Achievements and Awards for the A.Y 2023-2024

 1.Received Ph.D on 10/8/2023.

2. Certified UHV Course

3.Attended ATAL FDP

4. NITTR Certificate

DCMT Question Bank

 

ANDHRA LOYOLA INSTITUTE OF ENGINEERING AND TECHNOLOGY

DC MACHINES AND TRANSFORMERS(R20)

QUESTION BANK

IIND YEAR I SEM

 

Unit-1

 

 

1.Find expression for magnetic force developed in a doubly-excited translational magnetic system. (10M) [L1]

2.Explain the construction and principle of operation of DC generator. (10M)  [L1]

3. Find expression for magnetic force developed in a singly-excited translational magnetic system. (10M)

4.  Explain characteristics DC generators. (10M) [L1]

5) Determine (i) the total torque developed (ii) the useful torque of a 250 V, 4 pole series motor with 782 wave connected conductors developing 8 kW and taking 40 A with a flux per pole of 25 mWb. The armature resistance of the motor is 0.75 ohms.(5M)  [L2]

6.explain different types of excitation techniques of DC Generators(10M) [L1]

7. A 20 kW, 250 V, 6 pole lap connected dc generator runs at 1250 rpm. Armature has 550 conductors. For full load armature – ohmic loss of 250 Wfind the useful flux per pole. Take 2 V as the brush drop at full load.and give applications(5m)  [L2]

8.Derive EMF equation  of DC Generators(10M)  [L1]

 

 

 

 

Unit-2

 

 

1.Explain the construction and principle of operation of DC Motor. (10M)  [L1]

2.Explain the mechanical and electrical characteristics of dc cumulative and differential dc motors. (10M) [L1]

3. Determine (i) the total torque developed (ii) the useful torque of a 250 V, 4 pole series motor with 782 wave connected conductors developing 8 kW and taking 40 A with a flux per pole of 25 mWb. The armature resistance of the motor is 0.75 ohms.  (5M) [L2]

4.explain the concept of armature reaction of dc motor   [L1] (10M)

5.explain the concept of losses and efficiency and application of dc motor  (5M)  [L1]

6.explain the concept of 3 point starter with neat diagram  (10M) [L1]

7. explain the concept of 4 point starter  (10M)  [L1]

8. A 230 V DC Shunt motor takes 32 A at full-load. Find the back emf on full load  if the resistances of motor armature and shunt field windings are 0.22 Ω and 120Ω respectively. (5M) [L2]

 

 

 

 

 

 

 

 

 

 

Unit-3

 

1.explain speed control technquies of DC shunt motor (10M) [L1]

2. Explain the procedure of conducting brake-test on DC machine with a neat  circuit   diagram(10M) [L1]

3. . Explain the procedure of conducting  swinbunes -test on DC machine with a neat  circuit   diagram(10M) [L1]

4. . Explain the procedure of conducting  hopkinsonson -test DC machine with a neat  circuit   diagram

     And  Explain concept of separation of losses(10M) [L1]

5. In a retardation test on a D.C motor, with its field normally excited, the speed fell from 1525 to 1475 in 25 seconds. With an average load of 1 kW supplied by the armature, the same speed drop occurred in 20 seconds. Find the moment of inertia of the rotating parts in kg.m2(5M) [L2]

 

 

 

 

 

 

 

 

MID – II Q.B

 

Unit - 3

 

7.. Discuss the working principle of single-phasetransformer and also explain the  constructional details(10M) [L1]

8..Derive the emf equation of a transformer. (5M) [L1]

9.explain the concept of transformer on no load condition with phasor diagram[L1] (10M)

10.explain the concept of transformer on  load condition with phasor diagram[L1] (10M)

11. .explain the concept of transformer on unity power factor with phasor diagram [L1] (10M)

12.draw and explain equivalent circuit of transformers [L1] (10M)

13. In a 50-kVA, 11-kV/400-V, single-phase transformer, the iron and copper losses  are 500 W and 600 W, respectively under rated conditions. Calculate (a) the efficiency at unity power factor at full-load, (b) the load for maximum efficiency, and (c) the iron and copper losses for this load. [L2] (5M)

 

Unit-4

 

1.A 4.5 kVA, 400/210 V,50 Hz single phase transformer has the following test

data:

O.C. test (l.v.side) 210V, 1A,70 W

S.C. test (h.v.side) 15 V, 10.8A, 100 W

Calculate (i) Equivalent circuit referred to l.v side and

(ii) Secondary load voltage on full load at 0.8 power factor lagging

(iii) Efficiency of transformer at ¾ th load and 0.7 power factor (lag).  [L2]  (10M)

 

 

2. In a 400 V, 50 Hz transformer, the total iron loss is 2300 W.  When the supply voltage and the frequency reduced to 200 V and 25 Hz respectively the corresponding loss is 800 W.  Calculate the eddy current loss at normal voltage and frequency. [L2] (5M)

 

 

3. A 2-winding 10 kVA, 440/110 V transformer is reconnected as a step-down 550/440 V autotransformer. Compare volt-ampere rating of the autotransformer with that of original 2-winding transformer. Calculate power transferred to the load: (i) inductively (ii) conductively [L2] (5M)

 

 

 

4. The voltage per turn of a single-phase transformer is 1.1V. When the primary  winding is connected to a 220V, 50Hz A.C supply, the secondary voltage is found to be 550V. Find:  i) Primary and secondary turns ii) Core area if the maximum flux density is 1.1 Wb/m2 [L2] (5M)

 

 

5. A 220 V , 2.8 KVA single phase transformer has an iron loss of 120 W at  45 Hz and 70 W at 35 Hz. Find the hysteresis and eddy current losses at 50 Hz  [L2] (5M)

 

 

6. A single phase, 25 KVA, 2000/200V transformer has iron loss is 350W and full  load copper loss is 400W. Calculate the efficiency at unity power factor on full load and half load? [L2] (5M)

 

 

7. What is sumpner’s test and explain its principle with a neat circuit diagram  [L1] (5M)

 

8. What is O.C and S.C  test and explain its principle with a neat circuit diagram L1] (5M)

 

 

9.explain the operation of parallel operation of transformers [L1] (5M)

 

 

10. Two transformers A and B are connected in parallel to supply a load having an impedance of (2 + j 1.5)O. The equivalent impedances referred to the secondary windings are (0.15 + j 0.5)O and (0.1 + j 0.6)O respectively. The open-circuite.m.f. of A is 207 V and of B is 205 V. Calculate

(i) the voltage at the load

(ii) the power supplied to the load

(iii) the power output of each transformer and

(iv) the kVA input to each transformer. [L2] (10M)

 

 

 

Unit-5

1. What is the total load capacity of V-V bank as compared with a delta-delta bank?  (5M)

 [L1]

 

2. Explain the concept of Scott connection (three phase to two phase) conversion with a neat circuit diagram. (5M)

 [L1]

 

3. Derive an expression for saving of copper in it when compared to ordinary two winding transformer? (5M) [L1]

 

4. Explain open delta (or V – V ) connection with neat diagrams (5M) [ L1]

 

5. In a Sumpner’s test on two identical single – phase transformers rated 750 kVA, 11/0.5 kV, 50 Hz the wattmeter reading on h.v side is 8000 W and on the l.v side is 16000W. Find the efficiency of each transformer on half full load and 0.78 power factor. What will be its maximum efficiency (5M)

 [ L2]

 

6. A balanced 3-phase, 100 kW load at 400V and 0.8 p.f. lag is to be obtained from a balanced 2-phase, 1100V lines. Determine the kVA rating of each unit of the Scott-connected transformer. [ L2]

 

7. A balanced 3-phase, 100 kW load at 400V and 0.8 p.f. lag is to be obtained from a balanced 2-phase, 1100V lines. Determine the kVA rating of each unit of the Scott-connected transformer.  (5M)

 [ L2]

 

8. What are the advantages of poly-phase transformers? Give different  configurations (5M)

 [ L2]