(SEM VI) THEORY EXAMINATION 2024-25 SPECIAL ELECTRICAL MACHINES

BEE061 – SPECIAL ELECTRICAL MACHINES

Time: 3 Hours | Max Marks: 70

SECTION A – Short Answer Questions

(2 × 7 = 14 marks | Attempt ALL)

 Write definitions / lists / neat block diagrams

a. Applications of DFIG (Doubly Fed Induction Generator)       Wind energy conversion systems

Variable-speed wind turbines                                                    Grid-connected renewable energy plants

Power generation with reduced converter rating

b. Constant Power Drive (Super-Synchronous Mode)               Rotor speed > synchronous speed

Power flows from rotor to grid through converters                Stator delivers power directly to grid
Draw block diagram with power flow arrows

c. Motors Exhibiting Detent Torque                                           Permanent Magnet Stepper Motor

Hybrid Stepper Motor

d. Rotor Position Sensing Methods in SRM                               Hall effect sensors

Optical encoders                                                                        Resolver

Sensorless methods (flux/current based)

e. Saliency of PMSM

Interior PMSM: Saliency present (Ld ≠ Lq) → Reluctance torque + PM torque

Surface PMSM: No saliency (Ld ≈ Lq) → Only PM torque

f. Applications of PMAC and PCB Motors

PMAC Motors:                                                                             CNC machines

Robotics                                                                                         Electric vehicles

PCB Motors:                                                                               Cooling fans

Small appliances                                                                          Compact electronic devices

g. Applications of Universal Motor                                               Electric drills

Mixers and grinders

SECTION B – Medium Answer Questions

(7 × 3 = 21 marks | Attempt ANY THREE)
Write working principle + characteristics + equations

a. Singly Excited Induction Generator (Isolated Mode)      Self-excitation using capacitors

Magnetization curve provides voltage build-up

Torque–speed characteristic similar to induction motor but operating as generator
Explain with magnetization and torque-speed curves

b. Stepper Motor Numerical (Very Important)                    Given: Step angle = 1.8°

Resolution:

Resolution=3601.8=200 steps/rev\text{Resolution} = \frac{360}{1.8} = 200 \text{ steps/rev}Resolution=1.8360​=200 steps/rev

Steps for 50 revolutions:

200×50=10,000 steps200 \times 50 = 10{,}000 \text{ steps}200×50=10,000 steps

Speed (f = 5000 pulses/sec):

Speed=5000200×60=1500 rpm\text{Speed} = \frac{5000}{200} \times 60 = 1500 \text{ rpm}Speed=2005000​×60=1500 rpm 

c. Power Converters for SRM

Asymmetric half-bridge converter

C-dump converter

Split-DC converter

Explain energization and demagnetization of phases

d. Differentiation

(i) Electronic vs Mechanical Commutator

(ii) BLDC vs DC Motor

e. 1-Phase Hysteresis Motor

Smooth rotor without slots

Torque produced due to hysteresis effect

Constant torque from standstill to synchronous speed
Explain torque-speed characteristic

SECTION C – Long Answer

(7 × 1 = 07)

a. Linear Induction Motor (LIM)         Construction: Primary winding + secondary reaction rail

Working: Traveling magnetic field produces linear force

Force equation:

F=PvF = \frac{P}{v}F=vP​                    Explain applications: metros, conveyors

OR

b. Two-Phase Servomotor                Control winding + reference winding

Torque proportional to control voltage

Good dynamic response                  Explain with torque-speed characteristics