Prepared by M.P.RAJESH, Sr.Lecturer in EEE Under the guidance of Smt CH.JYOTHI , Head of EEE

1. e – Lessons Student Module for C-16 Curriculum State Board of Technical Education & Training Andhra Pradesh Year/Semester : III Semester Branch : Electrical & Electronics Engg Subject : EE-302,DC Machines & Measuring Instrument Topic : Basics of D.C.Generator Sub Topic :Losses in D.C. Generators Duration : 50 Minutes Prepared by M.P.RAJESH, Sr.Lecturer in EEE Under the guidance of Smt CH.JYOTHI, Head of EEE At ESC.Government Polytechnic , NANDYAL C-16, EE- 302.1.9

2. OBJECTIVE On completion of this period , you will be able to: • Know the losses incurred in the D.C. Generator C-16, EE- 302.1.9

3. RECAP Different types of windings in a D.C. Generator. • Armature winding • Shunt field winding • Series field winding • Inter pole winding C-16, EE- 302.1.9

4. LOSSES IN D.C MACHINES C-16, EE- 302.1.9

5. Total Losses In D.C Machines Mechanical losses Copper losses Iron losses Frictional losses Windage losses Series copper losses Armature copper losses Shunt copper losses Hysteresis losses Eddy current losses C-16, EE- 302.1.9

6. COPPER LOSS What do you understand by the word copper loss ? Copper loss is the term often given to heat produced by electrical currents in the conductors of transformer windings, or other electrical devices C-16, EE- 302.1.9

7. COPPER LOSS Copper loss occur in three ways : 1. Armature copper loss 2. Field copper loss 3. Brush contact copper loss C-16, EE- 302.1.9

8. ARMATURE COPPER LOSS • Armature COPPER LOSS = Ia2 Ra • Where ‘Ra’ = Armature resistance • This loss is up to 30 to 40% of full load losses C-16, EE- 302.1.9

9. FIELD COPPER LOSS IN SHUNT GENERATORS : Field copper loss = Ish2Rsh = V Ish Where Rsh is the shunt field resistance SHUNT FIELD COPPER LOSS IS PRACTICALLY CONSTANT IN SERIES GENERATORS : • Field copper loss =I2se Rse • Where Rse - is the resistance of the series field winding • Field copper loss is about 20 to 30 % of full load losses C-16, EE- 302.1.9

10. BRUSH CONTACT POWER LOSS • This loss is due to brush contact resistance • It is usually included in the armature copper loss C-16, EE- 302.1.8

11. Iron losses or Core losses or Magnetic losses C-16, EE- 302.1.9

12. IRON LOSS • Due to rotation of the iron core in the magnetic field some losses take place in the armature core and they are known as iron or core losses . • Iron losses consists of : 1. Hysteresis losses. 2. Eddy current loss C-16, EE- 302.1.9

13. Hysteresis Loss Hysteresis means Lag behind C-16, EE- 302.1.9

14. HYSTERESIS LOSS • Hysteresis loss is a heat loss caused by the magnetic properties of the armature. • This loss is due to reversal of magnetization of armature core • When an armature core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field C-16, EE- 302.1.9

15. HYSTERESIS LOSS • When the armature core is rotating, its magnetic field keeps • changing direction. The continuous movement of the magnetic • particles, as they try to align themselves with the magnetic field, • produces molecular friction. • This in turns produces heat so the power is lost in the form of heat C-16, EE- 302.1.9

16. HYSTERESIS LOSS DEPENDS ON 1) Volume of iron core (v) 2) Maximum volume of flux density (Bmax ) 3) Frequency of magnetic reversals (f) 4) Grade of iron C-16, EE- 302.1.9

17. HYSTERESIS LOSS GIVEN BY STEINMETZ FORMULA Hysteresis loss Wh = ŋ B1.6max f v watt Where v = Volume of iron in m3 ŋ= Steinmetz hysteresis co-efficient f = frequency of magnetic reversals C-16, EE- 302.1.9

18. TO REDUCE HYSTERESIS LOSS • Silicon steel laminations are used • Annealing process also reduces the hysteresis loss CRGO Electrical Steel Laminations (Cold Rolled Gain Oriented ) C-16, EE- 302.1.9

19. CRGO SHEETS Sheets placed in shape Sheets in process Cut Sheets C-16, EE- 302.1.9

20. Eddy current loss Eddy means Swirl or Whirl pool C-16, EE- 302.1.9

21. EDDY CURRENT LOSSES • When the armature core rotates it cuts the magnetic flux • Hence an emf is induce in the body of the core due to laws of electromagnetic induction C-16, EE- 302.1.9

22. EDDY CURRENT LOSSES • This emf sets up large current in the body of the core due to its small resistance • This current is known as eddy current • The power loss due to flow of eddy current is known as eddy current loss C-16, EE- 302.1.9

23. EDDY CURRENT LOSSES • Eddy current loss is considerably high if solid iron core is used. • Eddy current loss can be reduced by using laminated core. SOLID IRON CORE LAMINATED IRON CORE C-16, EE- 302.1.9

24. EDDY CURRENT LOSS DEPENDS ON 1. Volume of Iron core 2. Maximum value of flux density 3. Frequency of magnetic reversals 4. Thickness of each lamination C-16, EE- 302.1.9

25. EDDY CURRENT LOSS FORMULA We=K B2maxf² t² *v watt Where Bmax=maximum flux density f = frequency of magnetic reversals t = thickness of each lamination v = volume of armature core C-16, EE- 302.1.9

26. CORE LOSSES • Hysteresis and eddy current losses are practically constant for • shunt and compound wound generators, as the field current is approximately constant. • Iron loss is about 20 to30% of full load losses. C-16, EE- 302.1.9

27. This loss consist of 1.Friction loss 2. Windage Loss MECHANICAL LOSS Mechanical loss is about 10 to 20% of full load losses. C-16, EE- 302.1.9

28. FRICTION LOSS • Friction loss occur at bearings and commutator or brushes. • Friction losses depends on • 1. Brush pressure • 2. Coefficient of friction • 3. Type of bearing • 4. Lubrication • 5. Speed • Friction loss is approximately proportional to speed. C-16, EE- 302.1.9

29. WINDAGE LOSS OR WIND FRICTION LOSS • It is the power required to circulate air through the machine and ventilating ducts. • Windage loss is approximately proportional to the square of the speed. C-16, EE- 302.1.9

30. STRAY LOSSES Magnetic( iron) and Mechanical losses are collectively know as stray losses Stray losses can also be called as rotational losses C-16, EE- 302.1.9

31. CONSTANT LOSSES or STANDING LOSSES • Field copper losses is constant for shunt and compound generators • Stray losses and shunt copper loss are together known as constant losses or standing losses ( Wc ) C-16, EE- 302.1.9

32. VARIABLE LOSSES • Armature copper loss (Ia2 Ra) is known as variable loss because it varies with load current C-16, EE- 302.1.9

33. TOTAL LOSS IN A D.C GENARATOR Total loss =Variable loss + constant loss C-16, EE- 302.1.9

34. SUMMARY we have understand the various Losses in a D.C generator Copper loss Gross loss Mechanical loss. C-16, EE- 302.1.9

35. QUIZ 1) Hysteresis loss is proportional to a) Square of the frequency b) Frequency c) Square of the flux density d) Thickness of each lamination : C-16, EE- 302.1.9

36. QUIZ 2) Armature copper loss can also be called as a) Stray losses b) Constant losses c) Variable losses d) Magnetic losses C-16, EE- 302.1.9

37. Frequently Asked Question 1) Explain the various losses that occur in D.C. Generators. 2) What are constant losses in D.C. Generators. C-16, EE- 302.1.9

38. End of Lesson 9 In Chapter - 1 of EE 302 C-16, EE- 302.1.9