Unit 5

Computer Aided Design

N.Pandiarajan

Associate Professor/EEE

SSNCE

Advantages ofComputer Aided Design

� The modern practice is computer aided design of electrical apparatus.

� This method has the advantages of flexibility, speed and accuracy.

� Graphical display of the machine as well as its view from different angles is possible.

� Modification of design can be done very quickly.

� Vast quantities of data can be handled and large number of logical steps can be executed accurately within short time.

Computer Aided Design

� Conventional design involves cumbersome hand calculations.

� Computer Aided Design involves computer program.

� Data is given to computer and computer gives all values of design solution.

� If data is modified, the corresponding design solution is obtained in a fraction of a second.

� Thus design work is made easy, fast and flexible.

Objectives of Machine Design

1. Lower Cost

2. Smaller Size

3. Wider temperature limits of operatability

4. Lower Weight

with judicious use of materials at the

disposal.

Computers in design

1. Once a program has been developed and fully

implemented on a computer, all future designs are

nothing but routine computations largely

independent of designer's skill.

2. Highly trained designers are thus relieved of routine

tasks and may be utilised for developmental work.

3. A computer can only work on exact information.

Though it can reduce empiricism and handle non-linearities, it has neither the 'feel' nor the intuition of

a designer. Feeding exact information to the computer

means formulation of mathematical relationships

between various functional variables, keeping in mind

their relative importance in the design.

4. Thus, for successful use of computer in design it is

essential that the design principles are thoroughly

understood. Again, computer can be effectively used

as a means of this understanding, so that the

mathematical relationships between the variables

could be more correct.

5. The designer breaks down the complete design

process into several parts, assigns routine tasks to

the computer to obtain a series of intermediate

results. He uses his skill and judgment for the

decisions which are dependent upon experience and

human ability to detect trends, based on which

he feeds further information to the computer to arrive

at a final solution.

Thus it combines skill and judgment of the

designer with the fast computing power of the

computer.

6. Computer's ability to furnish optimum design by sorting

through a large number of different combinations. is a

good feature However, optimisation through

total computerisation is often difficult specially with

electrical machines due to a large number of available

frame sizes, magnetic materials, and wire gauges.

Such approach involves so much logic that

formulation is difficult. and often requires storing of

relatively large amount of data. Moreover, it is very

difficult to let a number of designers to agree with one

formulation of logic as the best one.

On the contrary, optimisation through continuous

interaction between the designer and the computer

has been more effective specially for electrical

machines, though it is costlier in terms of operation

time.

Major Divisions of Design

1. Design of electric circuit

2. Mechanical design

3. Design of magnetic circuit

4. Thermal design

with performance analysis.For computer-aided design, the above problems are often treated

separately, even broken down into simple elements and considered as individual problem.

The results are then combined.

1. Given specification consists of performance

requirements as defined by customer's need and

Indian Standard Specifications.

2. Based on given specification, the designer chooses

materials-magnetic, conducting and insulating, for

electrical design and other materials for frame,

bearing, etc. For this, the designer must be

conversant with the characteristics, availability and

cost of materials needed as to feed the computer with

relevant information.

3. Assumption of basic design parameters such as, flux-

density (Specific magnetic loading), ampere-conductor

per meter (Specific electric loading), space factor,

stacking factor, etc. is then made and fed to the

computer.

4. Design process consist of analysis calculations to

determine the various dimensions of magnetic and

electric circuits, thermal and mechanical designs.

5. Predetermination of performance of the machine is then

made based on the calculated dimensions. This means

calculation of machine parameters from mechanical

dimensions obtained through the design process

followed by calculation of performance under no-load and

load conditions, determination of temperature-rise, cost etc.

6. The next procedure is the comparison between the

calculated performance and customer's requirement. If not

satisfactory (which is generally the case at the first

instance), the designer has to modify the basic

assumptions so as to bring the final design closer tt the

objective. Such modification is not generally a simple task

for there are many input parameters can bechanged an

and needs skill and intuition of a designer.

Numerical Methods

� The deficiency in analytical methods has

been largely eliminated in numerical

methods such as the

1. Finite Difference Method

2. Finite Element Method

Finite Difference Model

� The finite difference model of a problem

gives point wise approximation to the

governing equation.

� In the finite difference model, a solution

region is envisaged as an array of grid

points.

Finite Element Model

� A finite element model gives piecewise approximation to the governing- equation.

� In the finite element model, the solution region can be analytically modeled or approximated by replacing it with an assemblage of discrete elements.

� The advantage is that these elements can be used to represent exceedingly complex shapes since they can be put together in a variety of ways.

� Thus, for problems with irregular solution region or unusual specification of boundary conditions, finite element method is

particularly well-suited requiring fewer nodes.

Finite Element Method

1. Defining the elements

2. Selection of Interpolation function

3. Matrix Equation for elements

4. Assembly of elements

5. Numerical solutions of equations

formed in above steps