DC GENERATOR

04/08/23 GROUP I1

Aa Bb “DC”Ee FfGg..… The New Alphabet

04/08/23 GROUP I2

What is DC GENERATOR?

Direct Current generators are dc machines used as a dc generator. As with AC generator, DC generators are energy converters. The DC generator

converts mechanical energy (in horsepower, HP) to electrical energy (in watts, W). Three basic methods of field excitation with DC generators are explored. These

include the separately excited, self-excited shunt, and the compound connected case. In the case of the separately excited DC generator, the excitation is provided by supplying DC current to the field coil which is separate from the armature. In the case of the self-excited shunt, the armature supplies its own field excitation while connected in parallel with the field coil. A variable resistance in series with the field coil is used to vary the amount of excitation. Some residual magnetism which exists in the magnetic circuit gets the process started. For a compound connected DC generator, the series field and the shunt field with a variable series resistance are used in combination. Two forms of the compound connected generator exist which are called the long(commulatively-compounded) and the short shunt(differentially-compounded). The connection employed in this lab is the short-shunt compound connection where the field coil with variable series resistance is connected directly across the armature with the series field between the positive terminal of the armature and the output terminal.

04/08/23 GROUP I3

For all practical purposes, the direct current generator is only used for special applications and local dc power generation. This limitation is due to the commutate required to rectify the internal generated at voltage, thereby making large scale dc power generators not feasible consequently, all electrical energy produced commercially is generated and distributed in the form of three phase ac power. The current use of solid state converters makes conversion to dc economical. However, the operating characteristics dc generators are still important, because most concepts can be applied to all other machines.

04/08/23 GROUP I4

Principles of DC GENERATOR

   It is based on the principle of production of dynamically (or motionally) induced e.m.f (Electromotive Force). Whenever a conductor cuts magnetic flux, dynamically induced e.m.f. is produced in it according to Faraday's Laws of Electromagnetic Induction. This e.m.f. causes a current to flow if the conductor circuit is closed. Hence, the basic essential parts of an electric generator are :

             A magnetic field and              A conductor or conductors which can so move as to

cut the flux.

04/08/23 GROUP I5

Classification of DC GENERATOR

Generators are usually classified according to the way in which their fields are excited and they are classified according

to the manner which their field flux is produced. The field windings provide the excitation necessary to set

up the magnetic fields in the machine. There are various types of field windings that can be used in the generator or motor circuit. In addition to the following field winding types, permanent magnet fields are used on some smaller DC products. There are 5 major types of dc generator,

Separately-Excited DC generator Shunt or Self –Excited DC generator Series DC generator Commulatively Compounded DC generator Differentially Compounded DC generator

04/08/23 GROUP I6

Separately-ExcitedDC GENERATOR

In separately-excited dc generator, the field flux is derived from separate power source independent of the generator itself.

Circuit diagram

04/08/23 GROUP I7

Self-ExcitedDC GENERATOR

In self-excited dc generator, the field flux is derived by connecting the field circuit directly across the terminals of the generator.

Circuit Diagram

04/08/23 GROUP I8

Shunt wound(self-excited)In this field winding is connected in parallel with the armature conductors and have the full voltage of the generator applied across them.The field coils consist of many turns of small wire. They are connected in parallel with the load. In other words, they are connected across the output voltage of the armature.

Current in the field windings of a shunt-wound generator is independent of the load current (currents in parallel branches are independent of each other). Since field current, and therefore field strength, is not affected by load current, the output voltage remains more nearly constant than does the output voltage of the series-wound generator.

In actual use, the output voltage in a dc shunt-wound generator varies inversely as load current varies. The output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E = IR).

04/08/23 GROUP I9

Series DC GENERATOR

In series dc generator, the field flux is produced by connecting the field circuit in series with the armature of the generator.

Circuit Diagram

04/08/23 GROUP I10

Series-wound generatorIn the series-wound generator, the field windings are connected in series with the armature. Current that flows in the armature flows through the external circuit and through the field windings. The external circuit connected to the generator is called the load circuit.

A series-wound generator uses very low resistance field coils, which consist of a few turns of large diameter wire.

The voltage output increases as the load circuit starts drawing more current. Under low-load current conditions, the current that flows in the load and through the generator is small. Since small current means that a small magnetic field is set up by the field poles, only a small voltage is induced in the armature. If the resistance of the load decreases, the load current increases. Under this condition, more current flows through the field. This increases the magnetic field and increases the output voltage. A series-wound dc generator has the characteristic that the output voltage varies with load current. This is undesirable in most applications. For this reason, this type of generator is rarely used in everyday practice.

04/08/23 GROUP I11

Commulatively Compounded DC GENERATOR

In commulatively compounded dc generator, both shunt and series field are present and their effects are additive.

Circuit Diagram

04/08/23 GROUP I12

Differentially CompoundedDC GENERATOR

In differentially compounded dc generator, both shunt and series field are present, but their effects are subtractive.

Circuit Diagram

04/08/23 GROUP I13

Compound-wound generator :Compound-wound generators have a series-field winding in addition to a shunt-field winding. The shunt and series windings are wound on the same pole pieces. They can be either short-shunt or long-shunt as shown in figures. In a compound generator, the shunt field is stronger than the series field. When series field aids the shunt field, generator is said to be commulatively-compounded. On the other hand if series field opposes the shunt field, the generator is said to be differentially compounded.

In the compound-wound generator when load current increases, the armature voltage decreases just as in the shunt-wound generator. This causes the voltage applied to the shunt-field winding to decrease, which results in a decrease in the magnetic field. This same increase in load current, since it flows through the series winding, causes an increase in the magnetic field produced by that winding.

By proportioning the two fields so that the decrease in the shunt field is just compensated by the increase in the series field, the output voltage remains constant. Which shows the voltage characteristics of the series-, shunt-, and compound-wound generators. As you can see, by proportioning the effects of the two fields (series and shunt), a compound-wound generator provides a constant output voltage under varying load conditions.

04/08/23 GROUP I14

Overview of the circuit diagram

04/08/23 GROUP I15

Parts of DC machine

In describing the various constructional parts of a dc machine, we can divide the machines into two parts:

Stationary Part (Stator)The role of stator is to serve as the seat of magnetic flux

that is to penetrate the armature care. Except for small special machines  using permanent magnets, the field circuit usually consist  of cylindrical yoke or frame to which a set of electromagnets, the field poles are bolted .

In small machines where weight is not a prime consideration, cost iron is used for the yoke frame. In larger machines cast steel is used since this materials makes it possible to reduce the weight considerably and still maintain the reluctance of the magnetic circuit. The yoke has abased with feet or a supporting bracket upon which the entire structure rests.

04/08/23 GROUP I16

Field Poles The field poles are constructed of laminated steel of about

0.025 to 0.045 inch thick per lamination and having good magnetic permeability. The stacks of laminations are equal to or somewhat shorter than the axial length of the armature core and are held together by rivets driven through holes in the laminations.

The shape of this assembled pole core is such that a smaller cross sectioned exists so that the field windings can be inserted on them. The pole shoe is curved and is widen than the pole core, to spread the flux more uniformly over a larger projecting area, at the same time creating a ledge upon which the field winding can have mechanical support.

04/08/23 GROUP I17

Parts of DC machine

Rotating Part (Rotor)

The rotor holds the insulated armature winding and it is built up of high grade laminated steel core. A shaft through the core supports this structure as well as the commutator. The latter is located such that the brushes in the stationary brush rigging line up and rest on it the brushes are spring located and have a rounded contact surface to ensure good contact with the commutator.

04/08/23 GROUP I18

04/08/23 GROUP I19

04/08/23 GROUP I20

04/08/23 GROUP I21

04/08/23 GROUP I22

04/08/23 GROUP I23

04/08/23 GROUP I24

04/08/23 GROUP I25

04/08/23 GROUP I26

04/08/23 GROUP I27

04/08/23 GROUP I28

04/08/23 GROUP I29

04/08/23 GROUP I30

04/08/23 GROUP I31

04/08/23 GROUP I32

PRESENTED BY:

FAJARDO, MICHAEL MEJIA, JOSEPH

SALAZAR, CHRISTIAN PASCO, RAYMOND ECHIPARE, AARON

HAEL, ALLAN CEPE, JOHN VINCIT

CRUZ, MARK ANTHONY CRUZ, KELVIN

RAMOS, LELIBETH CALINOG, AIRINA

04/08/23 GROUP I33

PRESENTED TO:

ENGR. ARTURO TADEO CABUYAGAN