(SEM V) THEORY EXAMINATION 2024-25 ELECTRICAL MACHINES-II

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=4.44fNϕkc​kd​

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.