8/7/2019 lab 1 - Copy

http://slidepdf.com/reader/full/lab-1-copy 1/4

Lab Report                                        

LAB # 1: PARTS OF DC MACHINES.

Theory: 

Electro-mechanical devices: convert electrical energy to mechanical motion/work and

vice versa. The direct current (dc) machine can be used as a motor or as a generator.Atypical DC generator or motor consists of an armature core, an air gap, poles, and a yoke

which form the magnetic circuit; an armature winding, a field winding, brushes and acommutator which form the electric circuit; and a frame, end bells, bearings, brush

supports and a shaft which provide the mechanical support.

Parts:

1) Field System

2) Armture

3) Commutator

1) Field system:

Yoke

The  yoke  is   a  circular  steel  ring,   whichsupports the field,  poles mechanically  and

provides the necessary magnetic path between

the pole. The yoke can be solid or laminated.

In many DC machines, the yoke also serves asthe frame.

Pole core/Field Poles

The pole cores can be made from solid steel

castings or from laminations. At the air gap,

the pole usually fans out into what is knownas a pole head or pole shoe. This is done to reduce the reluctance of the air gap. Normally

the field coils are formed and placed on the pole cores and then the whole assembly is

mounted to the yoke.

Pole coils/Field Coils

The field coils are those windings, which are located onthe poles and set up the magnetic fields in the machine.

They also usually consist of copper wire are insulated

from the poles. The field coils may be either shuntwindings (in parallel with the armature winding) or 

series windings (in series with the armature winding) or 

a combination of both.

8/7/2019 lab 1 - Copy

http://slidepdf.com/reader/full/lab-1-copy 2/4

Lab Report                                        

Pole-pitch

It may be defined as :

i) The periphery of the armature divided by the number of poles of the generator i.e. the

distance between two adjacent poles.

ii) It is equal to the number of armature conductors (or armature slots) per pole. If there

are 48 conductors and 4 poles, the pole pitch is 48/4 = 12.

Conductor

The length of a wire lying in the magnetic field and in which an e.m.f. is induced, is

called a conductor (or inductor).

Coil-span or Coil-pitch (YS)

It is the distance measured in terms of armature slots(or armature conductors) between two sides of a coil.

If the pole span or coil pitch is equal to the pole pilch

(as in the case of coil A in Fig. 26.24 where pole-

pitch of 4 has been assumed), then winding is calledfull-pitched. It means that coil span is 180 electrical

degrees. In this case, the coil sides lie under opposite

poles, hence the induced e.m.fs. in them are additive.

Therefore, maximum e.m.f. is induced in the coil as a

whole, it being the sum of the e.m.f.s induced in thetwo coil sides. For example, if there arc 36 slots and 4

poles, then coil span is 36/4 = 9 slots. If number of slots is 35. then Ys = 35/4 = 8 because it is customary

to drop fractions.

If the coil span is less than the pole pitch (as in coil B

where coil pitch is 3/4th of the pole pitch), then thewinding is fractional-pitched. In this case, there is a phase difference between the e.m.fs.

in the two sides of the coil. Hence, the total e.m.f. round the coil which is the vector sum

of e.m.fs. in the two coil sides, is less in this case as compared to that in the first case.

Pitch of a Winding (Y)

It may be defined as the distance round the armature between two successive conductors

which are directly connected together. Or, it is the distance between the beginnings of 

two consecutive turns.

Y   = YB – YF ………… for lap winding

8/7/2019 lab 1 - Copy

http://slidepdf.com/reader/full/lab-1-copy 3/4

Lab Report                                        

= YB + YF ………….. for wave winding

Back Pitch (YB)

The distance, measured in terms of the armature

conductors, which a coil advances on the back of the armature is called back pitch and is denoted byYB As  seen  from  Fig.  26.28.  element  I  is

connected on the back of the armature to clement

8. Hence. YB = (8 – 1) = 7.

Front   Pitch   (YF) The   number   of   armatureconductors or elements spanned by a coil on the

front (or commutator end of an armature) is called

the front pitch and is designated by YF Again in Fig. 26.28, element 8 is connected to

clement 3 on the front of the armature, the connections being made at the commutator 

segment. Hence.   YF = 8-3 = 5.Alternatively, the front pitch may be defined as thedistance (in terms of armature conductors) between the second conductor of one coil and

the first conductor of the next coil which are connected together at the front ie.commutator end of the armature. Both front and back pitches for lap and wave-winding

are shown in Fig. 26.25 and 26.26.

Resultant Pitch (YR )

It is the distance between the beginning of one coil and the beginning of the next coil to

which it is connected .

Armature

Armature Core

Armature core is cylindrical in shap.The armature core is made up thin magnetic steel

laminations stamped from sheet steel with a blanking die.

8/7/2019 lab 1 - Copy

http://slidepdf.com/reader/full/lab-1-copy 4/4

Lab Report                                        

Armature Winding 

The armature winding is the winding, which fits in the armature slots and is eventuallyconnected to the commutator. It either generates or receives the voltage depending on

whether the unit is a generator or motor. The armature winding usually consists of copper 

wire, either round or rectangular and is insulated from the armature stack.

3)Commutator 

The commutator  is  the  mechanicalrectifier ,

which

changes   the   AC   voltage   of   the   rotating

conductors  to  DC  voltage.  It  consists  of  anumber  of  segments   normally  equal  to   the

number of slots. The segments or commutator 

bars are made of silver bearing copper and areseparated from each other by mica insulation.

Commutator Pitch (YG)

It is the distance (measured in commutator bars or segments) between the segments to

which the two ends of a coil arc connected. From Fig. 26.25 and 26.26 it is clear that for lap winding. YG is the difference of YB and Yf whereas for wavewinding it is the sum of 

YB and Yf Obviously, commutator pitch is equal to the number of bars between coil leads.

A = rotor 

B = bar collector 

C = brushes and brush holder 

D = shaft.