Course: B.Tech (Semester V) Subject Code: KEE-503
Subject Name: Electrical Machine–II Time: 3 Hours
Total Marks: 100
Note: Attempt all sections. Choose missing data suitably.
Section A – Short Answer Questions (2 × 10 = 20 Marks)
Answer all briefly:
Two main types of synchronous generators based on rotor construction.
Why alternators are rated in kVA and not in kW. Function of a synchronous condenser.
Effects of hunting in a synchronous motor. Define slip in induction motor.
Advantages of squirrel cage induction motor. Meaning of crawling in an induction motor.
Methods of speed control in 3-phase induction motor.
Types of single-phase induction motors. Define a universal motor.
Section B – Descriptive Questions (10 × 3 = 30 Marks)
Attempt any three:
Explain Potier’s Triangle Method for voltage regulation of a cylindrical rotor alternator.
Why synchronous motors are not self-starting — explain with diagram and damper winding starting method.
Working principle of a 3-phase induction motor; construction of squirrel cage and wound rotor types.
Need of a starter in 3-phase induction motor and explain autotransformer starting method with neat sketch.
A 230 V, 50 Hz capacitor-start single-phase induction motor at standstill gives:
Main winding: 100 V, 2 A, 40 W
Auxiliary winding: 80 V, 1 A, 50 W
→ Find capacitance for maximum starting torque.
Section C – Long Answer Questions (10 × 5 = 50 Marks)
Question 3
(a) A 230 V, 3-phase, 5 kVA star-connected salient pole alternator with Xd=12ΩX_d = 12 ΩXd=12Ω, Xq=7ΩX_q = 7 ΩXq=7Ω delivers full-load current at unity p.f. → Calculate excitation voltage, neglecting resistance.
or
(b) Explain with phasor diagrams the operation of synchronous motor at constant load and variable excitation. Draw and explain V-curves and their significance.
Question 4
(a) Derive EMF equation of an alternator. Define and derive pitch factor and distribution factor.
or
(b) A 3-phase, 1500 kVA, 2300 V alternator has Rdc=0.08Ω/phaseR_{dc} = 0.08 Ω/phaseRdc=0.08Ω/phase. Assume effective resistance = 1.5 × RdcR_{dc}Rdc.
Field current of 70 A → short circuit current = full-load current of 376 A.
Same field current → emf = 700 V (open circuit).
→ Determine synchronous reactance and full-load regulation at 0.8 p.f. lag.
Question 5
(a) Sketch and derive Torque–Slip characteristics of a 3-phase induction motor; derive conditions for maximum torque and maximum starting torque.
or
(b) An 18.65 kW, 4-pole, 50 Hz, 3-phase induction motor has friction and windage loss = 2.5% of output, full-load slip = 4%. Find:
Rotor copper loss
Rotor input
Shaft torque
Question 6
(a) Purpose, construction, and working of deep-bar cage rotors.
or
(b) Explain cascade operation of a 3-phase induction motor. Derive speed relations for cumulative and differential cascading.
Question 7
(a) Explain Double Revolving Field Theory and operation of single-phase induction motor using it.
or
(b) Explain working of:
(i) Shaded Pole Motor
(ii) Capacitor Start–Capacitor Run Motor
