SPECIAL ELECTRICAL MACHINE (NEE603)
Section-wise Solved Answers & Notes
SECTION – A (10 × 2 = 20 Marks)
Very short & precise answers
(a) Types of stepper motor • Variable Reluctance (VR) stepper motor
• Permanent Magnet (PM) stepper motor • Hybrid stepper motor
(b) Applications of Printed Circuit Board (PCB)
• Electronic circuits mounting • Computers and mobiles
• Control circuits • Power electronics equipment
(c) Resolution
Resolution of a stepper motor is the smallest angular movement of the rotor per input pulse, usually expressed in degrees per step.
(d) Modes of operation of Switched Reluctance Motor (SRM)
• Motoring mode • Generating mode
• Braking mode
(e) Slewing
Slewing is the maximum speed at which a stepper motor can start, stop, or reverse without losing steps.
(f) Use of deep bar cage rotor and double cage rotor
They are used to: • Improve starting torque
• Reduce starting current • Provide good speed regulation
(g) Reversal of direction of PMDC motor
Direction can be reversed by reversing the polarity of supply voltage to the armature.
(h) Types of single-phase induction motors • Split-phase motor
• Capacitor start motor • Capacitor run motor
• Shaded pole motor
(i) Classification of magnetic materials • Diamagnetic
• Paramagnetic • Ferromagnetic
(j) Holding torque
Holding torque is the maximum torque a stepper motor can hold when energized but not rotating.
SECTION – B (Attempt Any Five) (5 × 10 = 50 Marks)
(a) Shaded pole induction motor
Construction: • Stator with salient poles
• Copper shading ring on each pole • Squirrel cage rotor
Working: Shading coil creates a weak rotating magnetic field, producing starting torque.
Torque-speed characteristics: • Low starting torque
• Nearly constant speed
Applications: • Fans
• Clocks • Small household appliances
(b) Deep bar induction motor
Construction: Rotor bars are deep and narrow, placed in rotor slots.
Working: • At start → high rotor reactance → high torque
• At running speed → low rotor resistance
Advantages:
• Good starting torque • Reduced starting current
(c) Two-phase AC series motor
Construction:
• Two stator windings in quadrature • Series field and armature
Working:
Produces unidirectional torque due to phase displacement.
Torque-speed characteristics:
• High starting torque • Speed decreases with load
(d) Switched Reluctance Motor (SRM)
Construction:
• Salient pole stator with windings • Salient pole rotor without windings
Principle:
Rotor moves to position of minimum reluctance.
Advantages: • Simple construction
• High efficiency • Rugged design
(e) Hybrid stepper motor
Principle: Combination of PM and VR stepper motor principles.
Characteristics: • High resolution
• High torque • Accurate positioning
Applications: • CNC machines
• Robotics • Printers
(f) Permanent Magnet DC Motor (PMDC)
Construction: • Permanent magnets on stator
• Armature winding on rotor
Working: Torque produced due to interaction of armature current and magnetic field.
Advantages: • No field winding losses • Compact size
(g) Hysteresis motor
Construction: • Smooth cylindrical rotor made of hard magnetic material
• Uniform air gap
Working: Torque produced due to hysteresis loss in rotor.
Characteristics:
• Constant torque
• Smooth and noiseless operation
(h) Linear induction motor
Construction: • Unrolled stator of induction motor • Conductive secondary
Working:
Produces linear motion instead of rotational motion.
Applications:
• Maglev trains • Conveyors
SECTION – C (Attempt Any Two) (2 × 15 = 30 Marks)
Q3. Static slip power recovery scheme
Principle:
In slip-ring induction motor, slip power from rotor circuit is recovered and fed back to supply, instead of being wasted as heat.
Advantages:
• Improved efficiency • Speed control above synchronous speed
• Reduced rotor losses (Neat diagram required in exam)
Q4. Numerical (Induction motor performance)
Given:
• Poles = 6 • Torque developed = 150 Nm
• Rotor frequency = 1.5 Hz • Mechanical loss torque = 10 Nm
Steps: Synchronous speed
Ns=120fPN_s = \frac{120f}{P}Ns=P120f
Slip
s=frfs = \frac{f_r}{f}s=ffr
Shaft torque Tshaft=150−10=140 NmT_{shaft} = 150 - 10 = 140 \, \text{Nm}Tshaft=150−10=140Nm
Shaft power P=2πNT60P = \frac{2\pi NT}{60}P=602πNT
Rotor copper loss
Prc=s×PairgapP_{rc} = s \times P_{airgap}Prc=s×Pairgap
Input power and efficiency calculated accordingly
(Show full steps in exam for full marks)
Q5. Variable reluctance stepper motor
Construction: • Stator with phase windings • Soft iron toothed rotor
Working: Rotor aligns with energized stator pole, moving step by step.
Advantages: • Simple construction • Low cost
Applications:
• Position control systems • Robotics • Disk drives
