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Construction of D.C. Machines

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To define a D.C. machine, we must know its two main parts. The two main parts are as follows:

(i) Stationary part– It is deliberate mainly for generating a magnetic flux.

(ii) Rotating part– It is called the armature, where mechanical energy is transformed into electrical (electrical generator), or contrariwise, electrical energy into mechanical (electric motor).

Air gap separates the stationary and rotating parts. The stationary part of a D.C. machine comprises of main poles, deliberate to create the magnetic flux, commutating poles interposed between the main poles and designed to confirm sparkles procedure of brushes at the commutator (in very small machines with a lack of space com mutating poles are not used) ; and a frame/yoke,

Above figure shows the structure of generator or motor magnetic structure.

The armature is a cylindrical body revolving between the poles and includes a slotted armature core, a winding inserted in it, a commentator, and brush gear.

1.1 FRAME-

This figure shows the sectional view of four pole D.C. machine

  • The frame is the immovable part on which the main and commutating pole are fixed by means of which the machine is attached to its bedplate. The ring shaped part is the yoke. It is like a path between the main and commutating pole flux.End-shields or frame-heads which carry the bearings are also attached to the frame. Of these main structural elements of the machine the yoke, the pole cores, the armature core and the air-gap between the armature core and the pole core are known to form the magnetic circuit while the pole coils, the armature windings, the commutator and brushes should form the electric circuit of the machine.

Earlier, the frame used cast iron in early machines but now it has been substituted by cast material. Cast iron possesses a flux density of 0.8 Wb/m2whereas cast steel is at about 1.5 Wb/m2Thus;weight is reduced considerably upon using cast steel. This is one of the disadvantages of using cast iron. Additional drawback with cast iron is its mechanical and magnetic properties which are indeterminate due to the existence of casting blow holes,lately; rolled steel yokes have been established with the developments in the welding techniques. The benefits being a steady magnetic as well as mechanic properties.

1.2 FIELD POLES

It is better to use cast steel for large and medium sizes, for small size cast iron is preferred. The additional poles, end shields which carry the bearings will be attached to the frame if the armature diameter will be up to 35 to 45 cm. And when it exceeds 1 m, it is common exercise to use pedestal-type bearings, mounted separately, on the machine bed plate outside the frame.

Refer the figure and look at the end shield bearings and the pedestal bearings looks like a ball or roller. Plain rings are used frequently. Machines with large diameter a brush-like holder is fixed.

Previously the poles were cast essential with the yoke. This is still followed by small machines. These days’ machines use poles which are completely laminated. Also solid steel poles are being used which are laminated.

Let’s find out why lamination is necessary. Laminated construction is essentialas of the poundings of field power that result when the jagged armature rotor magnetic structure goes through the pole shoe. Differences in field powerout come in internal eddy currents being created in a magnetic structure. Eddy currents often cause damages, so to prevent such damage to cause lamination of magnetic structures is important. Laminated structures permit magnetic flux to go through the length of the laminations, but it does not allow currents to flow from the structure from one lamination to another. The accumulated pile of laminations is held together as a unit by suitably placed fasteners. Close to the inner wall of the main frame there is a laminated pole which is curved in shape.

We can look at the figure which shows the field pole with details of its construction. The pole shoe can be considered as a backing to the field coils and extents out the flux in the air gap and also being of bigger cross-section decreases the hesitancy of the magnetic path.

1.3 COMMUTATING POLES

An interpole which is similar to main pole and comprises of core terminating in a pole shoe have many shapes and coils mounted to it.

The commutating poles are organized harshly midway among the main poles and are attached to the yoke.

Commutating poles are made up of solid steel, but for machines operating on abruptly fluctuating as they are made up of steel.

1.4 ARMATURE

The armature can be defined as the magnetic material which comprises of core and winding. Armature core is made up of iron which is a magnetic material. Iron is considered as a good conductor of heat and electricity. Eddy currents generates when solid iron core rotates in magnetic fields. When the eddy currents travel through the core leads it causes problems like heat dissipation and also wastage of energy. To minimize these chances the core is being made up of skinny laminations.

We can look at the figure of armature of D.C. machines to see the skinny lamination which is done by silicon steel. The laminations are usually 0.4 to 0.5 mm thick and are shielded with glaze.

The laminations in small machines are connected to shaft directly and held tightly between the flanges which support the winding of armature as well. One end flange reposes against a shoulder on the shaft, the laminations are fitted and other end is pushed on the shaft and engaged by a key.

The core (except in small size) is allocated into number of packets by radial ventilation spacers. The spacers are usually sections bonded to thick steel laminations and arranged to pass centrally down each tooth.

In the case of small machines the laminations are perforated in one place. We can see it in the figure as well. These laminations are made up straight on the shaft. By this arrangement, it is necessary to deliver axial ventilation holes so that air can pass into ventilating ducts.

In case of medium sized machines which have more than four poles, the armature laminations are fabricated on a spider. Refer the figure and you will see that laminations are punched in one place and directly keyed.

Now, for the large machines lamination is quite difficult. Thin sections are not easy to handle as they can become wavy when put together. There is a possibility that it gets twisted. We can see in the figure that to form a part of complete ring, circular laminations are being cut in pieces. Thus, we can say that to complete a circular lamination up to four to six or even eight segments are required. To make such laminations strong two key ways are jagged together in each segment.

The conductors of each coil are so spread out that when one side of the coil is under a north pole, the contrary is under a south pole.

Fig. 10 shows the arrangement of conductors and insulation in a slot.

1.5 COMMUTATOR

In D.C. machines, two layers are used which are diamond shaped. The coils are usually past wound. In small machines, the coils are detained in position by band of steel wire, wound under tension along the core length. In large machines, it is advantageous to employ wedges of fibre or wood to hold coils in place in the slots. Wire bands are employed for holding the overhang. The equalizer connections are located under the overhang on the side of the commutator. Fig. 11 shows a typical arrangement for equilizers. The equilizers can be lodged on the other end of the armature also.

  • A commutator transforms alternating voltage to a direct voltage.
  • A commutator is made up of hard copper which looks like a cylindrical structure built up of segments. These segments are disconnected from one another and from the frame of the machine by mica strips. The segments are linked to the winding through risers. The risers have air spaces between one another so that air is drawn across the commutator thereby keeping the commutator cool.

We can see the figure which shows the mechanisms of a commutator. The general appearance of a commutator when completed is as shown in Fig. 13 (a). The commutator and armature assembly is shown in Fig. 13 (b).

1.6 BRUSH GEAR

To gather current from a rotating commutator or to feed current to it use is made of brush-gear which consists of:

  1. Brushes
    ii.            Brush holders
    iii.            Brush studs or brush-holder arms
    iv.            Brush rocker
    v.            Current-collecting busbars.

Brushes– The brushes used for D.C. machines are divided into five classes:

(i)  Metal graphite

(ii) Carbon graphite

(iii) Graphite

(iv) Electro-graphite

(v) Copper.

  • The allowable current density at the brush contact varies from 5 A/cm2 in case of carbon to 23 A/cm2 in case of copper.
  • The use of copper brushes is made for machines designed for large currents at low voltages. Unless, very carefully oiled, they cut the commutator very speedily and in any case, the wear is hasty. Graphite and carbon graphite brushes are self-lubricating and, are, therefore, broadly used. Even with the softest brushes, however, there is a gradual wearing away of the commutator, and if mica between the commutator segments does not wear down so rapidly as the segments do, the high mica will cause the brushes to make meager contact with segments, and glowing will result and resulting damage to commutator.

So to stop this, the mica is frequently ‘undercut’ to a level below the commutator surface by means of a thin milling cutter.

Brush holders. Box type brush holders are used in all ordinary D.C. machines. A box type brush holder is shown in Fig. 14. At the outer end of the arm, a brush box opens at top and bottom is attached. The brush is pressed on to the commutator by a clock
spring. The pressure can be adjusted by a lever arrangement provided with the spring. The brush is connected to a flexible conductor called pig tail. The
flexible conductor may be attached to the brush by a screw or may be soldered.

  • The bush boxes are usually made of bronze casting or sheet brass. In low voltage D.C. machines where the commutation conditions are easy galvanized steel box may be used.
  • Some manufacturers use individual brush holders while others use multiple holders, i.e., a number of single boxes built up into one long assembly.

Brush rockers. Brush holders are fixed to brush rockers with bolts. The brush rocker is arranged concentrically round the commutator. Cast iron is usually, used for brush rockers.

  • With small machines roller bearings are used at both ends.
  • For larger machines roller bearings are used for driving end and ball bearings are used for non-driving (commutator) end.
  • The bearings are housed in the end shields.
  • For large machines pedestal bearings are used.

1.7 ARMATURE SHAFT BEARINGS AND ARMATURE WINDINGS

The armature winding is very vital element of a machine, as it directly takes part in the adaptation of energy from one form into another. The requirements which a winding must meet are diverse and often of a conflicting nature. Among these requirements the following are of chiefprominence.

  • The winding must be designed with the most advantageous utilization of the material in respect to weight and efficiency.
  • The winding should provide the necessary mechanical, thermal and electrical strength of the machine to ensure the usual service life of 16-20 years.
  • For D.C. machines proper current collection at the commutator (i.e., absence of detrimental sparking) must be ensured.
  • According to the degree of closure produced by winding, armature windings are of the following two types:
  1. Open coil winding
  2. Closed coil winding:

The closed armature windings are of two types:

(i)                           Ring winding

(ii)                        Drum winding

In general, we can say there are two types of drum armature windings:

(i)                           Lap winding

(ii)                        Wave winding.

“Lap winding” is preferred for reasonably low voltage but high current generators
whereas “wave of winding” is used for high voltage, low current machines.

– In ‘lap winding’ the finish of each coil is linked to the start of the next coil so that winding or commutator pitch is unity.

–  In ‘wave winding’ the finish of coil is associated to the start of another coil well away from the fixed coil.

 

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