# C-14 DIRECT CURRENT DC MACHINE Subject code : EE-302

D.C. MACHINES

Subject Title             :           D.C. Machines

Subject Code           :           EE-302

Periods/Week            :           04

Periods/Semester    :           60

TIME SCHEDULE
Unit Test- 1 Syllubus

 Sl. No. Major Topics Periods Weightage Short Questions Essay Questions 1. Fundamentals of D.C  Generators 14 29 03 02 2. Armature Reaction and Characteristics of D.C Generators. 16 26 02 02 TOTAL 30 55 05 04

Unit Test- 2 Syllubus

 Sl. No. Major Topics Periods Weightage Short Questions Essay Questions 1. Fundamentals of D.C Motors 12 26 02 02 2. Speed Control and Starters for D.C Motors 10 16 02 01 3. Characteristics and Testing of D.C Motors 8 13 01 01 TOTAL 30 55 05 04

UNIT TEST-I
MODEL PAPER-I
PART-A
NOTE  :  Answer all questions                                                     2 X 3=6
Each question carries two marks
1. Write the working principle of DC generator.
2. Difference between lap and wave winding.
3. What is armature reaction?
PART-B
NOTE:   Answer any two questions                                             2 X 7=14
Each question carries seven marks
4 . What are the essential part of dc generator and explain.
5 .A shunt generator delivers 200A at 250V.The armature and shunt field resistances are
0.25ohm and  50 ohm respectively. The total stray losses amounts to 900W.Calculate
a) Copper losses  b) Commercial efficiency.
6. A 4pole lap wound dc Generator with 450 armature conductors supplies a current of
150A.The brushes have been displaced through 3 angular degrees from the geometrical
neutral axis. Calculate a) Demagnetising AT/pole  b) Cross magnetising AT/pole c) The
additional field current for neutralising the demagnetisation if the field winding has 1000
turns per pole.

UNIT TEST-I
MODEL PAPER-II
PART-A
NOTE :  Answer all questions                                                     2 X 3=6
Each question carries two marks

1. . State the Fleming’s right hand rule
2. Draw the schematic diagram of separately excited dc generator.
3. Draw external characteristic curve for a d.c shunt generator
PART-B
NOTE:    Answer any two questions                                             2 X 7=14
Each question carries seven marks

4. Derive the generated emf equation of a dc generator.
5. A short shunt d.c generator supplies a current of 100A at a voltage of 220V.If the
Resistance  of the shunt field is 50ohm,series field is 0.025ohm and armature is
0.05ohm.The total brush drop 2V and iron and frictional losses amount to 1KW.
Find i)The Generated EMF ii)The CU losses iii) The Generator efficiency.

6. .Describe various method to improve the commutation

UNIT TEST-II
MODEL PAPER-I
PART-A
NOTE :  Answer all questions                                                     2 X 3=6
Each question carries two marks

1. State the Flemings left hand rule.
2. State factor that affect the speed of a dc motor
3. What is meant by regenerative test

PART-B
NOTE :   Answer any two questions                                             2 X 7=14
Each question carries seven marks

4. Derive the equation for armature torque and shaft  torque in D.C Motor.
5. A 200V shunt motor has armature resistance of 0.04ohm and runs at 1000 rpm. When the
armature current is 60A, if the torque remains unchanged ,Find the speed and armature
current when the field strength is reduced by 20% .
6. Explain how break test is conducted on D.C shunt motor with a neat sketch and also write
merits and demerits of the test.

UNIT TEST-II
MODEL PAPER-II
PART-A
NOTE :    Answer all questions                                                     2 X 3=6
Each question carries two marks

1. Why D.C series motor having high starting torque?
2. State the necessity for starter in dc motor?
3. What are the disadvantages of Swinburne’s test
PART-B
NOTE :   Answer any two questions                                             2 X 7=14
Each question carries seven marks

4. A 250V,4pole shunt motor has wave wound armature with 600 conductors. The armature
and field resistance are 0.25ohm and 125ohm respectively and the flux per pole is
0.02weber.If the motor draws 15A from the mains neglecting armature reaction,
Calculate i) Speed and torque developed in the armature II ) Shaft torque when rotational
losses is equal to 300W
5. Draw the schematic diagram of a 3 Point starter and Explain the working with protective
devices.
6. The Hopkinson’s test on two similar D.C shunt machines gave the following results
Time voltage 220V
Time current excluding field current = 40A
Armature current of motoring machine = 200A
Field current of motoring machine = 6A
Field current of generating machine = 7A
Calculate the efficiency of each machine at the given load conditions. The armature
resistances of each of the machine is 0.05ohm

BOARD DIPLOMA EXAMINATIONS, (C-14)
DEEE-THIRD SEMISTER EXAMINATIONS
MODEL PAPER-I
EE-302 , D.C. MACHINES
Time: 3Hours                                                                                                   Total marks: 80
PART-A

NOTE:    Answer all questions.                                         3 X 10=30

Each question carries three marks.

1. Write about dynamically induced EMF?

2. Compare Lap and wave winding in different aspects?

3. Draw the separately excited D.C generator and name the parts?

4. List the effect of armature reaction in D.C machines?

5. Draw the external characteristic curve of a D.C shunt generator?

6. State the Fleming’s left hand rule?

7. Why series motor is having high starting torque?

8. State the necessity of speed control of d.c. motor

9. State the necessity of starter in d.c motor?

10. What is meant by Regenerative Test.

PART-B                                                             5 X 10=50

Each question carries ten marks.

11. Explain the working of simple loop D.C Generator?

12. A shunt generator delivers 195A, at terminal p.d of 250V. The armature resistance and
shunt field resistance are 0.02ohm and 50ohm respectively. The iron and friction losses
equal 950 W .Find (i) Emf generated (ii) Copper losses (iii) o/p of prime mover
(iv) Electrical efficiency?

13. Derive the formula for De-magnetising AT/Pole & Cross magnetising A T/Pole for a d.c
generator?

14. A 4pole lap wound dc generator with 450armature conductors supplies a current of
150amps. The brushes have been displaced through 3 angular degrees from the
geometrical neutral axis. Calculate (i) Demagnetising AT/pole (ii) Cross magnetising
AT/pole (iii) The additional field current for neutralising the demagnetisation is the field
winding has 1000 turns/pole?

15. (I) explain the principle operation of D.C Motor with neat sketches?

(ii) Explain the significance of back EMF in dc motors and its formula?

16. Explain the different losses & power stages in d.c motor?

17. A 200V shunt motor has armature resistance of 0.04ohm and runs at 1000 rpm, when
the armature current is 60A.If the torque is unchanged; find the speed and armature
current when the field strength is reduced by 20%?

18. Explain how break test is conducted on d.c shunt motor with a neat sketch, also write
the merits and de-merits of the test?

BOARD DIPLOMA EXAMINATIONS, (C-14)
DEEE-THIRD SEMISTER EXAMINATIONS
MODEL PAPER-I
EE-302 , D.C. MACHINES
Time: 3Hours                                                           Total marks: 80
PART-A
NOTE:    Answer all questions.                                       3 x10=30
Each question carries three marks.
1. State the Fleming’s right hand rule?
2. Define (a) Pole pitch (b) Back pitch (c) Front pitch?
3. Draw the power flow diagram of d.c generator?
4. State the causes of sparking at brush of d.c generator?
5. List the advantages of parallel operation of d.c generator?
6. A 4 pole 500V, shunt motor has 720 wave wound armature conductors. The full load
armature current is 60A, and the flux/pole is 0.03Wb.The armature resistance is 0.2ohm and
the contact drop is 1V/Brush, Calculate the full load speed of the motor?
7. State the different losses in D.C motors?
8. State the factors that affect the speed of a D.C motor?
9. Draw the connection diagram for speed control of d.c.series motor
10. What are the disadvantages of Swinburne’s Test.

PART-B                                                          5 x10=50
Each question carries ten marks.
11. Explain the construction of a dc generator with neat sketch?
12. . Classify the D.C generators based on excitation along with voltage and current
equations, showing diagrammatically?
13. A100kw, 400V, 6pole dc shunt generator has a lap wound armature with 36 slots and 28
Conductors per slot. The shunt field resistance is 100 ohms. The brush is given a lead of
12°. Calculate the Demagnetising and cross magnetising AT/Pole?
14. Explain O.C.C, Internal and external characteristics of D.C Shunt generator?
15. (i) Classify DC motors in to different types with figures of each. Also write voltage and
Current  equation?
(ii) Derive an equation for armature torque in dc motors?
16. (i) Derive the condition for maximum efficiency of D.C motor?
(ii) A 220V, dc shunt motor takes 5A at no load when running at 700r.p.m. the field
Resistance is 200ohms.aThe resistance of armature is stand still gave a drop of 5V
Across armature terminals when 10Amps wave passed through it. Calculate (i) speed
on load (ii) torque in N.M (iii) Efficiency.
17. Draw the schematic diagram of a 3-point starter and explain the working with protective
devices?
18. A 250V 15KW Shunt motor has a maximum efficiency of 88% and a speed of 750 r.p.m
when delivering 75% of its rated output. The resistance of its shunt field is 100ohms.
Calculate the efficiency when the motor draws a current of 78Amp from mains?