Complete information on D.C. Generator

A general term applied to m/cs which are used for the transformation of mechanical energy in to electrical energy.

If a coil or conductor is rotated in the magnetic field and the conductor cuts the magnetic flux, an e.m.f. is induced in the conductor. This is due to Faraday’s Law of electromagnetic induction.

(A) Field Yoke or Frame:

Yoke is a stationery part consists of cast iron or steel frame.

(B) Poles with Field Coils:

Poles are bolted with the yoke Fig. 6.30. Poles shoes are made of thin sheet stampings (laminations) to produce better magnetic condition. Field coils are wound on the poles for electromagnet.

(C) Armature:

Within the pole shoes a roating part known as armature rotate. Armature is supported by bearings at the ends. Armature is made of lami­nated iron core having siots for winding of conductors.

(D) Commutator:

Commutator is mounted upon the armature shaft. The ends of the coils of armature conductors are connected to the copper seg­ments of commutator. The commutator is made up of hard copper. Commutator segments are separated from each other by thin sheet of mica.

As e.m.f. induced in the armature conductor, is of alternating current (A.C.) type. The function of the commutator is to convert A.C. into D.C. and facilitate for collection of current through brushes on the commutator from armature conductor (Fig.6.32)

(d) Brush and brush holder (e) Rocker arm

The purpose of the commutator is to take in and give out the currents from the armature. It keeps the negative and positive terminal constant in the circuit. Hence, commutator converts A.C. to D.C.

(E) The Brush Gear:

It consists of two or more sets of stationary carbon brushes held in box-type brush holder. The brushes make contact on the commu­tator surface with a certain amount of pressure (about 0.9 kg per sq. cm of brush area), supplied by a spring in brush-holder. The brush-holders are clamped on the spindles which are insulated from the shaft and mounted on the brush rocker.

The rocker is capable of movement so as to obtain best position for the brushes on the commutator.

The following may be the reasons for not building up the voltage of D.C. Generators:-

1. The direction of rotation of generator may be in opposition, for this change the-field connection or reverse the direction of rotation.

2. There may be no residual magnetism in poles or weak magnetism. The field should be excited through a separate D.C. source of low voltage supply or battery.

3. There may be break or open circuit in the field winding or field regular, or short circuit to the field winding.

4. Wrong position of the brushes.

5. Bad contact of brushes. The proper brush pressure and good contact with the commutator is made.

6. The resistance in the field circuit is high. If the commutator surface is dirty, which will introduce high resistance in the field circuit and pre­vent self-excitations, i.e., prevent to flow the sufficient current in the field circuit with the small voltage produced by the residual magnetism. Clean the commutator.

7. The speed of the generator is very less.

Overheating of DC machine may arise due to the following reasons:

1. Over loading of machines. The heat produced is due to the power loss i.e. (I²R). If the rate current is increased, the power loss or generation of heat will also increase by square times.

2. Defective bearings:

3. High brush friction.

4. Internal short circuit.

5. Insulation leakage.

The following may be the reasons for reversed polarity of DC generators:

1. Wrong direction of rotation.

2. Reverse connections c f volt-meter on the generator panel.

3. Reverse connections field (self -excited) or exciter in case of separately excited.

4. Reverse of residual magnesium in the iron pole pieces.

(i) Maintenance of Commutator:

As long as the commutator shows a satisfactory and well polished surface, no maintenance work like grinding, smoothing the lie will be required. It is not advisable to treat the commutator with many. It is, however, recommended to wipe the commutator with a Clean rag, free from dust, etc.

If the commentator no longer smooth or shows pitted spots, it must be polished the fine emery cloth; all copper and emery dust may be removed by blowing If the commentator is out of true to a flat grind, stone or emery dismantling of the armature be turned on a lathe. Thereafter the mica is re-undercut and the edges of the segments be leveled slightly.

(ii) Carbon brushes:

The Carbon brushes must be checked periodically for proper contact with the commutator and for good movability in the holders. Defective brushes should be renewed in time, special care should be taken to use only type of burst which was supplied with the machine; using a variety of brushes of different type and manufacture leads to trouble.

New brushes should be ground down with the aid of an emery cloth. To this a strip of emery cloth with a width equal to the length of the commutators introduced vertically into a mica lamination somewhat more deeply under cut and ma ked with the works and is drawn through under the brushes while turning the armature slowly and uniformly by hand or mechanically in the direction of rotation After grinding the accumulated carbon dust should be removed by blowing.

(III) Brush Holders:

If the brush holders have to be exchanged or renewed, the following point should be observed when replacing the brush holder:-

The radial clearance between holder and commutator should be approximately 1.5mm.

The brush holders should be perfectly in line With the segments in axial direction and should be staggered axially by pole pairs in such a way as to ensure uniform wear of the commutator.

Type of DC Generators:

Following are the different types of D.C. generators:

1. Separately Excited. 2 Self-excited.

Self-excited generators are further subdivided into

(a) Series wound generator.

(b) Shunt wound generator.

(c) Compound wound generators are again subdivided into.

(i) Short shunt compound.

(ii) Long shunt compound.

(iii) Short shunt compound generator can be of two types:

(a) cumulative short shunt and

(b) Differential short shunt,

(II) Long shunt compound generator can also be of two types:

(a) Cumulative long shunt and (b) differential long shunt.

By changing the number of turns in the series field, it is possible to obtain three types of compound generators. These are; (1) over compounded generator (2) a flat-compounded generator, and (3) and under compounded generator

These generators are designed as such and have the desired series turn to obtain the characteristics described below

1. If the turns of the series field are increased over the number necessary to give the same voltage output at all loads, the generator will be over compounded, means that as the load is increased, the generated voltage increases.

At no load, normal voltage is obtained, but as the load is increased to full load the voltage rises approximately 5 percent. This is desirable when the generator is located at some distance from the load. The rise in generated voltage compensates ‘or the voltage drops in the line.

2. If the number of turns is decreased, a flat compounded generator is obtained. In this generator, the voltage produced at full load is of the same value as voltage at no load. This generator is used where the load is nearby, such as m the same building.

3. If the turns in the series winding are further decreased, an under compounded generator will result. In this type, the voltage at no load is normal the load is increased; the voltage drops considerably, until at full load it is approximately 20 percent below normal. This generator is useful where a short circuit might occur, as in a welding machine.

(a) Shunt-wound Generator:

In this generator the field windings are connected in parallel with the armature conductors and are made with many turns of fine wire. Shunt generators with regulators are used for lighting and power supply purposes. These are also used for charging batteries because these generators supply the suitable charging voltage according to the condition of the battery.

(b) Series-wound Generator:

The field coils are wound with few turns of thick wire of large cross-section and are connected in series with the armature. Before the machine excites, the external circuit must be closed. There are used for specific purposes, e.g., boosters. In this type the voltage increases as the load increases.

(c) The Compound Generator:

There are several types of compound generators, the most common being the (i) short-shunt cumulative generators. This has the shunt field connected across the armature, and the current flow in the shunt field is in the same direction as in the series field, (ii) This generator can also be connected as long shunt compound.