Transformers and generators Solved Previous year Question Paper with answer

**MODULE 1**

1. Explain the Operation Practical Transformer on-load, with the help of phasor diagrams.

2. A single-phase, 250/500V Transformer gave the following results:

Open circuit Test : 250V, 1A, 80W on low voltage side.

Short circuit Test : 20V, 12A, 100W on high voltage side.

Calculate the circuit constants and show them on an equivalent circuit.

3. A 5 KVA, 500/250V, 50Hz, 1Ø transformer gave the following readings:

O.C. Test : 500V, 1A, 50W (HV side)

S.C. Test : 25V, 10A, 60W(LV side shorted)

Determine: i) Circuit constants, ii) The efficiency on full load, 0.8 lagging p.f, iii)The voltage regulation on full load, 0.8 lagging p.f iv) Draw the equivalent circuit.

3. Compare a single unit three – phase transformer with a bank of three single – phase transformer.

4. Develop the exact equivalent circuit of a single-phase transformer. From this derive the approximate and simplified equivalent circuits of the transformer. State the assumptions made.

5. Develop an exact equivalent circuit diagram of a single-phase transformer referred to both primary and secondary sides indicating all the parameters.

5. Explain how the open and short circuit tests are conducted on transformers to calculate the equivalent circuit parameters.

6. Calculate the ratings and turns ratio of a three-phase transformer to transform 10,000 KVA from 220 KV to 6600V, if the transformer is to be connected in i) Y-**Δ** ii) **Δ**-Y.

7. Mention the advantages of single 3 φ transformers over bank of single phase transformers.

8. Explain with the help of connection and phasor diagrams, how Scott connections are used to obtain two phase supply from three-phase supply mains.

9. A 3-phase,1000 KVA, 6600/1100V, the transformer is delta connected on the primary and star connected on the secondary, the primary resistance per phase is 1.8Ω and the secondary resistance per phase is 0.025Ω. Determine the efficiency when the secondary is supplying full load at 0.8 p.f. and the iron loss is 15KW. Also, determine the efficiency on full load unity p.f.

10. Write a note on voltage regulation and the significance of a single-phase transformer.

**MODULE 2**

1.Explain with a neat sketch, the Sumpner’s test on single-phase transformer.

2. Explain the necessity and necessary conditions for parallel operation transformers.

3. An auto transformer supplies a load of 3KW at 115 volts at UPF. If the applied voltage is 230V, calculate the power transferred to the load i) Inductively ii) Conductively.

4. Two transformers A and B are joined in parallel to the same load. Determine the current delivered by each transformer, given : Open circuit emf 6600V for A and 6400V for B. Equivalent leakage impedance in terms of secondary (0.3+j3)Ω for A and (0.2 + j1)Ω for B. The load impedance is (8+j6)Ω.

5. Derive an expression for the saving of copper in auto transformer as compared to an equivalent two winding transformer.

6. Explain with the help of a figure, the working of a load tap changing transformer.

7. Analyze the performance of transformers by conducting Sumpner’s test with relevant circuit diagram.

8. Derive an expression for the currents and load shared by two transformers connected in parallel supplying a common load, when no load voltages are unequal.

9. Two single-phase transformers A and B are connected in parallel. They have the same KVAratings but their resistances are respectively 0.05 and 0.1 per unit and their leakage reactance0.05 and 0.04 per unit. If A is operated on full load at a p.f. of 0.8 lagging, what will be the load and p.f.of B.

10. What is an Auto Transformer? Derive an expression for saving of copper in auto transformer compared to two windings transformer also write its applications.

11. With the help of neat sketches, explain the working ON load tap changer and OFF load tap changer.

**MODULE 3**

1. What are the purposes for which Tertiary windings are used?

2. Explain with the help of neat sketches, the effects of armature reaction in DC machines.

3. A 3-phase, 16-pole synchronous generator has a resultant air gap flux of 0.06 wb per pole. The flux is distributed sinusoidally over the pole. The stator has 2 slots per pole per phase and 4 conductors per slot are accommodated in two layers. The coil span is 150° electrical.Calculate the phase and line-induced voltages when the machine runs at 375 rpm.

4. What do you mean by three winding transformers? Deduce the equivalent circuit of a 3-winding transformer.

5. A 4-pole generator has a wave wound armature with 722 conductors and it delivers 100Aon full loads. If the brush lead is 8°, calculate the armature demagnetizing and cross-magnetizing ampere-turns per pole.

6. Derive the expressions for pitch factor and distribution factors in connection with alternator armature windings.

7. Draw and explain the equivalent circuit of a Tertiary transformer.

8. Discuss the armature reaction in D.C. machines and derive the equations for AT_{d} /pole and AT_{c} /pole.

9. A 6 pole, 148 A, dc shunt generator has 480 conductors and is wave wound. Its field current is 2A. Find the demagnetizing and cross-magnetising amper – turns/pole at full load if

i) The brushes are at the GNA.

ii) The brushes are shifted from GNA by 5° electrical.

iii) The brushes are shifted from GNA by 5° mechanical.

10. Explain in detail cooling of transformers.

11. Derive EMF equation of the synchronous generator and give the expression for pitch factor and distribution factor.

12. A, 3φ, 8 pole , star connected alternator has the armature coils short chorded by one slot. The coil span is 165″ electrical. The alternator is driven at the speed of 750 rpm. If there are12 conductors per slot and flux per pole is 50 mwb. Calculate the value of induced phase and line voltages.

**MODULE 4**

1. What are the necessary and necessary conditions for parallel operation of the Alternator? Explain the synchronization of Alternators by synchronizing lamp method.

2. Write a note on ‘V’ curves of Alternator.

3. Explain with the help of circuit diagram, the slip test on salient pole synchronous machine for the determination of direct and quadrature axis synchronous reactants.

4. Explain the generator load characteristics.

5. With the help of phasor diagram, explain the concept of two reaction theory for salient pole synchronous machine.

6. Derive the expression for synchronizing power.

7. Explain the short circuit ratio and its significance of an alternator.

8. Enumerate the various methods available for determining voltage regulation. Explain in details EMF and MMF methods.

9. Explain Zpf method of predetermination of voltage regulation of the alternator. Mention the advantages of this method.

10. The open circuit and short circuit test results for 3φ , star connected , 1000KVA,1905V,50Hz alternators are

I_{f} in Amps | 10 | 20 | 25 | 30 | 40 | 50 |

V_{oc} line volts | 760 | 1500 | 1700 | 1905 | 2300 | 2600 |

I_{sc} in Amps | – | 220 | – | 335 | – | – |

The armature reactance per phase is 0.2Ω. Draw the O.C and S.C characteristics and find the voltage regulation on full load 0.8 logging p.f. by

i) Amper_turn method. ii) Synchronous impedance method.

**MODULE 5**

1. Sketch and explain the open circuit and short circuit characteristics of a synchronous machine.

2. From the following test results, determine the voltage regulation of 2000V, 1 φ alternator delivering a current of 100A, at i) UPF ii) 0.8 leading p.fand iii) 0.71 lagging pf. Test results: Full load current of 100A is produced on the short circuit by a field excitation of2.5A. An emf of 500V is produced on the open circuit by the same excitation. The armatureresistance is 0.8Ω.

3. What are the causes and effects of hunting?

4. Define SCR (Short Circuit Ratio). What is the significance of SCR on the performance of synchronous machines?

5. Explain the Synchronous Impedance method of determining the voltage regulation of alternators.

6. What do you mean by Hunting in Alternators? How the effects of Hunting are reduced?

7. With a neat circuit diagram, explain the slip test on the salient pole synchronous machine to determine Xd and Xq from the slip-test.

8. The single phase alternators operating in parallel have induced emfs on open circuit of230L0″ and 230 L10° volts and respective reactances of j2Ω and j3Ω. Calculate i) Terminal voltage ii) Current iii) Power delivered by each of the alternators to a load of impedance 6Ω (reactive).

9. Discuss the concept of two – reaction theory in a salient pole synchronous machine with the help of phasor diagram.

10. Write a note on the capability curves of the synchronous generator.

11. Discuss about hunting in synchronous machines. Also, explain the role of damper windings.