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VTU Electrical and Electronic Engineering (Semester 4)
Transformers & Induction Machines
May 2016
Total marks: --
Total time: --
INSTRUCTIONS
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary

1(a) With neat schematic diagram, explain core and shell type transformer.
6 M
1(b) Discuss the qorking condition of 1-ϕ transformer on resistive load with vector diagram.
6 M
1(c) A 230/460 volts transformer has a primary resistance of 0.2Ω and a reactance of 0.5Ω and the corresponding values for the secondary are 0.75Ω and 1.8Ω repectively. Find the secondary terminal voltage when supplying i) 10A at 0.8 p.f. lagging; ii) 10A at 0.8 p.f leading.
8 M

2(a) Develop the equivalent of a 1-ϕ transformer referred to primary side from the fundamentals.
6 M
2(b) Discuss the different types of losses in transformer and derive efficiency of transformer.
6 M
2(c) A 200 kV A single phase transformer is in circuit continuously. For 8 hours in a day the load is 80 kW at unity power factor and for the remaining peroid of 24 ours if runs on no-load. Full load copper losses are 3.02 kW and the iron losses are 1.6 kW. Find all day efficiency.
8 M

3(a) Discuss the essential and desirable conditions to be fulfilled for operating two single phase transformers in parallel.
6 M
3(b) What is an auto transformer? State its merits and demerits over two winding transformer.
6 M
3(c) Two transformers have following characteristics:
Transformer 1 ----- % IR = 1.0% and % IX = 5.0%
Transformer 2 ----- % IR = 1.5% and % IX = 4.0%
How they will share a load of 100 kV A at 0.8 p.f lagging?
8 M

4(a) Draw the soft connection of transformer and mark the terminals. Explain its merits and demerits.
6 M
4(b) Briefly discuss the choice of transformer connections.
6 M
4(c) Give the detail analysis of load sharing between two three phase transformers operating in parallel.
8 M

5(a) Bring out clearly, with the help of neat sketches the difference between the 3-phase slip ring induction motor and three phase squirrel cage induction motor.
6 M
5(b) Explain the terms slip, slip frequency and give the relation between them.
6 M
5(c) A 746 kWm 3-phase, 50 Hz 16-pole induction motor has a rotor impedance of (0.02 + j0.15) Ω at stand still full-load torque is obtained at 360rpm. Calculate: i) The ratio of maximum to full-load torque ; ii) The speed for maximum torque and iii) The rotor resistance to be added to get maximum starting torque.
8 M

6(a) Enumerate various components of power loss in an induction motor and name the parts where in these occur.
6 M
6(b) Draw the circle diagram of a 3-phase mesh connected, 30 h.p, 500V, 4pole, 50Hz cage type induction motor. The table gives the measurement of line current and voltage and readings of two watt meters connected to measure the input power.
 No-load 500V 8.3V +2.85kW -1.35kW Blocked rotor test 100V 32A -0.75kW +2.35kW
Find from the diagram for full-load:
i) Line current
ii) Power factor
iii) Efficiency and
iv) Maximum output.
14 M

7(a) With neat sketch, explain the construction of deep bar cage rotor motor.
6 M
7(b) Draw the torque-speed characteristics double cage rotor motor. How these characteristics are different from squirrel cage induction motor?
8 M
7(c) Describe with a neat diagram the principle of operation of induction generator.
8 M

8(a) Why the starter is necessary to start an induction motor? Mention the various methods of starting and discuss the limitations of these methods.
6 M
8(b) With neat schematic diagram, explain the method of star-delta transformer.
6 M
8(c) With neat sketches, explain the construction working principle of i) Split phase and ii) Capacitor start single phase induction motor.
8 M

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