Experiment Name: Dismantle of DC Machine.

 

Experiment Name: Dismantle of DC Machine.

Experiment Number: 01

Experiment Date:

Submission Date:

Aim/Objective:

1.      To dismantle  the DC Machine.

2.      To Study the construction of  DC Machine.

 

Theory:

A  D.C.  machine  is  a  device  that  converts  Mechanical  energy  into

Electrical  energy  and  vice-versa.  When  the  device  acts  as  a generator,

the  mechanical  energy  is  converted  into  electrical  energy.  On  the  other

hand, when the device acts as a motor, the electrical energy is converted into mechanical energy.

So, DC Machine s two type-

DC Generator and DC Motor

Constructional features:

Generator and motor are very similar to each other for their essential parts

and construction. The essential parts of dc machine, as shown in figure 1,

can  be  grouped  into  two  categories  depending  upon  their  placement  as

follows

STATOR:

Magnetic frame or Yoke:

The  outer  frame  of  the  DC  Machine  is  called  the  Yoke  and  is

normally  made  of  cast  Iron  for  small  machine,  while  for  large  machines

cast  steel  or  rolled  steel  is  employed.  Yoke  generally  serves  two

purposes.

 

It provides mechanical support to the poles and acts as a protecting cover for the whole machine.

 

It provides a return path of low reluctance to the magnetic fluxproduced by the poles.

 

Field  System :

The  object  of  the  field  system  is  to  create  a  uniform magnetic field within which the armature rotates. Field system consists of three parts namely

pole  cores,  pole  shoes and magnetizing  coils  or field coils.

The field magnets consist of pole cores and pole shoes.

Pole core is usually of circular section & is used to carry the coils of insulated wires carrying the exciting current.

Pole shoe acts as a support to the field coils & spread out the flux in the air gap & also being a larger cross-section reduces the reluctance

of the magnetic path. Both pole cores and pole shoes are laminated and are  bolted  to

the  frame  of  yoke.  The  thickness  of  the  lamination  varies from 1mm to 0.25mm.

 

Field coils or field windings which consists of copper wire or stripwound around the pole pieces. When current passes through the coil, the coil magnetizes the poles, which produce the necessary flux that is cut by revolving armature conductors.

 

Interpoles   &   Interpole   Winding:These  poles  are  comparatively smaller  in  size  than  the  main  poles.  They  are  placed between  adjacent main poles. The flux provided by interpoles givesspark- less commutation

&  hence  they  are  also  called  as  commutating  poles.  The  exciting  coils around  Interpoles  is  normally  connected  in  series  with  the  armature circuit & carries the entire armature current.

Air gap:

Radial spacing between main pole & surface of the armature is called air-gap. The length of air gap may be set between 1mm & 5mm

 

ROTOR:

Armature:

The purpose of armature is to rotate the armature conductors in the uniform magnetic field. It is further divided into two parts namely:

Armature core and Armature winding.

Armature core:

it is cylindrical in shape mounted on the shaft as shown in figure. It consists of slots on its periphery & the air ducts to permit the  air  flow  through  armature  which  serves  cooling  purpose  It is  made from  high  permeability  silicon- steel  stampings,  each  stamping  being

separated from its neighboring one by thin paper or varnish as insulation.

The function of armature core is:

1.      To provide house for armature winding

2.      To  provide  a  path  of  low  relucta nce  to  the  magnetic  flux produced by the field winding

 

Armature winding:

It is nothing but the interconnection of the armature conductors  placed  in  the  slots  provided  on  the  armature core  periphery.The function of armature winding is:

 

1.      To generate e.m.f in the armature winding in case of generator

2.      To carry the current supplied in case of dc motor

Commutator:

The  basic  nature  of  e.m.f  induced  in  the  armature conductor is alternating. This needs rectification in case of d.c. generator, which is possible by d evice called as commutator. It is cylindrical in shape and  is made up of wedge shape  segment  of  hard  drawn, high conductivity copper. These segments are insulated  from  each  other  byvthin  layer of mica  as shown  in figure . It is connected to the armature conductor by means of copper lug or strip.  The function  of  commutator is:

1.      To collect current from the armature conductors

2.      To convert internally developed alternately e.m.f into unidirectional e.m.f.

Brushes:

The function of brushes is to collect current from commutator or to drawn current to commutator. They are usually made of carbon and are  in  the  shape  of  rectangular  block.  These  brushes  are      housed  in brush  holders  usually  of  the  box  type  as  shown  in  the  fig.  The  brushes are  in  contact  with  the  commute r  by  a  spring  whose  tension  can  be

adjusted by changing the position of lever in the notches.

Bearings:

Bearing  is  a  device,  which  gives  free  &  smooth  rotation  of armature.  Ball bearing  is  frequently  employed  due  to  their  reliability, though for heavy duties roller bearings are preferable.

 

 

 

 

 

 

 

Circuit/Image:

Specification:

Add the specification of Your Machine

Voltage(Volt)	Current(Full Load in Amp )	Power(watt)	Speed(RPM)
	Field current(Amp)
Insulation CLass	Duty	Ambient Temp

 

Procedure:

1. Open the enclosure of the machine by the guidence of the  instructor.

2. First place the machine on a clean, dry and non- slip place.

3. Using screw driver, open the nuts of the machine from both side and observe inner construction.

4. Using spanner set , open the frame and separate armature from the frame . If necessary use mallet .

5.place the armature on a dry and clean place and observe  it and also observe stator part.

6. Assemble the machine carefully in reverse manner.

Apparatus Used:

Screw driver, Hammer/Mallet ,Spanner set

Precautions:

1. Free from any type of live connection.

2. Place the machine first on a non- slip, dry and clean place.

3. Don't hit the coil .

4. Don't  hit the ball- bearing.

5.place the armature very carefully in its house maintaing the air gap.

6. Check the whole to your guide or instructor.

Conclusion:

Viva Voice Questions:

1.      What is dc generator and dc motor?

2.      What is the function of yoke?

3.      What do you mean by armature and field ?

4.      What is the function of pole shoe?

5.      What is the function of brush?

6.      What do you mean by commutator and what is the function of it?

7.      Which type of voltage is generated in DC generator?

8.      How many terminal points are you observed in a shunt type dc generator

 

 

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