About this previous year paper
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
| Electronic | Mechanical |
|---|---|
| No sparking | Sparking present |
| High speed | Limited speed |
| Low maintenance | High maintenance |
(ii) BLDC vs DC Motor
| BLDC | Conventional DC |
|---|---|
| Electronic commutation | Mechanical commutation |
| High efficiency | Lower efficiency |
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