Electrical Machine Design

INTRODUCTION – Electrical Machine Design

Electrical machine design involves application of science and technology to produce cost-effective, durable, quality and efficient machines.  Also the machines should be designed as per standard specifications. The requirements like low cost and high quality will be conflicting in nature and so a compromise should be made between them.

The electrical machines can be classified into static and dynamic machines.  The transformer is a static (stationary) machine.  The motors and generators are dynamic (rotating) machines.  The transformer converts electrical energy from one voltage level to another voltage level.  The rotating machines converts electrical energy to mechanical energy or vice-versa.

The conversion in any electrical machine takes place through magnetic  field. The required magnetic field is produced by an electromagnet which requires a core and winding. The basic principle of operation of all electrical machine is governed by Faraday’s law of electromagnetic induction.

CONSTRUCTIONAL ELEMENTS OF TRANSFORMER

The transformer is a static electromagnetic device used to transfer electrical energy from a high potential (voltage) circuit to low potential (voltage) circuit or vice-versa.  It consists of two or more windings which link with a common magnetic field.  An iron core serves as a path for magnetic flux.

The constructional elements of the transformer are windings, core, tank and cooling tubes or radiators.  A simple transformer has two windings and they are called high voltage winding and low voltage winding.  One of the winding is connected to supply and it is called primary.  The other winding is connected to load and it is called secondary.

The two different types of transformer constructions are core type and shell type.  In core type type transformer the windings surround the core and in shell type transformer the core surround the windings.  The core and winding assembly is housed in the tank. Cooling tubes or radiators are provided around the tank surface in order to increase the effective cooling surface.

CONSTRUCTIONAL ELEMENTS OF ROTATING MACHINES

The rotating electrical machines converts electrical energy to mechanical energy or vice-versa.  The energy conversion takes place through magnetic field.  Every rotating machine have the following three quantities. The presence of any two quantities, will produce the third quantity.The rotating electrical machines converts electrical energy to mechanical energy or vice-versa. The energy conversion takes place through magnetic field.  Every rotating machine have the following three quantities. The presence of any two quantities, will produce the third quantity.

  • Magnetic field – I (Field)
  • Magnetic field – II (Armature)
  • Mechanical force

In generator, the armature is rotated by a mechanical force inside a magnetic field or the magnetic field is rotated by keeping armature stationary.  By Faraday’s law of induction, an emf is induced in the armature.  When the generator is loaded,  the armature current flows, which produce another magnetic field (armature magnetic field).  Hence in a generator, by the presence of a magnetic field and mechanical force, an another magnetic field is produced.

The mechanical force developed by the motor is due to the reaction of two magnetic fields.  A current carrying conductor has a magnetic field around it. When it is placed in another magnetic field it experiences a mechanical force due to the reaction of two magnetic field.  Hence in a motor by the presence of two magnetic fields a mechanical force is developed.

From the above discussion it is clerar that any rotating machine requires two magnetic field and one of the field is rotating.  Hence a rotating machine will have a stationary and rotating electromagnet, each consisting of a core and winding.  The stationary electromagnet is called stator and the rotating electromagnet is called rotor.

The basic constructional elements of a rotating electrical machine are stator and rotor.  In dc machines the stator consists of field core and winding.  The rotor comprises of armature core and winding.  In ac machines the stator has armature core and winding.  The rotor consists of field core and winding.  The constructional elements of various electrical machines are listed here.

Constructional elements of dc machine

Stator   –   Yoke or Frame           Rotor     –   Armature core

  • Field pole –   Armature winding
  • Pole shoe     –  Commutator
  • Field winding    Others     –  Brush
  • Interpole –  Brush holder

Constructional elements of salient pole synchronous machine

Stator  –  Frame     Rotor    –  Field pole

  • Armature core –  Pole shoe
  • Armature winding –   Field windin   –    Damper winding

Constructional elements of cylindrical rotor synchronous machine

Stator  –  Frame                 Rotor –  Rotor core

  • Stator core                   –  Rotor bars
  • Stator winding               –   End ring

Constructional elements of slip-ring induction motor

Stator  –  Frame      Rotor   –  Rotor core

  • Stator core –  Rotor winding
  • Stator winding  –  Slip rings

CLASSIFICATION OF DESIGN PROBLEMS

The design of an electrical machine involves solution of many complex and diverse engineering problems.  The design problems may be classified under the following four headings.

  • Electromagnetic design
  • Mechanical design
  • Thermal design
  • Dielectric design

Each problem may be solved separately and the results are combined to give overall solution.  Each of these three major problems may be further divided into simple problem are combined to give the solution of a major design problem.

The electromagnetic design problem in rotating machines involves the design of stator & rotor core dimensions, stator & rotor teeth dimensions, air-gap length, stator and rotor windings.  In transformer it is the problem of designing the core and the windings.

The mechanical design in rotating machine involves the design of frame (enclosure), shaft and bearings.  In transformer it is the design of tank (i.e., housing for core and winding assembly).

The thermal design in rotating machine involves the design of cooling ducts in core and cooling fans.  In case of large machines coolants like air or hydrogen may be forced to circulate in the ducts and air-gap.  In transformer it involves the design of cooling tubes or radiators.

Another important design problem, that may require great attention is the design of insulations (Dielectric design).  Dielectric material are used to insulate one conductor from other and also the windings from the core.  The dielectric materials are designed to withstand high voltage stresses.  The breakdown of dielectric materials may lead to failure of machine.

STANDARD SPECIFICATIONS – Electrical Machine Design

Every country has a standards organization to fix standard specification for the manufacturers.  The specifications are guidelines for the manufacturers to produce economic products without compromising quality.  The manufacturers who are compiling with the standards will be issued a certification for their products.  The quality of the certified products will be periodically monitored by the standards organisations.

The standard specifications issued for electrical machines includes the following.

  • Standard ratings of machines.
  • Types of enclosure.
  • Standard dimensions of conductors to be used.
  • Method of marking ratings and name plate details.
  • Performance specifications to be met.
  • Types of insulation and permissible temperature rise.
  • Permissible loss and range of efficiency.
  • Procedure for testing of machine parts and machines.
  • Auxiliary equipments to be provided.
  • Cooling methods to be adopted.

In India, the Indian Standards Organisation (ISO) has laid down their specifications (ISI) for various produicts. The standards will be amended time to time, in order to include the latest developments in technology. Recently they have released revised standards ISO 9002,  to comply with international standards electrical machine design.

Electrical Machine Design

The name plate of a rotating machine has to bear the following details as per ISI specifications.

  • kW or kVS rating of machine
  • Rated working voltage
  • Operating speed
  • Full load current
  • Class of insulation
  • Frame size
  • Manufacturers name
  • Serial number of the product

Some of the Indian standard specifications numbers along with year of issue for elect4rical machines are listed here.

IS 325 – 1966  :   Specifications for three phase induction motor.

IS 1231-1974  :   Specification for foot mounted induction motor.

IS 4029-1967 ;  Guide for testing three phase induction motor.

IS 996-1979   :  Specification for single phase ac and universal motor.

IS 1885-1993 :  Specifications for electric and magnetic circuits.

IS 9499-1980  :  Conventions concerning electric and magnetic circuits.

IS 7538-1996  :   Specifications for three phase induction motor for centrifugal pumps and Agricultural applications.

IS 12615-1986:  Specifications for energy efficient induction motor.

Is 9320-1979 :    Guide for testing dc machines.

IS 4722-1992 :    Specification for rotating electrical machines.

IS 12802-198 :     Temperature rise measurement of rotating electrical machines.

IS 4889-1968  :    Method of determination of efficiency of rotating electrical machines.

IS 13555-1993:   Guide for selection and application of three phase induction motor for different types of driven equipment.

IS 7132-1973 :   Guide for testing synchronous machines.

IS 5422-1996  :   Turbine type generators.

IS 7572-1974  :   Guide for testing single-phase ac and universal motors.

IS 8789-1996  :  Values of performance characteristics for three phase induction motors.

IS 12066-1986 :  Three phase induction motors for machine tools.

IS 1180-1989  :  Specifications for outdoor 3-phase distribution transformer upto 100 kVA (Sealed And Non-sealed type)

IS 2026-1994  :  Specifications of power transformers.

IS 11171-1985:  Dry type power transformers

IS 5142-1969  :  Continuously variable voltage auto transformers.

IS 10028-1985:  Code of practice for selection, installation and maintenance of transformers

IS 10561-1983 :   Applications guide for power transformers.

IS 13956-1994 :  Testing transformers.

IS 9678-1980 :  Methods of measuring temperature rise of electrical equipment.

IS 12063-1987  :  Classification of degree of protections provided by enclosures of electrical Equipment.

IS 3855-1966  :  Standard dimensions of rectangular enameled copper conductor.

IS 449-1962  :   Standard dimensions of enameled round copper conductor (oleo resinous enamel).

IS 1595-1960  :  Standard dimensions of enameled round copper conductor (synthetic enamel).

IS 1897-1962  :  Standard dimensions of bare copper strip.

IS 1666-1961  :  Standard dimension of paper covered rectangular copper conductor for transformer Windings.

IS IS 2068-1962  : Standard dimensions of cotton covered rectangular copper conductor for Transformer windings.

IS 3454-1996  :  Standard dimensions of paper covered round conductors used for transformer Windings.

IS 450-1964:  Standard dimensions of cotton covered round conductors used for transformer Windings.

GENERAL DESIGN PROCEDURE – Electrical Machine Design

     In general any electrical machine has two windings.  The transformer has primary and secondary winding.  The dc machine and synchronous machine has armature and field winding.  The induction machine has stator and rotor winding.  The basic principle of operation of all electrical machine is governed by Faraday’s law of induction.  Also in every electrical machine the energy is transferred through the magnetic field.   Hence a general design procedure can be developed for the design of electrical machines.

The general design procedure is to relate the main dimensions of the machine to its rated power output.  An electrical machine is designed to deliver a certain amount of power called rated power.  The rated power output of a machine is defined as the maximum power that can be delivered by the machine safely.  In dc machine the power rating is expressed in kW and in ac machine in kVA.  In case of motor the output power is expressed in HP.

In electrical machine the core and winding of the machine are together called active part.  (Because the energy conversion takes place only in the active part of the machine).  A general output equation can be developed for dc machine which relates the power output to volume of active part (D2L),speed magnetic and electric loading.  Similarly a general  output equation can be developed for ac machine which relates kVA rating to volume of active part(D2L),speed, magnetic and electric loading.

MAIN DIMENSIONS OF ROTATING MACHINES

In rotating machine the active part is cylindrical in shape.  The volume of the cylinder is given by the product of area of cross section and length.  If D is the diameter and L is the length of cylinder, then the  volume is given by πD2L/4.  Therefore D and L are specified as main dimensions.

In case of dc machine, D represent the diameter of armature and L represent the length of armature.  In case of ac machine,  D represent the inner diameter of stator and L represent the length of stator core.  The fig1.1 shows the main dimensions of rotating machines.

Here,  Dr  =  Diameter of rotor.

Lg =  Length of air-gap.