Principles of electrical machine design notes

Principles of electrical machine design notes



Principles of electrical machine design notes


The magnetic flux in all electrical machines (generators, motors and transformers) plays an
important role in converting or transferring the energy. Field or magnetizing winding of rotating machines produces the flux while armature winding supplies either electrical power or mechanical power. In case of transformers primary wing supplies the power demand of the secondary.

The  basic  design  of  an  electrical  machine  involves  the  dimensioning  of  the  magnetic  circuit,
electrical circuit, insulation system etc., and is carried out by applying analytical equations.

A designer is generally confronted with a number of problems for which there may not be one solution,  but  many  solutions.  A  design  should  ensure  that  the  products  perform  in  accordance with the requirements at higher efficiency, lower weight of material for the desired output, lower
temperature rise and lower cost. Also they are to be reliable and durable.

A practical designer must effect the design so that the stock (standard frames, punching etc.,) is
adaptable to the requirements of the specification. The designer must also affect some sort of compromise between the ideal design and a design which comply with manufacturing conditions.
A electrical designer must be familiar with the,

  1. National and international standards

Indian Standard (IS), Bureau of Indian Standard (BIS), India British Standard (BS), England
International Electrotechnical Commission (IEC)
NEMA  (The National Electrical Manufacturers Association).

  1. Specifications (that deals with machine ratings, performance requirements etc., of the consumer)
  2. Cost of material and labour
  3. Manufacturing constraints etc.


A designer can refer to Design Data Handbook (Electrical Machine Design Data Book,  authored
by A Shanmugasundaram and others , New Age International Publishers, Reprint 2007, or any other such handbooks) which is a source of design procedure, properties of materials, ranges of
design parameters etc., and manufacturer’s brochure.

As the design involves a number of assumptions and constraints, final design values can be obtained only by iterative methods. Computer plays a vital role in arriving at the final values. By
Finite Element Method (FEM), the effect of a single parameter on the dynamical performance of
the  machine  can  be  studied.  Furthermore,  some  tests,  which  are  not  even  feasible  in  laboratory
setup, can be virtually performed by Finite Element Method.

The design problems, that have been considered to solve in the latter chapters, are of different nature from the  design  worked  out in detail in  respect of any machine. However, these test

problems provide adequate elementary skills in design, which is an indication that a student has a fair knowledge to deal with the entire design.

Factors for consideration in electrical machine design

The basic components of all electromagnetic apparatus are the field and armature windings supported by dielectric or insulation, cooling system and mechanical parts. Therefore, the factors for consideration in the design are,

    1. Magnetic circuit or the flux path: Should establish required amount of flux using minimum mmf. The core losses should be less.
    2. Electric circuit or windings: Should ensure required emf is induced with no complexity in winding arrangement. The copper losses should be less.
    3. Insulation: Should ensure trouble free separation of machine parts operating at different potential and confine the current in the prescribed paths.
    4. Cooling system or ventilation: Should ensure that the machine operates at the

specified temperature.

    1. Machine parts: Should be robust.

The art of successful design lies not only in resolving the conflict for space between iron, copper, insulation and coolant but also in optimization of cost of manufacturing, and operating and maintenance charges.
The factors, apart from the above, that requires consideration are

      1. Limitation in design ( saturation, current density, insulation, temperature rise etc.,)
      2. Customer’s needs
      3. National and international standards
      4. Convenience in production line and transportation
      5. Maintenance  and repairs
      6. Environmental conditions etc.
Limitations in design

The  materials  used  for  the  machine  and  others  such  as  cooling  etc.,  imposes  a  limitation in
design. The limitations stem from saturation of iron, current density in conductors, temperature, insulation, mechanical properties, efficiency, power factor etc.

  1. Saturation:  Higher flux density reduces the volume of iron but drives the iron to operate beyond knee of the magnetization curve or in the region of saturation. Saturation  of  iron  poses  a  limitation  on  account  of  increased  core  loss  and excessive

excitation required to establish a desired value of flux. It also introduces harmonics.

  1. Current   density: Higher current density reduces the volume of copper but increases

the losses and temperature.

  1. Temperature: poses a limitation on account of possible damage to insulation and other materials.
  2. Insulation (which is both mechanically and electrically weak): poses a limitation on account of breakdown by excessive voltage gradient, mechanical forces or heat.


  1. Mechanical strength of the materials poses a limitation particularly in case of large and high speed machines.
  2. High efficiency  and high power factor poses  a limitation on account of  higher capital

cost.  (A  low  value  of  efficiency  and  power  factor  on  the  other  hand  results  in  a  high
maintenance cost).

  1. Mechanical Commutation in dc motors or generators leads to poor commutation.

Apart from the above factors Consumer, manufacturer or standard specifications may pose a limitation.

Materials for Electrical Machines


The main material characteristics of relevance to electrical machines are those associated with conductors for electric circuit, the insulation system necessary to isolate the circuits, and with the specialized steels and permanent magnets used for the magnetic circuit.

Conducting materials

Commonly  used  conducting  materials  are  copper  and  aluminum.  Some  of  the  desirable
properties a good conductor should possess are listed below.

  1. Low value of resistivity or high conductivity
  2. Low value of temperature coefficient of resistance
  3. High tensile strength
  4. High melting point
  5. High resistance to corrosion
  6. Allow brazing, soldering or welding so that the joints are reliable
  7. Highly malleable and ductile
  8. Durable and cheap by cost

Some of the properties of copper and aluminum are shown in the table-2.






Resistivity at 200C

0.0172 ohm / m/ mm2

0.0269 ohm / m/ mm2


Conductivity at 200C

58.14 x 106S/m

37.2 x 106S/m


Density at 200C





Temperature coefficient (0-100oC)

0.393 % per 0C

0.4 % per 0C

Explanation: If the temperature increases by 1oC, resistance increases by 0.4% in case of aluminum


Coefficient of linear expansion (0-100oC)

16.8x10-6 per oC

23.5 x10-6 per oC


Tensile strength

25 to 40 kg / mm2

10 to 18 kg / mm2


Mechanical property

highly malleable and ductile

not highly malleable and ductile


Melting point




Thermal conductivity (0-100oC)

599 W/m 0C

238 W/m 0C



can be easily soldered

cannot be soldered easily






For the same resistance and length, cross-sectional area of aluminum is 61% larger than that of the copper conductor and almost 50% lighter than copper.
Though the aluminum reduces the cost of small capacity transformers, it increases the size and
cost of large capacity transformers. Aluminum is being much used now a days only because copper is expensive and not easily available. Aluminum is almost 50% cheaper than Copper and not much superior to copper.

Magnetic materials


The magnetic properties of a magnetic material depend on the orientation of the crystals of the
material  and  decide  the  size  of  the  machine  or  equipment  for  a  given  rating,  excitation
required, efficiency of operation etc.
The some of the properties that a good magnetic material should possess are listed below.

  1. Low reluctance or should be highly permeable or should have a high value of relative


  1. High saturation induction (to minimize weight and volume of iron parts)
  2. High electrical resistivity so that the eddy emf and the hence eddy current loss is less
  3. Narrow hysteresis loop or low Coercivity so that hysteresis loss is less and efficiency of

operation is high

  1. A high curie point. (Above Curie point or temperature the material loses the magnetic

property      or becomes paramagnetic, that is effectively non-magnetic)

  1. Should have a high value of energy product (expressed in joules / m3).

Magnetic  materials  can  broadly  be  classified  as  Diamagnetic,  Paramagnetic,  Ferromagnetic,
Antiferromagnetic and  Ferrimagnetic materials.  Only  ferromagnetic materials have properties
that are well suitable for electrical machines. Ferromagnetic properties are confined almost entirely to iron, nickel and cobalt and their alloys. The only exceptions are some alloys of manganese and some of the rare earth elements.

The relative permeability         r of ferromagnetic material is far greater than 1.0. When ferromagnetic materials are subjected to the magnetic field, the dipoles align themselves in the direction of the applied field and get strongly magnetized.
Further the Ferromagnetic materials can be classified as Hard or Permanent Magnetic materials
and Soft Magnetic materials.

  1. Hard  or  permanent  magnetic  materials  have  large  size  hysteresis  loop  (obviously

hysteresis loss is more) and gradually rising magnetization curve. Ex: carbon steel, tungsten steal, cobalt steel, alnico, hard ferrite etc.

  1. Soft  magnetic  materials  have  small  size  hysteresis  loop  and  a  steep  magnetization

Ex: i) cast iron, cast steel, rolled steel, forged steel etc., (in the solid form).
-Generally used for yokes poles of dc machines, rotors of turbo alternator etc., where steady or dc flux is involved.

ii) Silicon steel (Iron + 0.3 to 4.5% silicon) in the laminated form. Addition of silicon in proper percentage eliminates ageing & reduce core loss. Low silicon content steel or dynamo grade steel is used in rotating electrical machines and are operated at high flux density. High content silicon steel (4 to 5% silicon) or transformer grade steel (or high resistance steel) is used in transformers. Further sheet steel may be hot or cold rolled. Cold rolled grain oriented steel (CRGOS) is costlier and superior to hot rolled. CRGO steel is generally used in transformers.

Special purpose Alloys:

Nickel iron alloys have high permeability and addition of molybdenum or chromium leads to improved magnetic material. Nickel with iron in different proportion leads to

    1. High nickel permalloy (iron +molybdenum +copper or chromium), used in

current transformers, magnetic amplifiers etc.,

    1. Low nickel Permalloy (iron +silicon +chromium or manganese), used in transformers, induction coils, chokes etc.
    2. Perminvor (iron +nickel +cobalt)
    3. Pemendur (iron +cobalt +vanadium), used for microphones, oscilloscopes, etc.
    4. Mumetal (Copper + iron)
Amorphous alloys (often called metallic glasses):

Amorphous alloys are produced by rapid solidification of the alloy at cooling rates of about a million degrees centigrade per second. The alloys solidify with a glass-like atomic structure which is non-crystalline frozen liquid. The rapid cooling is achieved by causing the molten alloy to flow through an orifice onto a rapidly rotating water cooled drum. This can produce sheets as thin as 10µm and a metre or more wide.
These alloys can be classified as iron rich based group and cobalt based group.


µ x 10-3

in tesla



Ωm x 108

3% Si grain oriented






2.5% Si grain non -oriented






<0.5% Si grain non oriented






Low carbon iron






78% Ni and iron






50% Ni and iron






Iron based Amorphous






Insulating materials

To avoid any electrical activity between parts at different potentials, insulation is used. An ideal insulating material should possess the following properties.

  1. Should have high dielectric strength.
  2. Should with stand high temperature.
  3. Should have good thermal conductivity
  4. Should not undergo thermal oxidation
  5. Should not deteriorate due to higher temperature and repeated heat cycle
  6. Should have high value of resistivity ( like 1018 cm)
  7. Should not consume any power or should have a low dielectric loss angle
  8. Should withstand stresses due to centrifugal forces ( as in rotating machines), electro dynamic or mechanical forces ( as in transformers)
  9. Should withstand vibration, abrasion, bending
  10. Should not absorb moisture
  11. Should be flexible and cheap
  12. Liquid insulators should  not evaporate or volatilize


Insulating materials can be classified as Solid, Liquid and Gas, and vacuum. The term insulting material is sometimes used in a broader sense to designate also insulating liquids, gas and vacuum.

Solid: Used with field, armature, transformer windings etc. The examples are:

  1. Fibrous or inorganic animal or plant origin, natural or synthetic paper, wood, card board, cotton, jute, silk etc.,

rayon, nylon, terelane, asbestos, fiber glass etc.,

  1. Plastic or resins. Natural resins-lac, amber, shellac etc.,

Synthetic resins-phenol formaldehyde, melamine, polyesters, epoxy, silicon resins,
bakelite, Teflon, PVC etc

  1. Rubber : natural rubber, synthetic rubber-butadiene, silicone rubber, hypalon, etc.,
  2. Mineral : mica, marble, slate, talc chloride etc.,
  3. Ceramic : porcelain, steatite, alumina etc.,
  4. Glass : soda lime glass, silica glass, lead glass, borosilicate glass
  5. Non-resinous : mineral waxes, asphalt, bitumen, chlorinated naphthalene, enamel etc.,

Liquid: Used in transformers, circuit breakers, reactors, rheostats, cables, capacitors etc., & for impregnation. The examples are:

  1. Mineral oil (petroleum by product)
  2. Synthetic oil askarels, pyranols etc.,
  3. Varnish, French polish, lacquer epoxy resin etc.,

Gaseous: The examples are:

  1. Air used in switches, air condensers, transmission and distribution lines etc.,
  2. Nitrogen use in capacitors, HV gas pressure cables etc.,
  3. Hydrogen though not used as a dielectric, generally used as a coolant
  4. Inert gases neon, argon, mercury and sodium vapors generally used for neon sign lamps.
  5. Halogens like fluorine, used under high pressure in cables

No insulating material in practice satisfies all the desirable properties. Therefore a material which satisfies most of the desirable properties must be selected.

Classification of insulating materials based on thermal consideration


The insulation system (also called insulation class) for wires used in generators, motors transformers and other    wire-wound electrical components is divided into different classes according the temperature that they can safely withstand.
As per Indian Standard ( Thermal evaluation and classification of Electrical Insulation,IS.No.1271,1985,first revision) and other international standard insulation is classified by letter grades A,E,B,F,H (previous Y,A,E,B,F,H,C).


Insulation class

operating temperature
in ℃

Typical materials






Cotton,  silk,  paper,  wood,
impregnation or oil immersed








The  material  of  class  Y  impregnated  with  natura
cellulose esters, insulating oils etc., and also laminated wood,
varnished paper etc.




Synthetic  resin  enamels  of  vinyl  acetate  or  nylon
cotton and paper laminates with formaldehyde bonding etc.,




Mica,  glass  fiber,  asbestos  etc.,  with  suitable  bon
substances, built up mica, glass fiber and asbestos laminates.




The  materials  of
bonding materials










Glass  fiber  and  asbestos  materials  and  built  up  mica
appropriate silicone resins




Mica,  ceramics,  glass,  quartz  and  asbestos  with  binder
resins of super thermal stability.


l  resins,

tapes, ding stance with
s  or


The maximum operating temperature is the temperature the insulation can reach during operation and is the sum of standardized ambient temperature i.e. 40 degree centigrade, permissible temperature rise and allowance tolerance for hot spot in winding. For example, the maximum temperature of class B insulation is (ambient temperature 40 + allowable temperature rise 80 + hot spot tolerance 10) = 130oC.

Insulation is the weakest element against heat and is a critical factor in deciding the life of electrical equipment. The maximum operating temperatures prescribed for different class of

insulation are for a healthy lifetime of 20,000 hours. The height temperature permitted for the machine parts is usually about 2000C at the maximum. Exceeding the maximum operating temperature will affect the life of the insulation. As a rule of thumb, the lifetime of the winding insulation will be reduced by half for every 10 ºC rise in temperature. The present day trend is to design the machine


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Principles of electrical machine design notes


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Principles of electrical machine design notes