(SEM VI) THEORY EXAMINATION 2024-25 ELECTRICAL MACHINE DESIGN

BEE062 – ELECTRICAL MACHINE DESIGN

Section-Wise Solved Answers (2024–25)

SECTION A

(Attempt all questions) – 2 × 7 = 14 marks

(a) What will happen if the induction motor air gap increases?

If the air gap of an induction motor increases, the magnetizing current required to establish the air-gap flux increases significantly. This results in a higher no-load current, reduced power factor, and increased copper losses. The torque produced by the motor decreases because the magnetic coupling between stator and rotor becomes weaker. Overall efficiency drops and performance deteriorates.

(b) What is voltage regulation of transformer?

Voltage regulation of a transformer is defined as the change in secondary terminal voltage from no-load to full-load condition, expressed as a percentage of full-load voltage, while the primary voltage remains constant. It indicates the ability of the transformer to maintain constant output voltage under varying load conditions.

(c) What are the various constraints of design of a machine?

The design of an electrical machine is constrained by electrical limitations (current density, insulation), magnetic limitations (flux density, saturation), thermal limits (temperature rise), mechanical strength, cost, efficiency, and space requirements. All these factors must be balanced to achieve an economical and reliable design.

(d) What are specific electric and magnetic loadings?

Specific electric loading is defined as the total current carried by all conductors per meter of armature periphery. Specific magnetic loading is the average magnetic flux density in the air gap. These parameters play a key role in determining the size, output, and performance of electrical machines.

(e) Why rotor slots of the induction motor are skewed?

Rotor slots are skewed to reduce magnetic locking between stator and rotor teeth, minimize cogging torque, reduce harmonic torques, and decrease noise. Skewing also improves smoothness of operation and reduces vibrations.

(f) What is Computer Aided Design (CAD)?

Computer Aided Design (CAD) is the use of computer systems to assist in the design, analysis, optimization, and modification of electrical machines. CAD improves accuracy, reduces design time, allows optimization, and enables simulation before actual manufacturing.

(g) What is the ratio of no-load to full-load current of a three-phase induction motor?

The no-load current of a three-phase induction motor is typically 25% to 40% of the full-load current, depending on motor size and design.

 SECTION B

(Attempt any three) – 7 × 3 = 21 marks

(a) Discuss the various classes of insulation in terms of temperature

Electrical insulation materials are classified based on their maximum permissible operating temperature.
Class A insulation can withstand temperatures up to 105°C and includes materials like cotton and paper.
Class B insulation is suitable up to 130°C and uses mica and glass fiber.
Class F insulation operates up to 155°C and is commonly used in modern machines.
Class H insulation can withstand temperatures up to 180°C and is used in high-performance machines.
Higher insulation class allows higher loading and longer machine life.

(b) Derive expression of KVA rating of three-core type transformer

The KVA rating of a transformer depends on window area, space factor, current density, and flux density.
Output power is proportional to the product of window area and core area.
Using standard transformer design equations, the KVA rating is expressed as:

Q = K × Aw × Ac × Bm × J × f

where Aw is window area, Ac is core area, Bm is maximum flux density, J is current density, and f is frequency.

(c) Difference between core type and shell type transformer

In a core type transformer, windings surround the core, while in a shell type transformer, the core surrounds the windings. Core type has better cooling, whereas shell type offers better mechanical protection and lower leakage flux. Shell type transformers are preferred for high-power applications.

(d) Factors influencing choice of specific magnetic and electric loading

Specific magnetic loading is influenced by core material, cooling method, and losses. Higher magnetic loading increases iron losses.
Specific electric loading depends on current density, cooling, and permissible temperature rise. High electric loading increases copper losses. A balanced choice ensures efficiency and reliability.

(e) Discuss no-load current, voltage regulation and efficiency of transformer

No-load current consists of magnetizing and core loss components. Voltage regulation indicates voltage variation with load. Transformer efficiency is the ratio of output power to input power and depends on copper and iron losses. Maximum efficiency occurs when copper losses equal iron losses.

SECTION C

Q3. Attempt any one

(a) Derive expression of KVA rating of three-phase shell type transformer

In shell type transformers, both windings are placed on the central limb.
The output equation is derived considering window area, space factor, current density, and flux density.
The KVA rating is given by:

Q = 3.33 × f × Bm × J × Kw × Aw × Ac

This shows that output depends on magnetic and electric loadings and transformer dimensions.

(b) Explain core and armature design of DC and 3-phase AC machines

In DC machines, the armature is the rotating part carrying conductors, and the core is laminated to reduce eddy current losses.
In 3-phase AC machines, the stator acts as the armature and produces rotating magnetic field, while the rotor may be squirrel cage or wound type. Proper core design ensures efficient magnetic flux path and reduced losses.

Q4. Attempt any one

(a) Explain synthesis and hybrid methods of CAD

Synthesis method starts from output requirements and designs dimensions step by step.
Hybrid method combines analytical equations with optimization techniques. It offers better accuracy and flexibility than traditional methods.

(b) Prove that emf/turn of single-phase transformer is proportional to √Q

EMF per turn is proportional to flux. Since output KVA is proportional to square of linear dimensions, EMF per turn becomes proportional to square root of output KVA.

Q5. Attempt any one

(a) Explain the design of field system of DC machine

The field system consists of poles, field windings, yoke, and pole shoes.
Its design ensures uniform flux distribution, reduced losses, and proper magnetic circuit. Pole shape and material are selected to avoid saturation.

(b) Explain modern trends in design and manufacturing techniques

Modern techniques include CAD, finite element analysis, CNC machining, automation, and improved insulation materials. These reduce cost and improve efficiency and reliability.

Q6. Attempt any one

(a) Explain selection and guidelines of number of poles of DC machine

Number of poles depends on speed, voltage, current rating, and machine size. High-speed machines use fewer poles, while low-speed machines use more poles to reduce armature reaction.

(b) Explain design of salient and non-salient pole rotor of alternator

Salient pole rotors are used for low-speed alternators and have projecting poles.
Non-salient pole rotors are cylindrical and used for high-speed turbo alternators. Design depends on speed and application.

Q7. Attempt any one

(a) Limitations of traditional design and need of CAD

Traditional methods are time-consuming and less accurate. CAD allows optimization, simulation, and faster design with better accuracy and reduced cost.

(b) Explain cooling system designs of transformers

Transformers use air natural (AN), air forced (AF), oil natural air natural (ONAN), and oil forced air forced (OFAF) cooling methods. Proper cooling improves efficiency and life.