Subject Code: BEE503
Subject Name: Electrical Machines-II
Course: B.Tech (Semester V)
Maximum Marks: 70
Duration: 3 Hours
Exam Year: 2024–25
Sections: A, B, and C
SECTION A – Short Answer Questions (2 × 7 = 14 Marks)
Attempt all questions briefly.
Why is a rotating field system used instead of a stationary field in alternators?
What are the effects of hunting in a synchronous motor?
Why can’t an induction motor run at synchronous speed?
Explain how a universal motor works on both AC and DC supply.
Define and explain cogging and crawling in 3-phase induction motors.
Define slip in induction motors.
Given: 3-phase, 50 Hz, 4-pole motor with 4% slip → find speed in r.p.m.
What is the function of a synchronous condenser?
SECTION B – Medium-Length Questions (7 × 3 = 21 Marks)
Attempt any three of the following.
Derive the EMF equation of an alternator.
Explain armature reaction under unity, lagging, and leading power factors.
Why does a synchronous motor have no starting torque?
Describe its principle and methods of starting.
Derive the torque equation of a 3-phase induction motor and find the condition for maximum torque.
Example: 746 kW, 50 Hz, 16-pole, rotor impedance (0.02 + j0.15) Ω.
Explain high-torque cage motors and describe Deep-bar and Double-cage motors.
Why are single-phase induction motors not self-starting?
Explain Double revolving field theory.
SECTION C – Long / Analytical Questions (7 × 5 = 35 Marks)
Attempt one part from each question.
Q3. Alternators
a. Explain voltage regulation in synchronous generators.
Given: 3-phase, 10 kVA, 400 V, 50 Hz, 0.8 PF lagging; impedance (0.5 + j10) Ω → calculate regulation using synchronous impedance method.
b. Explain parallel operation of alternators, reasons, and necessary conditions.
Q4. Synchronous Motors
a. Explain:
(i) V-curves of synchronous motors
(ii) Synchronous condenser operation
b. Explain Two-Reaction Theory for salient pole synchronous machines with phasor diagram.
Q5. Three-Phase Induction Motors
a. Describe the rotating magnetic field in 3-phase induction motors and explain its working principle with phasor diagram and equivalent circuit.
b. A 6-pole, 50 Hz motor develops 150 Nm torque at 1.5 Hz rotor frequency.
Find:
(i) Shaft power output
(ii) Rotor copper loss
(iii) Input power and efficiency, if stator loss = 700 W and friction torque = 10 Nm.
Q6. Starters and Speed Control
a. Explain the need for starters in 3-phase induction motors.
Describe DOL starter and Auto-transformer starter with diagrams.
b. Discuss methods of speed control for 3-phase induction motors.
Q7. Single-Phase Motors
a. Why are single-phase induction motors not self-starting?
Explain Double revolving field theory.
b. Explain:
(i) Split-phase motor
(ii) Capacitor-start, capacitor-run motor
(iii) Shaded-pole motor.
Key Topics for Preparation
EMF Equation of Alternator & Armature Reaction
Voltage Regulation (EMF, MMF, Potier methods)
Parallel operation of alternators
V-curves & Power factor control using Synchronous Condenser
Torque equation & Maximum torque condition of Induction Motors
Starting methods (DOL, Star-Delta, Auto-transformer)
Cogging, Crawling, and Hunting effects
Double Revolving Field Theory
Types of Single-Phase Motors (Split-phase, Capacitor, Shaded-pole)
Study Tips
Draw and label diagrams neatly — alternator phasors, equivalent circuits, torque-speed curves.
Revise key formulas:
Ns=120f/PN_s = 120f/PNs=120f/P, Slip=(Ns−N)/NsSlip = (N_s - N)/N_sSlip=(Ns−N)/Ns, E=4.44fNϕkckdE = 4.44 f N \phi k_c k_dE=4.44fNϕkckd
Solve numerical problems on voltage regulation, torque, and speed.
Understand differences: synchronous vs induction, single-phase vs three-phase motors.
Memorize advantages/disadvantages of double cage and deep-bar rotors.
