from or to the machine.

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