Principles of electrical machine design notes

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.

2

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

permeability

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

curve.
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.

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).

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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