JNTU
JNTU B.Tech II ELECTRICAL MACHINES II Previous Question Paper (February - 2024 - R15)
Question paper consists of Part A, Part B.
Part A is compulsory, which carries 25 marks. In Part A, answer all questions.
In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions.
Part A
What is the condition for maximum efficiency in a single phase transformer?
Answer
The condition for maximum efficiency in a single-phase transformer occurs when the variable copper losses are equal to the constant iron (core) losses.
Mathematically, this is expressed as I22Req2 = Pi, where I2 is the load current, Req2 is the equivalent resistance referred to the secondary side, and Pi is the constant iron loss.
Draw the equivalent circuit of transformer referred to primary.
Answer
The equivalent circuit of a transformer referred to the primary side represents all secondary parameters (resistance, leakage reactance, and load) transformed to the primary circuit by accounting for the turns ratio a = N1/N2. The circuit consists of the following components connected in series and parallel:
- The primary winding resistance R1 and leakage reactance X1 are connected in series with the input supply.
- The magnetizing branch, consisting of the core loss resistance Ro and magnetizing reactance Xm, is connected in parallel across the primary terminals to account for no-load current Io.
- The secondary resistance R2 and leakage reactance X2 are referred to the primary as R'2 = a2R2 and X'2 = a2X2, connected in series with the primary circuit.
- The load impedance ZL is referred to the primary as Z'L = a2ZL, completing the series path.
Why SC test is normally performed on HV side?
Answer
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Write the conditions for parallel operation of single-phase transformers
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Write the advantages of Auto transformers
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Draw a circuit diagram for conversion of 2 winding transformer into auto transformer in additive polarity
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The induction motor is called as asynchronous motor. Justify
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Write about crawling in an induction motor.
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What happens when induction motor runs above the synchronous speed?
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Mention any two speed control methods from stator side of induction motor
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Part B
With the help of phasor diagram explain the operation of single-phase on NO-load.
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A single-phase transformer has 1000 turns on the primary and 200 turns on the secondary. The no load current is 3A at a p.f of 0.2 lagging. Calculate the primary current and p.f. When the secondary current is 280A at a p.f. of 0.8 lagging.
Answer
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Explain the constructional details of single-phase transformers.
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How the iron losses vary with the change in supply frequency in transformer? Explain.
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Explain the procedure for conducting Sumpner’s test along with all precautions to be taken while conducting the test with neat diagram.
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Describe the parallel operation of two transformers when no-load voltages of these transformers are not equal.
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Describe the procedure to separate the core losses of a transformer.
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What are the distinguishing features of Y-Y and Y- Δ three phase connections? Explain.
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A bank of three-phase transformers has it’s HV terminals connected to 3 wire 3 phase, 11 kV system. It’s LV terminals are connected to a 3 wire 3 phase load rated at 1500 kVA, 2200V. Calculate the voltage, current and KVA ratings of each transformer for both HV and LV windings for Y- Δ connection.
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Two identical transformers each of rating 5KVA, 200/100V, 50 Hz transformers are connected in open delta. Calculate the KVA rating of the open delta bank when HV side is used as primary.
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Describe the construction of a three-phase cage type induction motor with neat diagram.
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A 3-phase induction motor is wound for 4 poles and is supplied from 50 Hz system. Calculate i) The synchronous speed ii) The speed of the motor when slip is 4% and iii) The rotor current frequency when the motor runs at 600 rpm.
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Explain how starting performance of three-phase squirrel cage type induction motor is improved by means of double cage rotor windings.
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Derive the expression for the rotor EMF and rotor current of an induction motor.
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Explain the pole changing method of speed control in induction motor.
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Explain the principle of induction generator.
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A three phase, 400 V, 50 Hz, 4-pole, delta connected, squirrel cage induction motor has the following data: No-load: 400 V, 3.0 A, 300 W, at 0.3 p.f; Blocked rotor: 120 V, 7.0 A 500 W, at 0.85 p.f. Draw the circle diagram, determine starting torque, maximum torque and efficiency when the motor works with a slip of 5%. The stator effective resistance per phase is equal to 4Ω.
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