unit -Ipptx Introductionof Electrical Machine Apparatus

Electrical machine design

1. EE-2355 Design of Electrical Machines – UNIT -I 11/27/2014 1 St.Joseph's College of Engineering

2. Major consideration in design Design of machines: Design is defined as a creative physical realization of theoretical concepts. Engineering design is application of science, technology and invention to produce machines to perform specified tasks with optimum economy and efficiency. The problem of design and manufacture of electric machinery is to build, as economically as possible , a machine which fulfils a certain set of specifications and guarantees. 11/27/2014 2 St.Joseph's College of Engineering

3. Contd.., The major design considerations to evolve a good design are as follows: Design Base : Matching the existing experience with R & D; bringing in the latest material technology; limitations in design; convenience in production line and transportation; working safety and reliability; maintenance and repair; environmental conditions ; cost economy; optimization. Specification: Meeting with the customer’s requirements, guarantees; satisfy the national and international standards. Design Transfer: Transfer of design to factory foreman i.e. drawings, processes, instructions, job-flow, meeting the delivery schedule. Information updating: Technical journals, R and D papers and reports, interaction in meetings and seminars. 11/27/2014 3 St.Joseph's College of Engineering

4. Contd., It is impossible to design a machine which is cheap and is also durable at the same time. Long life span – high quality materials and advanced manufacturing techniques → high cost Good design – machine has reasonable operating life (20-30 years) and reasonable initial cost. 11/27/2014 4 St.Joseph's College of Engineering

5. Areas of design Main areas of design: Knowing the characteristics and specifications that a machine has to satisfy, the main areas of design are: The magnetic circuit; core, yoke, airgap etc., The electric circuit; the windings The insulation (Dielectric circuit) Heating and cooling circuit (Thermal circuit) The mechanical construction The designer work lies in suitably allocating the space to frame, core, airgap , winding, insulation and cooling medium in the machine. 11/27/2014 5 St.Joseph's College of Engineering

6. Limitations Limitations: Saturation of magnetic parts : Increased core losses and excitation at higher flux density. Temperature rise : increased temperature rise under higher output deteriorates the insulation and affects the life of the machine. Insulation: Breakdown due to high voltage gradient; temperature rise limitations, mechanical damage. Mechanical strength: specially in turbo-machine due to large size and high speed. Efficiency: If high efficiency is the aim, the machine becomes costly, for lower efficiency higher running cost and temperature rise with associated problems. Customer’s specifications: Impose limitations to identify criterion for best design. Commutation: In d.c . machine output is limited because of commutation problem Power Factor: P.F imposes a limitation specially in case of 3-ph induction motor. Because of p.f . limitation higher flux density cannot be taken in airgap which would have otherwise resulted in more compact size motor. 11/27/2014 6 St.Joseph's College of Engineering

7. The design of electric machines- science and art. The process of design of a single machine may be divided into three major design problems, such as: Electromagnetic design Mechanical design Thermal design 11/27/2014 St.Joseph's College of Engineering 7

8. Various Principles of Machine Basic principles: All electric machines are electromagnetic devices. Among them the rotating machines are electromechanical energy conversion devices but transformer is a static machine where energy conversion does not take place, only the level of voltage changes. All electrical machines are based upon three principles: 1)Induction 2)Interaction 3) Alignment. 11/27/2014 St.Joseph's College of Engineering 8

9. Contd., Faraday’s Law of electromagnetic Induction: Emf induced in a electric circuit is equal to the rate of change of flux linkages Flux linkage Ψ = N Φ , e= -d Ψ / dt The change in flux linkages can be caused in three ways: The coil is stationary w.r.t flux and the flux varies in magnitude w.r.t time The flux is constant w.r.t time and is stationary and the coil moves through it. Both the changes mentioned above occur together i.e., the coil moves through a time varying field. In method ( i ) transformer or pulsational emf is produced . No energy conversion only energy transference – Transformer In (ii)method motional emf due to motion of conductor- electromechanical energy conversion - D.C.Machines , induction and synchronous machines. In method (iii) Tranformer and motional emf takes place. Both transfer and conversion of energy takes place – Commutator motors. 11/27/2014 St.Joseph's College of Engineering 9

10. RATING OF MOTORS The rating of the motor selected should be in accordance with the mech. work required by the driving machine. Motor selected is must not have excess temperature rise when it is fully loaded(damage the insulation). It should drive the load satisfactorily both under steady state as well as transient operation. Motor must be capable of withstanding short time overloads and must have enough starting torque to accelerate the motor to desired speed in given time.

11. Power loss and heating of motors Power losses are copper loss, core loss, mechanical losses. Copper loss – armature and field copper loss Core loss or iron loss – eddy current loss and hysteresis loss. Mech loss – windage and friction loss This power loss cause localised heating and responsible for temperature rise. Losses occurring in a machine are, W = W c + X 2 W v Constant loss – independent of current Variable loss – square of the load current X – load on the motor expressed as a function of rated load.

12. Heating and cooling curve The size and rating of a motor to be used as a drive element in a particular appln depends upon Heating effects in the motors Loading conditions and classes of duty Load inertia or inertias Environment conditions

13. Heating effects When a motor operates, heat is produced due to losses inside the machine and its temp rises. As the temperature increases beyond(exceeds) ambient value, a portion of heat produced, flows out to the surrounding medium. The amount of outflow of heat is a function of temp rise of motor above the ambient value. As motor temp rises, the heat outflow increases and equilibrium ultimately sets in when the heat generated becomes equal to heat dissipated to surrounding medium. Steady state temperature depends on power loss, which in turn depends on the output power of the machine. Steady state temperature is not same at various parts of the machine.(winding – high – loss density is high, dissipation is slow.

14. Too complex to predict the accuracy of heat flow and temperature distribution. Direction of heat flow does not remain same at all load conditions. At no-load and lightly loaded condition ,heat flows from iron parts to windings due to temp gradient. When load increases, heat generation in the winding is greater than iron core. Heating and cooling curves

15. simplifications Homogeneous body – uniform temp gradient. At all the points at which heat is generated have the same temp. At all the points at which heat is dissipated to cooling medium have the same temp. Heat dissipation is directly proportional to the difference of temp of the body and surrounding medium. No heat is radiated. Rate of dissipation of heat is constant at all temp.

16. CLASSES OF DUTY The rating of motor selected from the viewpoint of heating depends on the load conditions to which it is subjected. There are eight types of classes of duty. REF – S.K PILLAI

17. CONTINUOUS DUTY Operation at constant load of sufficient duration of thermal equilibrium to be reached. E.g.. Fan, conveyor etc..

18. SHORT TIME DUTY

19. INTERMITTENT DUTY

20. INTERMITTENT PERODIC DUTY WITH STARTING

21. INTERMITTENT PERODIC DUTY WITH STARTING AND BRAKING

22. CONTINUOUS DUTY WITH INTERMITTENT PERODIC LOADING

23. CONTINUOUS DUTY WITH STARTING AND BRAKING

24. CONTINUOUS DUTY WITH PERODIC SPEED CHANGES