DC Machines-Construction

Electromechanical energy conversion simply implies the conversion of electrical energy into mechanical energy or to E.E. from M.E.The conversion of electrical energy to mech energy is achieve by using some type of motor.An electrical energy is applied to the motor,which convert the corresponding energy to mech.E with the help of a shaft.

                                                  fig of electromechanical energy conversion

Classification of Motors
DC Motor-shunt motor,series motor,compound motor
AC Motor-3-phase induction Motor,1-phase induction motor,synchronous motor

An electric generator converts the mechanical energy applied at its input to an electrical energy.
A prime mover machine is require to rotate  the generator mechanically.

at the output we get electricity which can be ac generators or dc gens.The ac gens are called as alternators.

There are three parts of an electromechanical energy conversion devices are:
-Electrical system
-coupling field
-Mechanical system


 CONSTRUCTION OF D.C. MACHINES 

A D.C. machine consists Of two main parts . 

(i) Stationary part. It is designed mainly for producing a magnetic flux. 

(ii) Rotating part. It is called the armature, where mechanical energy is converted into 

electrical (electrical generator), or conversely, electrical energy into mechanical (electric motor). 

The stationary and rotating parts are separated from each other by an air gap. 

. The stationary part of a D.C. machine consists of main poles, designed to create the 

magnetic flux, commutating poles interposed between the main poles and designed to 

ensure sparkless operation Of the brushes at the commutator (in very small machines With 

a lack of space commutating poles are not used) ; and n frame/yokc. 

The armature is a cylindrical body rotating in the space between the poles and comprising 

a slotted armature core, a winding inserted in the armature core slots, a commutator, and 

brush gear. 

The frame is the stationary part Of a machine to which are the main commutating 

poles and by means of which the machine is bolted to its bed plate. 

The ring-shaped portion which serves as the path for the main and commutating pole fluxes 

is called the 'yoke'. 

Cast iron used to be the material for the frame/yoke in early machines but now it 

has been replaced cast steel. This is because cast iron is saturated by a flux density 

of about 0.8 Wb/m while saturation with cast steel is at about 1.5 Wb/m2. Thus the 

cross-section Of a cast iron frame is about twice that Of a cast steel frame for the same 

value of magnetic flux.Hence,if it is necessary to reduce the weight of machine, cast steel 
is used. Another disadvantage with the use ofcast iron is that its mechanical and magnetic 

properties are uncertain due to the presence of blow holes in the casting. Lately, rolled 

steel yokes have been developed With the improvements in the welding techniques. The 

advantages of fabricated yokes are that there are no pattern charges and the magnetic and 

mechanical properties Of the frame arc absolutely consistent. 

It may be advantageous to use cast iron for frames but for medium and large sizes 

usually rolled steel is used. 

If the armature diameter does not exceed 35 to 45 cm, then, in addition to the poles, end 

shields Or frame-heads which carry the bearings are also attached to the frame. When the 

armature diameter exceeds I m, it is common practice to use pedestal-type bearings, 

mounted separately, on the machine bed plate outside the frame. 

The end shield bearings, and sometimes the pedestal bearings, are of ball or roller type. 


However, more frequently plain pedestal bearings are used. 

In machines With large diameter armatures a brush-holder yoke is frequently fixed to the 

frame. 


 Field poles 
Formerly the poles were cast integral With the yoke. This practice is still being followed 

for small machines. But in present day machines it is usual to use either a completely 

laminated pole, or solid steel poles with laminated pole shoes. 

Laminated construction is necessary because of the pulsations of field strength that result 
when the notched armature rotor magnetic structure passes the pore shoe. Variations in 

field strength result in internal eddy currents being generated in a magnetic structure. 

These eddy currents cause losses ; they may be largely prevented by having laminated 

magnetic structures. Laminated structures allow magnetic flux to pass along the length 

Of the laminations, but do not allow electric eddy currents to pass across the structure 

from one lamination to another. The assembled stack of laminations is held together as 

a unit by appropriately placed rivets. The outer end Of the laminated pole is curved to fit 
very closely into the inner surface of the main frame. 

Fig. 3 shows the constructional details Of a field pole. The pole shoe acts as a support to 
the field coils and spreads out the flux in the air gap and also being of larger cross-section reduces the reluctance of the magnetic path.

• Different methods are used for attaching poles to the yoke. In case Of smaller sizes, the 
back of the pole is drilled and tapped to receive pole bolts (see Fig. 4). In larger sizes, 

a circular or a rectangular pole bar is fitted to the pole. This pole bar is drilled and ta



-pped and the pole bolts passing through laminations screw into the tapped bar (see Fig. 5)

Commutating Poles 
A commutating (also called interpole) is similar to a main pole and consists Of Core 

terminating in a pole shoe, which may have various shapes, and coil mounted on the core. 

The commutating poles are arranged strictly midway between the main poles and are 

bolted to the yoke. 

Commutating poles are usually made of solid steel, but for machines operating on sharply 

varying loads they are made Of sheet steel. 


Armature 
The armature consists of core and winding. Iron being the magnetic material is used for 

armature core. However, iron is also a good conductor of electricity. The rotation Of solid 
iron core in the magnetic field results in eddy currents. The flow Of eddy currents in the 

core leads to wastage of energy and creates the problem of heat dissipation. TO reduce the 

eddy currents the core is made of thin laminations. 

The armature of D. C. machines (see Fig. 6) is built up of thin laminations Of low loss sil

-icon steel. The laminations are usually 0.4 to 0.5 mm thick and arc insulated With varnish. 

The armature laminations, in small machines, are fitted directly on to the shaft and are 

clamped tightly between the flanges which also act as supports for the armature winding. 

One end flange rests against a shoulder on the shaft, the laminations are fitted and other 

end is pressed on the shaft and retained by a key. 

The core (except in small size) is divided into number of packets by radial ventilation spa

-cers. The spacers are usually I sections welded to thick steel laminations and arranged to

pass centrally down each tooth. 







The armature laminations of medium size machines (having more than four poles) are 

built on a spider. The spider may be fabricated. Laminations up to n diameter of about 

100 cm are punched in one piece and are directly keyed on the spider (see Fig. 8). 




In case of large machines, the laminations of such thin sections are difficult to handle 

because they tend to distort and become wavy when assembled together. Hence circular 

laminations instead of being cut in one piece are cut in a number of suitable sections or 

segments which form part of a complete ring (see Pig. 9). A complete circular lamination 

is made up of four or six or even eight segmental laminations. Usually two keyways arc 

notched in each segment and are dove-tailed or wedge shaped to make the laminations 

self-locking in position. 

The armature winding is housed in slots On the surface Of the armature. The conductors 

of each coil are so spaced that when one side of the coil is under a north pole, the opposi
-te is under a south pole. 







In D.C. machines two layer winding With diamond shaped coils is used. The coils are 

usually former wound. In small machines, the coils are held in position by band of steel 

Wire, wound under tension along the core length. In large machines, it is useful to employ 

of fibre or to hold coils in place in the slots. Wire bands are employed for 

holding the overhang. The equilizer connections are located under the overhang on the 

side of the commutator. Fig. 11 shows a typical arrangement for equilizers. The equilizers 

can be on the other end of the armature also. 


Commutator 
• A commutator converts alternating voltage to a direct voltage. 

. A is a cylindrical structure built up of segments made of hard drawn copper. 

These segments are separated from one another and from the frame of the machine by 

mica strips. The segments are connected to the winding through risers. The risers have 

air between One another so that air is drawn across the commutator thereby 

keeping the commutator cool. 

Brush Gear

TO collect current from a rotating commutator or to feed current to it use 


is made of brush-gear which consists of : 

(1) Brushes 

(2) Brush studs or brush-holder arms 

(3) Current-collecting busbars. 

(4) Brush holders 

(5) Brush rocker 


Brushes

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


(1) Metal graphite 

(2) Graphite 

(3) Copper. 

(4) Carbon graphite 

(5) Electro-graphite 

• The allowable current density at the brush contact varies from 5 in case Of carbon 

to 23 A./cm2 in case Of copper. 

• The use Of copper brushes is made for machines for large currents at lOW voltages. 

Unless, very carefully lubricated, they cut the commutator very quickly and in any case, 

the wear is rapid. Graphite and carbon graphite brushes are self-lubricating and, are

therefore, widely 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 poor 

contact with segments, and sparking will result and consequent damage to commutator. 

So to prevent this, the mica is frequently 'undercut' to a level below the commutator surfa

-ce by means of a narrow 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, open at top and botto
-m is attached. the brush is pressed on to the commutator by a clock spring. The pressure



can be adjusted by a lever arrangement is.provided with the spring. The brush is con- 


-nected 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 are usually made Of bronze casting or sheet brass. In low voltage D 

machines where the conditions are easy galvanised 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. 


 Armature Shaft Bearings 
• 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. 


Armature Windings

The armature winding is very important element Of a machine, 


as it directly takes part in the conversion Of energy from one form into another. The requi
-rements which a winding must meet are diverse and Often of a conflicting nature. Among the



 requirements the following are Of major importance. 


The winding must be designed With the most advantageous utilisation 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 (Le., 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 

The closed armature windings are of two types 

(i) Ring Winding 

2. coil winding. 

(E) Drum winding 

In general there are two types of drum armature windings 

(i) Lap winding 

(ii) Wave winding. 

"Lap winding" is suitable for comparatively low voltage but high current generators 

whereas "wave of wind is used for high vol low current machines. 

— In 'lap winding' the finish Of each coil is connected to the start Of the next So that 

winding or commutator pitch is unity. 

In 'Wave winding' the finish of coil is connected to the start of another coil well away fr
-om the fixed coil. 



 E.M.F. EQUATION OF A GENERATOR
p= number of poles, 

Φ =flux/pole, webers (Wb), 

Z= total number of armature conductors, 

number of slots x number of conductors/slot, 

N= rotational speed Of armature, r.p.m., 

a= number of parallel paths in armature, and 

Eg= generated e.m.f. per parallel path in armature. 

Average e.m.f. generated per conductor =dΦ /dt volt. 


Now, flux cut per conductor in one revolution, dΦ=pΦ Wb. 
Number Of revolutions per second=n/60 

Time for one revolution, dt=60/n seconds 

Hence, according to Faraday's laws of electromagnetic induction, 

E.m.f. generated per conductor =pΦn/60 volts. 


For a lap wound generator : 
Number of parallel paths, a =p 

Number of conductor (in series) in one path =z/p

Em. f. generated per path=Φpn/60 x Z/p=Φzn/60 

For a wave wound generator : 
Number of parallel paths, a= p 

Number Of conductor (in series) in one path =z/2

E .m.f. generator per path =Φpn/60 x z/2=Φpnz/120 volt. 

In general, e.m.f. generated Eg= Φzn/60 x p/a volts=Φznp/60a


where a= p for lap winding
      a=2 for wave winding