Electrical and Electronics Engineering department Electrical Machines-1

Electrical machines - 1 contains of Basic machines

1. Electrical Machines - 1 1

2. UNIT I ELECTROMECHANICAL ENERGY CONVERSION Fundamentals of Magnetic circuits- Statically and dynamically induced EMF - Principle of electromechanical energy conversion forces and torque in magnetic field systems- energy balance in magnetic circuits- magnetic force- co-energy in singly excited and multi excited magnetic field system mmf of distributed windings – Winding Inductances-, magnetic fields in rotating machines- magnetic saturation and leakage fluxes. Introduction to Indian Standard Specifications (ISS) - Role and significance in testing. 2

3. Electromechanical Energy Conversion A device which converts electrical energy into mechanical energy or mechanical energy into electrical energy is known as electromechanical energy conversion device. 3

4. Electromechanical Energy Conversion The electromechanical energy conversion takes place through the medium of a magnetic field. The magnetic field is used as a coupling medium between electrical and mechanical systems . It is because the energy storing capacity of a magnetic field is very high . Therefore , an electromechanical energy converter has three main parts. Mechanical system Coupling medium Electrical system 4

5. Electromechanical Energy Conversion When the electromechanical energy conversion takes place from electrical energy to mechanical energy, the converter is known as  Motor . Whereas , when the conversion takes place from mechanical energy to electrical energy, the device is known as  Generator . In the electrical machines, conversion of energy from electrical to mechanical or from mechanical to electrical results following two electromagnetic phenomena. When a conductor moves in a magnetic field, an EMF is induced in the conductor. When a current carrying conductor is placed in a magnetic field, a mechanical force acts on the conductor. 5

6. Electromechanical Energy Conversion These two effects occur simultaneously whenever energy conversion takes place from electrical to mechanical or vice-versa. In motoring action , electric current flows through the conductors placed in a magnetic field due to which a force is produced on each conductor. The conductors are placed on a rotor, which is free to move. Therefore, an electromagnetic torque is produced on the rotor so that the rotor starts rotating at some speed. The torque produced on the rotor is transferred to a shaft of the rotor and hence it can drive a mechanical load. Since the conductors are rotating in a magnetic field, thus an EMF is also induced in each conductor. 6

7. Electromechanical Energy Conversion 7

8. Electromechanical Energy Conversion In  generating action , in this case the rotor is driven by a prime mover. An EMF is induced in the rotor conductors due to which a current will flow and deliver electric power to the load. In addition to this, the current flowing through the conductors will interact with the magnetic field to produce a reaction torque, which will tend to oppose the torque developed by the prime mover. 8

9. Electromechanical Energy Conversion 9

10. Principle of Conservation of Energy The principle of conservation of energy states that “the energy can neither be create not destroyed. It can only be converted from one form to another”. In an electromechanical energy conversion device, the total input energy is equal to the sum of following three components − Energy dissipated, Energy stored, and Useful output energy. 10

11. Principle of Conservation of Energy Hence, the principle of electromechanical energy conversion is based on the following two equations − The energy balance equation or energy transfer equation for motoring action can be written as − 11

12. Principle of Conservation of Energy 12

13. Induced EMF When a magnetic flux linking a conductor or coil changes, an electromotive force (EMF) is induced in the conductor or coil, is known as  induced EMF . Depending upon the way of bringing the change in magnetic flux, the induced EMF is of two types − Statically Induced EMF Dynamically Induced EMF 13

14. Statically Induced EMF When the conductor is stationary and the magnetic field is changing, the induced EMF in such a way is known as  statically induced EMF  (as in a transformer). It is so called because the EMF is induced in a conductor which is stationary. The statically induced EMF can also be classified into two categories − Statically Induced EMF Mutually Induced EMF 14

15. Self-Induced EMF When an EMF is induced in the coil due to the change of its own magnetic flux linked with it is known as  self-induced EMF . Mathematically, self-induced EMF is given by, 15

16. Mutually Induced EMF When an EMF is induced in a coil due to changing magnetic flux of neighbouring coil is known as  mutually induced EMF . 16

17. Dynamically Induced EMF When the conductor is moved in a stationary magnetic field so that the magnetic flux linking with it changes in magnitude, as the conductor is subjected to a changing magnetic, therefore an EMF will be induced in it . The EMF induced in this way is known as  dynamically induced EMF  (as in a DC or AC generator). It is so called because EMF is induced in a conductor which is moving (dynamic ). 17

18. Excitation System In electrical machines and instruments, an  excitation system  is the one which receives electrical energy to produce the working magnetic flux. The excitation system is the crucial component every electromechanical energy conversion device such as electric motor. Depending upon the number of input supplies, any electromechanical energy conversion system can be grouped into the following two categories − Singly Excited System Doubly Excited System 18

19. Singly Excited System The electromechanical energy conversion system in which only one electrical input is given to the system to produce the working magnetic flux is known as a  singly excited system . It is so called because there is only one excitation provided in the system. 19

20. Singly Excited System The torque produced in the singly excited system is termed as  reluctance torque or saliency torque . 20

21. Doubly Excited System A   doubly excited system  is the one in which two separate sources of excitation are provided to produce the magnetic field. One example of a doubly excited system is the synchronous motor, where one electrical excitation is provided to stator and the other to the rotor. Here, the doubly excited system consists of a stator wound with a coil having a resistance of R1 and a rotor wound with a coil of resistance R2. Both the coils are excited by independent voltage sources. 21

22. Doubly Excited System 22

23. Electrical Safety - Indian Standards In the field of electrical engineering, engineers and other professionals get exposed to electricity indirectly during generation, transportation, installation and usage. Such conditions might cause hazards if accurate safety measures are not taken. To promote the safety and the right usage of equipment, there are certain rules and regulations formulated by the Bureau of Indian Standards (BIS). BIS follows the following five principles − Safety Ease of use and adaptability Simple technology Value for money products Energy efficiency and environment 23

24. Electrical Safety - Indian Standards BIS has published the following code of practice for public safety standards in order to promote the right to information, transparency and accountability in a proper manner to the public. Code of Practice for Electrical Wiring Installation IS  − 732 (1989) Section  − Electrical Installation Application  − Design of installation, selection and erection of equipment, inspection and testing of wiring system. 24

25. Electrical Safety - Indian Standards Code of practice for Earthing IS  − 3043 (1987) Section  − Electrical Installation Application  − Design, installation and calculation of Earthing system Lightning arrester for Alternating Current System IS  − 3070 (1993) Section  − Electro technical: Surge Arresters Application  − Identification, ratings, classification and testing procedure of Arrester 25