Electrical Machines: An Introduction

• The lecture serves as an introduction to electrical machines and electrical materials.
• Syllabus topics include:
  • Basics of EMT and electrical materials
  • Transformers
  • Electromechanical Energy Conversion
  • DC Machines
  • Synchronous Machines
  • Induction Machines
• The transformer syllabus includes construction, equivalent circuit, losses and efficiency, three-phase transformers, parallel operation, induced EMF, testing, voltage regulation, auto transformers, and harmonics.
• The DC machine syllabus includes construction, EMF and torque equations, equivalent circuit, speed control, electromechanical energy conversion, losses and efficiency, machine characteristics, starting & braking.
• The synchronous machine syllabus includes induced EMF, equivalent circuit, power angle equation, losses and efficiency, droop characteristics, synchronous motor, construction and physical phenomenology, voltage regulation methods, synchronous impedance, parallel operation, salient pole machines, and cascading of machines.
• The induction machine syllabus includes construction and rotating magnetic field, torque slip characteristics, tests, speed control, single-phase induction motor, equivalent circuit, power and efficiency, starting, and stability.
• Essential for machine study involve:
  • High weightage on theory and practice,
  • Using class notes and previous year questions for factual information.
  • practicing assignments, classwork and batch tests,
  • Consulting PYQ books such as YCT.
• A suggested class strategy involves:
  • Daily live classes,
  • Assignments,
  • Revising PDF notes plus short handwritten notes,
  • Weekend PYQ practice
• Topics in electrical materials include conducting, magnetic, and insulating materials.

Electrically Conducting Materials

• Electrically conducting materials are divided into high conductivity and high resistivity materials.
• High conductivity materials are used for windings, transmission, and distribution, such as copper, aluminum, gold, and silver.
  • σAg > σAu > σCu > σAl (conductivity comparison)
• High resistivity materials (alloys) with distorted crystal structures are used for making resistances and heating devices.
• Requirements for highly conducting materials include:
  • High conductivity
  • Low temperature coefficient of resistance (α),
  • Adequate mechanical strength and absence of brittleness,
  • Reusability and draw-ability for manufacturing small wires.
  • [R = R0[1+αΔT]]

Copper

• Copper has high electrical conductivity and adequate resistance to oxidation and corrosion.
• It's a malleable and ductile metal.
• Electrical machines employ winding of annealed highly conducting copper.
• Hard drawn copper wires are used because drawing increases mechanical strength.

Aluminum

• Aluminum is abundant but not found in free state, obtained from bauxite (mixture of monohydrate and trihydrate aluminum oxide).
  • (Al2O3.2H2O)
  • (Al2O3.3H2O)
• Bauxite is purified and dissolved in fused cryolite which is a double fluoride of aluminum and sodium.
• Aluminum's physical and mechanical properties include:
  • Silvery color and luster,
  • Melting point of 658°C,
  • Lightweight with a specific gravity of 2.7,
  • Ductile for thin wires in cables,
  • Sufficient ultimate strength (50 to 70 MPa),
  • Good conductor of heat and electricity.
• It forms a protective aluminum oxide layer (Al2O3), making it corrosion resistant.
• Pure Aluminum is soft but can be hardened with Ni, Mg, Si, and Co alloying elements.
• Cold-worked aluminum can be amended by heating it to 350°C and then cooling it in air.
• Property comparison of Copper vs Aluminum:
  • Cost: Copper (1), Aluminum (0.49)
  • Cross-section Area: Copper (1), Aluminum (1.62)
  • Diameter: Copper (1), Aluminum (1.27)
  • Volume: Copper (1), Aluminum (2.04)
  • Weight: Copper (1), Aluminum (0.49)
  • Breaking Strength: Copper (1), Aluminum (0.64)

Electrical Carbon

• Made from graphite or other forms of carbon.
• Carbon's conductivity is slightly less than metals and alloys.
• It is used to make brushes in electrical machines.
• Brush carbon is graphited to raise conductivity and reduce hardness.
• They should have a smooth surface to avoid wear of commutator.
• Carbon's negative temperature coefficient ensures constant brush voltage drop.

Magnetic Materials

• Magnetic materials are magnetizable and create a magnetic field.
• Originate from the motion of electrons and their magnetic moments (dipoles).
• Five classes of magnetic materials according to their magnetic behavior:
  • Diamagnetic substances
  • Paramagnetic substances
  • Ferromagnetic substances
  • Ferrimagnetic substances
  • Anti-ferromagnetic substances
• Magnetic effects exist outside when there is a net permanent magnetic moment.

Diamagnetic Materials

• Diamagnetic materials have a weak form of magnetism when placed in a magnetic field and become weakly magnetized in the opposite direction.
• They are repelled by a magnetic field.
• In the absence of an external field, diamagnetic materials do not have external fields, and magnetic dipoles are not present.
• When an external field is applied, dipoles are induced opposite to the external field.
• When the external field is removed, the dipoles vanish.
• Has negative magnetic susceptibility.
• Field lines move away from each other.
• Bin = μoμr H < μoH (μr < 1)
• B: magnetic flux density and closeness of magnetic field lines is reduced in this material.
• Examples include Cu, Ge, Si, and diamond.

Paramagnetic Materials

• These acquire weak magnetization in the same direction as the applied field due to less no. of unpaired è.
• Total dipole moment or magnetization is zero in absence of magnetic field.
• Dipoles are present but randomly oriented and align once an external magnetic field (H) is applied.
• The magnetic susceptibility (Xm) is > 0, with magnetization parallel to the applied field (M || H).
• Net dipole moment/volume is slightly greater than 1.
• Magnetic field lines become closer inside these materials.
• Examples include Potassium, Oxygen, Aluminum, Tungsten, Sodium and Calcium.

Ferromagnetic Material

• Characterized by the presence of permanent alignment of dipole moments.
• These consist of d-block and f-block elements.
• Ferro magnetic properties of transition elements is due to spin of unpaired electron.
• Common are Fe, Co, Ni, Gd, and Dy which are strongly attracted by mag. field.
• Magnetic Susceptibility, Xm >> 0,
• μr = 1 + Xm ; range: 3000 - 5000
• B= μoμr H >> μoH, field lines come "very" close to each other.

Hysteresis Curves & Definitions

• Retentivity: the residual flux density when the magnetic field is removed (y-intercept of hysteresis curve).
• Virginal Curve: The magnetization curve when a material is magnetized for the first time.
• Coercive Force: The magnetic field (Hc) required to reduce the flux density to zero (x-intercept of the hysteresis curve).

Domain Theory

• According to Weiss domain theory, a ferromagnetic material consists of magnetized domains.
• Magnetic moments in individual domains are parallel below the Curie temperature. Each domain is magnetically saturated and has a net magnetic moment.
• The direction of the net magnetic moment in each domain varies, resulting in no overall magnetic moment or magnetization.
• Above the Curie temperature, alignment is disrupted, ferro goes to Para.

Magnetostriction

• Describes change in material dimensions when exposed to a magnetic field.
• Under an alternating field, dimensions contract and expand in every half cycle.
• Magnetic oscillations in audible range create a humming sound.
• In a transformer, the material expands during the positive cycle and contracts during the negative cycle.

Anti-Ferro & Ferri-magnetic Materials

• Anti-Ferro: net mag moment = 0
• Ferri: net magnetic moment ≠0 - replace Fe by Mn in ferro mag
  • weak mag.
  • ρ(resistivity=high)
  • Pe = π² f² Bm² t² / 6 ρ
  • Pe=low
  • Used in high frequency T/F

Hard and Soft Magnetic Materials

• Soft Magnetic Materials
  • These have high permeability, low hysteresis losses, and low coercive force.
  • High permeability, low hysteresis losses, and low coercive force, example electromagnet and machines.
  • Characterized by a steeply ascending magnetization curve and a narrow B-H curve.
  • Easy to magnetize and lose magnetization quickly when the magnetizing force is removed.
  • Magnetization is rapid because domain walls move easily
• Hard Magnetic Materials
  • Have an ability to retain field.
  • They have high coercive force and sufficient permeability.
  • They have a high retentivity.
  • Commonly used for making permanent magnets.
  • They have a large coercive force, they have a large area in B-H curve or hysteresis loop comparison to soft magnetic materials.
  • B-H curve or hysteresis curve is almost rectangular.
• Difference between hard and soft magnetic material
  • Hard retains magnetism
  • Hard is difficult to demagnetized
  • Soft is easily magnetized and demagnetized
  • Hard materials retain magnetism even after the removal of applied magnetic field
  • Soft materials are easier to demagnetized as domain wall motion is easier.
  • Hard is used for making permanent magnets as domain wall motion is prevented.
  • Soft is suitable for temporary magnets and electromagnets.

Sheet Steel

• Electrical sheet steel is non-oriented steel.
• Early transformer cores used iron with low carbon content.
• Ageing decreases magnetic performance, increasing coercive force and hysteresis loss.
• Modern laminations in electrical machines and transformers are made of Si-steel, in the range of 0.3% to 4.5% by weight.
• Silicon reduces hysteresis loss and increases resistivity, reducing eddy current loss.
• With increased silicon content, there is a loss of permeability and ductility.
• Electrical sheet steels can be manufactured from hot or cold rolling.

Cold Rolled Grain Oriented Steel (CRGO)

• CRGO is used if iron is worked at a high flux density, as the mmf required would be too high without a material that can work at high flux density without saturation.
• Ordinary hot-rolled steel has crystals oriented randomly, resulting in low permeability.
  • μr = high in rolling direction
• The rolling direction is magnetically superior.
• CRGO steel is manufactured by a series of cold rolling and intermediate annealing steps.

Insulating Material

• An ideal insulating material has the following properties:
  • High dielectric strength, material should not break down at high electric fields
  • High resistivity means lower dielectric loss,
  • Low dielectric hysteresis that is lesser are of the hysteresis loop
  • Good thermal conductivity
  • High degree of thermal stability, can handle high termperatures
  • Non-hygroscopic, vibration, abrasion, and bending.
  • Should withstand chemical attack.
• Insulating materials are used to insulate winding from the core.