Principles of AC Machines.It include Transformer and Three phase Induction Motor

Principles Of AC machines

1. 1 Sanjivani College of Engineering, Kopargaon Department of MechatronicsEngineering (An Autonomous Institute) Affiliated to Savitribai Phule Pune University Accredited ‘A’ Grade by NAAC ________________________________________________________________________________________ Subject: Basic Electrical and Electronics Engineering (BEEE-BE104) UNIT-3: PRINCIPLES OF ALTERNATING CURRENT (AC) MACHINERY Topic: Introduction

2. Unit-III PRINCIPLES OF ALTERNATING CURRENT (AC) MACHINERY Hr COs Introduction to inductance, capacitance, and impedance . Introduction to single phase and poly phase supply . Principle of operation and construction of single phase transformers (core and shell type only). Losses, efficiency and voltage regulation, Working of auto-transformer (Only equation based numerical). Construction and working principle of three phase induction motors , types: torque slip characteristics; power stages and industrial applications (Only equation based numerical). 06 Hrs. CO1 CO3 Contents of Unit III Prof. Pritish M. Vibhute 2

3. Fundamental entities Amplitude Frequency Phase

4. Inductors & capacitors - Energy storage devices - Passive components - Electrostatic & electromagnetic

5. Introduction to single phase and poly phase supply Types of ALTERNATING Currents (AC) Single phase:- Domestic applications- low power appliances Only two wires [phase (230V-50Hz )and a neutral] Poly Phase:- In industrial applications- high power appliances Four wires[three phase wires(RYB- each 120degree phase difference) and one neutral] 1.732 (the square root of 3)times more power with the same current than single phase Reduces sizes of electrical construction eg wire size.

6. Principle of Working of transformer :- Mutual Inductance

7. Single phase transformers It is a static device which either steps-up (increase) or Steps-down (decrease) voltage without changing frequency Magnetic and electric circuits Laminated Core Limbs Yoke

8. Step up & Step Down transformer 11

9. Shell Type :- Efficient (2 cores) Difficult to repair as sandwiched winding Poor natural cooling Used in LV Power Supply Core type :- Easy to repair as separate windings Used in high voltage supply like DP

10. Losses in transformer and types

11. Transformer core Lamination

12. Transformer efficiency Efficiency do not have any unit as it’s a ratio Its factor(<1) in above formula If multiplied by 100, then unit is %

13. Regulation of transformer What is constant parameter stands for? What is regulation? Definition:- Change in terminal voltage from no load to full load expressed as percentage fraction of no load voltage is known as regulation of transformer. Note :- Total power = Active power + reactive power

14. Auto transformer An autotransformer is a electrical transformer with only one winding (one winding transformer) Where primary & secondary shares same common single winding Also known as Dimmerstat Dimmer Variac VaribaleAC supply Applications

15. Types & advantages of Autotransformer Advantages:- Lower losses hence higher efficiency Low cost & size as less copper used Can have variable output Better voltage regulation BUT No isolation

16. It is desired to have 4.13 mwb maximum flux in the core of a transformer operating at 110v and 50Hz. Determine Number of turns in the primary

17. If primary voltage is 230V, 50Hz, N1=500 and Bm =0.4144T then the crossectional area of the core will be

18. A step down transformer operates at 50 Hz when primary voltage is 230v with 500turns the maximum flux will be

19. A 3300/200V, 100KVA transformer has N2=80. find rated primary and secondary currents I1 andI2

20. A 3300/200V, 100KVA transformer has N2=80. find rated primary and secondary currents I1 and I2

21. 3 Phase Induction motor 80% of motors world wide are IM 50% power consumption No starter required Cheapest, efficient, reliable Robust motor and no maintenance Designed by Nikola Tesla (inventor of AC) Constant speed motor and good speed regulation High starting torque and overload capacity Synchronous motor

22. Working principle

23. Construction of 3 φ IM

24. Stator Parts Stator Frame Stator Core Stator Windings:- The 3-phase winding are arranged in such a manner that they produce one Rotating Magnetic Field(RMF) Delta or star connections

25. Rotor design:- Type 1. Squirrel cage Two types Squirrel cage Phase wound Rotor Parts:- Laminated Core Conductor/windings Parallel skewed End rings Very Common type of motor. 95% IM are of this type

26. 2. Sleep ring/ phase wound/ wound rotor Uses of external resistors :- To increase starting torque To decrease start current To control speed This motor has limited application. Only 5% IM in the world are of this type

27. Working of 3 Phase IM Overlapping windings of stator creates rotating magnetic field at synchronous speed Ns(speed at which rotating magnetic field rotates) Which in turn induces emf in rotor bars as bars are initially stationary [Faraday’s law] (that’s why the name Induction motor) Due to end rings a closed path found to the induced current, which produced magnetic flux. Hence as per Fleming's rule rotor starts rotating at speed Nr. As Nr tends to Ns, Nr < Ns and Ns α f(freq of I/P)

28. Concept of Synchronous speed and slip Speed increases as Potential Difference is higher and reduced with Potential Difference Induction motor can not run at Ns as at Ns relative moment becomes zero and hence no emf will be induced in rotor. Hence rotor decelerate, as rotor speed reduces relative moment reestablished and emf induced again hence speed again increase. Hence IM can not run at synchronous speed . Slip at rest=1 1 to 5% max

29. Torque-slip characteristics Three region Low slip(S~=0) T α S Medium slip T α 1/S High slip(s=1) T = 0

30. Torque-slip characteristics

31. 3 Phase IM characteristic

32. Power Stages of IM P 2 :Pc:Pm 1:s : (1-s) (iron & copper losses)

33. Efficiency of Induction motor Exactly same as that of transformer Maximum when losses are at its minimum

34. Applications of IM Squirrel cage IM has Constant speed and lower initial torque Hence applications are same as that of DC shunt motor Slip ring IM has Variable and controllable speed and Higher initial torque Hence Applications are same as that of DC series motor

35. Single phase induction motor Advantages:- simple in construction, cheap in cost, reliable and easy to repair and maintain Applications:- vacuum cleaners, fans, washing machines, centrifugal pumps, blowers, washing machines. Note:- Needs Auxiliary winding Starter is most

36. Numerical treatment Calculate the percentage slip for an 6 pole 50 Hz, 3 phase induction motor of rotates at 960 RPM. The power input to a 3 phase induction motor is 60kW. The net stator losses are 1kW. Find the total mechanical power developed and rotor copper loss per phase if motor is running with a slip of 3%.

37. Torque slip characteristics