ELECTROMECHANICAL ENERGY CONVERSION-I (EMEC-I)

ELECTROMECHANICAL ENERGY 

CONVERSION-I (EMEC-I)

2012

Uttarkhand Tech. university
 utu previous year question papers

Total marks :100

SECTION A

Q1:- Attempt any two of the following :

1. Define energy and co-energy. What is significance of co-energy? Show that the field energy in a linear magnetic system is given by W_f = 1/2Li² = 1/2 Ψi = (1/2L)Ψ².
2. Derive an expression for the torque in the doubly excited system having salient pole type of stator as well as well as rotor. State the assumption made.
3. For an electromagnetic system, show that the mechanical work done is equal to the area enclosed between two magnetization curve at open and closed position of the armature and  Ψ-I locus during the armature movement.

SECTION B

Q2:- Attempt any two of the following :

1. Explain "armature reaction" in a DC machine, indicating remedies to overcome its adverse effects.
2. Explain commutation processes in DC machines. What are the causes of pour commutation? Describe the role of inter-poles and compensating winding in a DC machine.
3. Draw the load characteristics of different DC generators. A DC generator delivers a current of 50 amp at 220 V to a load of a constant resistance while running at 1000 rpm. Its armature resistance is 0.5 ohm and that of the field is 50 ohm. Find the load current in case speed of DC generator falls to 100 rpm, field current remaining constant. Neglect effect of armature reaction.

SECTION C

Q3:- Attempt any four of the following :

1. Explain the Ward Leonard method of speed control with neat diagram.
2. A DC series motor is deriving a fan load where the load torque is proportional to cube of speed. The resistance of the armature and field in series is 1 ohm and the motor takes 10 amp and runs at 1000 rpm when operating from 200 V supply. Calculate the value of resistance to be inserted in series with the armature speed to 800 rpm.
3. The following is the OCC of the DC shunt generator at 300 rpm.
   The field resistance is adjusted to 354.5.
   Determine :
   
   (i) No load voltage
   (ii) Critical field resistance
   (iii) Critical speed for the given field resistance,
4. What is the necessity of starter in DC machines? Explain the working of 3 point starter of DC machines.
5. A 480 V, 20 kW shunt motor take 2.5 apm, when running with light load. Taking the armature resistance to be 0.6 ohm, field resistance to be 800 ohm and brush drop is 2 V, find the full load efficiency.

SECTION D

Q4:- Attempt any four of the following :

1. Define ALL day efficiency. A transformer has its maximum efficency of 0.98 kVA unity power factor load. Find the all day efficiency for the following loading cycle:
   (i) 2 kW at 0.5 p.f for 12 hours
   (ii) 12 kW at 0.8 p.f for 6 hours
   (iii) 18 kW at 0.9 p.f for 6 hours
2. What are the possible connections for a 3 phase transformer bank? Explain Y-4 connection.
3. Two transformer each rated 250 kVA, 11/2 kV and 50 Hz are connected in open delta on both the primary and secondary. Find the load kVA that can be supplied from this transformer connection.
4. A 100/400 V, 2kVA single phase transformer has 5% impedance. If it is connected as a 500/400 V auto-transformer, deduce from fundamental its kVA rating and percentage impedance as an auto-transformer.
5. What is the rush phenomena in a transformer? Derive an approximate expression for the magnitude of th maximum instantaneous current.
6. Explain open delta connection for a 3-phase transformer. What are the applications for this system? Discuss.

SECTION E

Q5:- Attempt any two of the following :

1. Derive an expression for the saving of copper in auto-transformer as compared with an equivalent two winding transformer.
2. Explain Scott connection and give its application.
3. What are the advantage of current and voltage harmonic in transformer? How this harmonics can be determined?

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