(SEM VI) THEORY EXAMINATION 2017-18 SPECIAL ELECTRICAL MACHINE

SPECIAL ELECTRICAL MACHINE (NEN014)

Section-wise Solved Answers

SECTION A – Short Answer Questions (2 × 10 = 20 marks)

(a) Why are low-speed machines preferable when frequent starting and stopping is required?

Low-speed machines are preferred because they have low inertia. Low inertia allows the motor to accelerate and decelerate quickly with less mechanical stress. This reduces heating, improves control, and increases life when frequent starting and stopping operations are required.

(b) Effect of single phasing on a three-phase induction motor

When one phase supply fails, the motor continues to run on the remaining two phases but at reduced torque. The current in the remaining phases increases, causing overheating. If load is high, the motor may stall and get damaged.

(c) Function of two stator windings in a single-phase induction motor

The two windings are:

Main winding

Auxiliary (starting) winding

They produce a phase difference in current, creating a rotating magnetic field necessary for starting torque, since a single-phase supply alone cannot produce starting torque.

(d) Advantages of low rotor inertia in a 2-phase servo motor

Low rotor inertia provides:

Fast response

Quick acceleration and deceleration

Better dynamic performance

High accuracy in control systems

This makes the motor suitable for servo and control applications.

(e) Slew range of a stepping motor

Slew range is the maximum frequency range at which a stepping motor can operate without losing synchronism, when the motor is already running.

(f) Single-mode working of Variable Reluctance Motor (VRM)

In single-mode operation, only one phase is energized at a time. The rotor aligns itself with the energized stator pole due to minimum reluctance, producing motion step by step.

(g) Applications of PMDC motor

Permanent Magnet DC motors are used in:

Toys                                                                  Windshield wipers

Portable tools                                                   Robotics

Automotive applications

They are compact and efficient.

(h) Single-phase hysteresis motor

A hysteresis motor uses a rotor made of magnetic material with high hysteresis loss. The rotor locks with the rotating magnetic field and runs at synchronous speed with smooth and silent operation.

(i) Numerical – Power factor calculation

Given:
Voltage = 200 V                                                Current = 3.1 A
Reactance = 15 Ω                                              Impedance Z = V / I = 200 / 3.1 ≈ 64.5 Ω

Power factor

cos⁡ϕ=RZ=1−(XZ)2\cos \phi = \frac{R}{Z} = \sqrt{1 - \left(\frac{X}{Z}\right)^2}cosϕ=ZR​=1−(ZX​)2​ =1−(1564.5)2= \sqrt{1 - \left(\frac{15}{64.5}\right)^2}=1−(64.515​)2​ ≈0.97 (lagging)\approx 0.97 \text{ (lagging)}≈0.97 (lagging) 

(j) Torque-speed characteristic of repulsion motor

The repulsion motor develops very high starting torque. Torque increases rapidly at low speed and then decreases as speed approaches synchronous speed. The curve is similar to a series motor.

SECTION B – Long Answer Questions (Attempt any 3)

(a) Slip power recovery using slip power converter

In wound rotor induction motors, slip power appears in the rotor circuit. Instead of wasting it as heat, a slip power recovery system converts this power and feeds it back to the supply.

At 600 rpm:
Slip = (Ns − N) / Ns
At low speed (60 rpm), slip is high, so slip power is maximum.

Slip power ∝ Slip × Rotor input
Thus, maximum slip power occurs at minimum speed.

(b) Equivalent circuit of single-phase induction motor (Double Revolving Field Theory)

According to this theory, a single-phase stator field is resolved into:

Forward rotating field

Backward rotating field

Each field produces torque in opposite directions. At standstill, torques cancel each other, so net torque is zero. When rotor is given a start, forward torque dominates and motor runs.

Equivalent circuit consists of two induction motors in parallel representing forward and backward fields.

(c) 2-phase 8/4 PM stepping motor

Number of stator poles = 8
Number of rotor poles = 4

Step angle:

θ=360m×Nr\theta = \frac{360}{m \times N_r}θ=m×Nr​360​ =3602×4=45∘= \frac{360}{2 \times 4} = 45^\circ=2×4360​=45∘

Excitation sequence determines clockwise or anticlockwise rotation.

(d) Three-phase brushless DC motor

A BLDC motor uses electronic commutation instead of brushes. The inverter supplies three-phase currents in sequence. Torque is produced by interaction between stator magnetic field and permanent magnet rotor.

Advantages:                                           High efficiency

No sparking                                           Low maintenance

SECTION C – Very Long Answer (Attempt any ONE from each question)

Q3(a) Static slip power recovery scheme

This scheme uses rectifiers and inverters to recover slip power from rotor circuit and return it to the supply. It improves efficiency and allows smooth speed control.

Advantages:                                          Energy saving

Improved efficiency                              Suitable for large drives

Q4(a) Capacitor start capacitor run motor

A capacitor-start capacitor-run motor uses two capacitors:

Large capacitor for starting                  Small capacitor for running

Starting capacitor has higher value because it must produce high starting torque.

Torque-slip curve shows high starting torque and smooth operation.

Q5(a) Drive circuit for stepping motor

Drive circuit must:                                Supply correct phase current

Provide fast switching                          Prevent overheating

Maintain step accuracy

VR and hybrid motors use transistorized switching circuits.

Q6(a) PMAC motor

PMAC motor uses permanent magnets on rotor and AC supply to stator. It operates at synchronous speed.

Advantages:                                          High efficiency

Compact size                                        High power density

Q7(a) Linear induction motor

A linear induction motor is an induction motor cut and laid flat. It produces linear motion instead of rotational motion.

Applications:                                         Metro trains

Conveyors                                             Magnetic levitation systems