SEM V THEORY EXAMINATION 2022-23 ELECTRICAL MACHINE-II

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