JNTU
JNTU B.Tech III ELECTRICAL MACHINES-III Previous Question Paper (November/December - 2017 - R13)
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. Each question carries 10 marks.
Part A
Answer all questions.
What happens to the value of synchronous reactance if air gap is increased?
Answer
If the air gap is increased, the synchronous reactance decreases. This occurs because increasing the air gap increases the reluctance of the magnetic circuit, which significantly reduces the armature reaction flux produced by the stator current. Since synchronous reactance is the sum of leakage reactance and armature reaction reactance, the reduction in armature reaction reactance leads to a decrease in the overall synchronous reactance.
A 50Hz alternator has a flux of 0.1 Wb/pole, sinusoidally distributed. Calculate the rms value of the emf generated in one turn of the winding, which spans 3/4 of a pole pitch.
Answer
The rms value of the induced emf in a single turn is given by the formula E = 4.44 × f × Φ × Kp, where f is the frequency, Φ is the flux per pole, and Kp is the pitch factor.
First, calculate the pitch factor (Kp). Since the coil spans 3/4 of a pole pitch, the coil span is 135 electrical degrees (3/4 × 180°). The pitch factor is calculated as:
Substituting the given values (f = 50 Hz, Φ = 0.1 Wb):
The rms value of the emf generated in the turn is 20.50 V.
Explain, why the field winding is placed on rotor, instead on stator of an alternator?
Answer
Sign in to view the complete answer paper.
What is synchronous reactance?
Answer
Sign in to view the complete answer paper.
Briefly explain the two main controls of an alternator.
Answer
Sign in to view the complete answer paper.
Give the condition for presence of no dc decaying component in the fault current of an alternator.
Answer
Sign in to view the complete answer paper.
Why synchronous motors are not self-staring? Explain.
Answer
Sign in to view the complete answer paper.
How are ‘V’ and ‘Ʌ’ curves of synchronous motor are defined?
Answer
Sign in to view the complete answer paper.
The full load slip of a single phase induction motor is higher than of corresponding 3 phase induction motor. Why?
Answer
Sign in to view the complete answer paper.
Give the working principle of universal motor.
Answer
Sign in to view the complete answer paper.
Unit I
What is armature reaction? With the help of neat diagrams explain its effect on main flux in synchronous machines.
Answer
Sign in to view the complete answer paper.
Explain the following terms related 3-phase a.c. windings: (i) Single-layer and double-layer windings, (ii) Full-pitch and short-pitch windings, (iii) Integral slot and fractional slot windings.
Answer
Sign in to view the complete answer paper.
Explain the experimental determination of synchronous impedance.
Answer
Sign in to view the complete answer paper.
Derive the relation between speed, frequency and number of poles in an alternator.
Answer
Sign in to view the complete answer paper.
Unit II
Derive an expression for finding regulation of salient - pole alternator using two reaction theory. Draw its Phasor diagram.
Answer
Sign in to view the complete answer paper.
Discuss about experimental determination of Xd and Xq of salient pole alternator using slip test.
Answer
Sign in to view the complete answer paper.
A 3- phase generator rated at 25 MVA, 13.8 kV is operating at normal terminal voltage and rated load at 0.8 pf lag. The direct axis synchronous reactance is 7.62 Ohm, Quadrature axis synchronous reactance is 4.57 Ohm and the armature resistance is 0.15 Ohm/ph. Determine the direct axis and quadrature axis components of armature current and internal induced voltage. Also find the regulation.
Answer
Sign in to view the complete answer paper.
Unit III
Explain the step by step procedure for synchronization of an alternator to the infinite bus system.
Answer
Sign in to view the complete answer paper.
Show that synchronizing power is essential for maintaining synchronism of two alternators running in parallel. Deduce the relevant expression for it.
Answer
Sign in to view the complete answer paper.
Explain the effect of change in excitation and mechanical power input on the alternator performance.
Answer
Sign in to view the complete answer paper.
Unit IV
Why at any load, the power factor decreases and the armature current increases if the field current is varied above and below the normal excitation.
Answer
Sign in to view the complete answer paper.
A 500V, 3-phase mesh connected motor has an excitation emf of 600V. The motor synchronous impedance is (0.4+j5) ohms while the wind age, friction and iron losses are 1200W. What maximum power output can it deliver?
Answer
Sign in to view the complete answer paper.
Explain how a synchronous motor can be operated as synchronous condenser.
Answer
Sign in to view the complete answer paper.
A synchronous motor has an equivalent armature reactance of 3.3 Ohm. The exciting current is adjusted to such a value that the open circuit emf is 950V. Find the p.f. at which the motor would operate when it takes 80kW from 800V supply line.
Answer
Sign in to view the complete answer paper.
Unit V
Draw the slip-torque characteristics of all types of single phase induction motors and compare their merits and demerits.
Answer
Sign in to view the complete answer paper.
Explain the working principle of split-phase and capacitor start single-phase induction motors.
Answer
Sign in to view the complete answer paper.
Draw the equivalent circuit diagram of single-phase induction motor.
Answer
Sign in to view the complete answer paper.
Explain the principle of operation of stepper motor and ac series motor.
Answer
Sign in to view the complete answer paper.