JNTU
JNTU B.Tech II ELECTRICAL MACHINES II Previous Question Paper (May – 2019 - R15)
Note: This question paper contains two parts A and B. Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit.
Part A
Draw the core type transformer and explain its principle of operation.
Answer
A core-type transformer consists of a rectangular magnetic core made of laminated steel sheets, with two vertical limbs around which the primary and secondary windings are wound. The windings are typically arranged as concentric cylindrical coils to minimize leakage flux.
The principle of operation is based on Faraday's law of electromagnetic induction. When an alternating voltage is applied to the primary winding, it produces an alternating magnetic flux in the core. This flux links with the secondary winding, inducing an electromotive force (EMF) in it according to the relation:
This mutual induction allows for the transfer of electrical energy from one circuit to another at the same frequency but at a different voltage level.
State and explain different type of losses in the transformer.
Answer
Losses in a transformer are broadly classified into two categories: core losses and copper losses.
- Core (Iron) Losses: These occur in the transformer core due to the alternating magnetic flux. They consist of hysteresis loss, caused by the reversal of magnetic domains in the core material, and eddy current loss, caused by circulating currents induced within the core laminations. Since the flux density and frequency remain essentially constant under normal operation, core losses are considered constant regardless of the load.
- Copper (Ohmic) Losses: These are the I2R losses occurring in the primary and secondary windings due to their electrical resistance. Unlike core losses, copper losses are variable and depend directly on the square of the load current.
Draw equivalent circuit of a 1-φ transformer.
Answer
Sign in to view the complete answer paper.
Draw the circuit diagram for conducting O.C test on 1-φ transformer.
Answer
Sign in to view the complete answer paper.
Explain the principle of operation of a single phase auto transformer.
Answer
Sign in to view the complete answer paper.
Draw a circuit diagram for conversion of 2-winding transformer in to auto transformer in additive polarity.
Answer
Sign in to view the complete answer paper.
Derive the condition for maximum torque at the time of starting in a 3-φ I.M.
Answer
Sign in to view the complete answer paper.
Derive the condition for maximum torque under running conditions in a 3-φ I.M.
Answer
Sign in to view the complete answer paper.
Explain the working principle of Induction generator.
Answer
Sign in to view the complete answer paper.
Explain the speed control of 3-φ IM using cascade connection.
Answer
Sign in to view the complete answer paper.
Part B
Explain the principle of operation of an ideal transformer.
Answer
Sign in to view the complete answer paper.
Derive the EMF equation of a single phase transformer.
Answer
Sign in to view the complete answer paper.
A 25 KVA single phase transformer has 250 turns on the primary and 40 turns on the secondary winding. The primary is connected to 1500 V, 50Hz mains. Calculate (i) primary and secondary currents on full load (ii) secondary emf (iii) maximum flux in the core.
Answer
Sign in to view the complete answer paper.
Draw the phasor diagrams for leading power factor load.
Answer
Sign in to view the complete answer paper.
When a transformer is connected to a 100V, 50 Hz supply the core loss is 100W of which 650W is hysteresis and 350W is eddy current loss. If the applied voltage is raised to 2000 V and the frequency to 100Hz, find the new core losses.
Answer
Sign in to view the complete answer paper.
With neat circuit diagram explain the of a short circuit test conducted on a transformer.
Answer
Sign in to view the complete answer paper.
A 10 KVA, 500/250 V single phase transformer has its maximum efficiency of 94% when delivering 90% of its rated output at unity pf. Estimate its efficiency when delivering its full load output at pf of 0.8 lagging.
Answer
Sign in to view the complete answer paper.
Derive and explain necessary condition for zero regulation of a transformer.
Answer
Sign in to view the complete answer paper.
Explain the parallel operation of two single phase transformers with unequal voltage ratios.
Answer
Sign in to view the complete answer paper.
With neat circuit diagram explain the procedure of conducting sumpners test on single phase transformers.
Answer
Sign in to view the complete answer paper.
Derive the condition for saving of a copper in a single phase auto transformer.
Answer
Sign in to view the complete answer paper.
A two winding transformer is rated at 2400/240 V, 50 KVA. It is reconnected as a step up auto transformer, with 2400 V input. Calculate the rating of the auto transformer and the inductively and conductively transferred powers while delivering the rated output at unity pf.
Answer
Sign in to view the complete answer paper.
Explain the scott connection operation with necessary circuit diagrams.
Answer
Sign in to view the complete answer paper.
Explain open-delta operation with necessary circuit diagrams.
Answer
Sign in to view the complete answer paper.
Two transformers are required for a scott connection operating from a 440V, 3-phase supply for supplying two single phase furnaces at 200 V on the two phase side. If the total output is 150 KVA, calculate the secondary to primary turn ratio and the winding currents of each transformer.
Answer
Sign in to view the complete answer paper.
Explain the torque slip characteristics of 3-φ IM.
Answer
Sign in to view the complete answer paper.
Derive and explain rotating magnetic field in a three phase induction motor.
Answer
Sign in to view the complete answer paper.
Briefly explain the working of double Cage Induction motor.
Answer
Sign in to view the complete answer paper.
The power input to the rotor of 440V, 50 Hz, 6 pole, 3-phase, and induction motor is 80 KW. The rotor electromotive force is observed to make 100 complete alterations per minute. Calculate (i) the slip (ii) the rotor speed (iii) rotor copper losses per phase.
Answer
Sign in to view the complete answer paper.
Explain the conducting procedure of No load test on three phase induction motor.
Answer
Sign in to view the complete answer paper.
Explain the working of y-Δ starter used in 3-φ induction motor.
Answer
Sign in to view the complete answer paper.
Explain the conducting procedure blocked rotor test on three phase induction motor.
Answer
Sign in to view the complete answer paper.
Explain the speed control of induction motor using Rotor resistance control.
Answer
Sign in to view the complete answer paper.