THEORY EXAMINATION (SEM–VI) 2016-17 SPECIAL ELECTRICAL MACHINE

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