1. 1 Teaching Innovation - Entrepreneurial - Global The Centre for Technology enabled Teaching & Learning, N Y S S, India DTEL DTEL (Department for Technology Enhanced Learning)

2. DEPARTMENT OF ELECTRICAL ENGINEERING IV-SEMESTER POWER DEVICES AND MACHINES 2 CHAPTER NO. - 5 THREE PHASE TRANSFORMERS AND THREE PHASE INDUCTION MOTOR

3. CHAPTER 5:-SYLLABUS FOR THREE PHASE TRANSFORMERDTEL. Construction 1 Different Connections (star-star, Delta-Delta, Star-Delta & Delta-star ) 2 Open Delta Connection, Scott connection 3 Parallel Operation 4 3

4. CHAPTER 5:-SYLLABUS FOR THREE PHASE INDUCTION MOTORDTEL. Principle of Operation 1 Necessity of starters 2 Direct Online Starter 3 Autotransformer Starter 4 4 Star-Delta Starter 5 Speed control techniques of three phase induction motor 6

5. CHAPTER-5 SPECIFIC OBJECTIVE / COURSE OUTCOMEDTEL To understand the basic concepts of three phase transformer, its construction, types, connections & applications. 1 To understand the basic concepts of three phase induction,motor, its construction, types & applications. 2 5 The student will be able to:

6. LECTURE 1:- ConstructionDTEL 6 6 PRELIMINARY CONSIDERATIONS  A Transformer is a device that converts one AC voltage to another AC voltage at the same frequency.  It consists of one or more coil(s) of wire wrapped around a common ferromagnetic core.  These coils are usually not connected electrically together. However, they are connected through the common magnetic flux confined to the core.

7. DTEL 7 LECTURE 1:- Construction Assuming that the transformer has at least two windings, one of them (primary) is connected to a source of AC power; the other (secondary) is connected to the loads. PRELIMINARY CONSIDERATIONS

8. DTEL 8 Power transformers Core form Shell form Windings are wrapped around two sides of a laminated square core. Windings are wrapped around the center leg of a laminated core. LECTURE 1:- Construction & Types

9. DTEL 9 Lamination types Laminated steel cores Toroidal steel cores Efficiency of transformers with toroidal cores is usually higher. Usually, windings are wrapped on top of each other to decrease flux leakage and, therefore, increase efficiency.

10. DTEL 10 Power transformers used in power distribution systems  A power transformer connected to the output of a generator and used to step its voltage up to the transmission level (110 kV and higher) is called a Unit Transformer.  A transformer used at a substation to step the voltage from the transmission level down to the distribution level (2.3 … 34.5 kV) is called a Substation Transformer.  A transformer converting the distribution voltage down to the final level (110 V, 220 V, etc.) is called a Distribution Transformer. LECTURE 1:- Construction & Types

11. DTEL 11 LECTURE 1 THANK YOU

12. DTEL 12 LECTURE 2:- C onnections THREE PHASE TRANSFORMER CONNECTION Four possible connections for a 3-phase transformer bank are: 1.Y-Y 2.Y-  3.  -  4.  -Y

13. LECTURE 2:- C onnectionsDTEL 13 1. Y-Y connection: The primary voltage on each phase of the transformer is The secondary phase voltage is The overall voltage ratio is

14. DTEL 14 LECTURE 2:- C onnections 2. Y-  connection: The primary voltage on each phase of the transformer is The secondary phase voltage is The overall voltage ratio is

15. DTEL 15 LECTURE 2:- C onnections 3.  -Y connection: The primary voltage on each phase of the transformer is The secondary phase voltage is The overall voltage ratio is

16. DTEL 16 LECTURE 2:- C onnections 4.  -  connection: The primary voltage on each phase of the transformer is The secondary phase voltage is The overall voltage ratio is

17. DTEL 17 LECTURE 2 THANK YOU

18. DTEL 18 LECTURE 3:- C onnections (CONTINUED) SCOTT CONNECTION  This connection uses two transformers with different rating.  One of the transformers having 50% taping is called main transformer and other one having 86.6% tapping is called teaser transformer  One end of the primary winding of the teaser transformer is connected to the centre tapping provided on the primary winding of the main transformer.  The two ends of the primary winding of the main transformer and 86.6 tapping point on the teaser transformer is connected to a balanced three phase supply.  The voltage per turn is same both in primary of both main and teaser transformer.  With the equal number of turns on secondary's of both the transformers, the secondary voltage will be equal in magnitude which results in symmetrical 2-phase system.

19. DTEL 19 SCOTT CONNECTION LECTURE 3:- C onnections (CONTINUED)

20. DTEL 20 Open Delta Connection In connection of three single phase transformers if one of the transformers is unable to operate then the supply to the load can be continued with the remaining tow transformers at the cost of reduced efficiency. The connection that obtained is called V-V connection or open delta connection. Consider the Fig.1 in which 3 phase supply is connected to the primaries. At the secondary side three equal three phase voltages will be available on no load. LECTURE 3:- C onnections (CONTINUED)

21. DTEL 21 The voltages are shown on phasor diagram. The connection is used when the three phase load is very small to warrant the installation of full three phase transformer. If one of the transformers fails in ∆ - ∆ bank and if it is required to continue the supply even though at reduced capacity until the transformer which is removed from the bank is repaired or a new one is installed then this type of connection is most suitable. When it is anticipated that in future the load increase, then it requires closing of open delta. In such cases open delta connection is preferred. Key point : It can be noted here that the removal of one of the transformers will not give the total load carried by V - V bank as tow third of the capacity of ∆ - ∆ bank. The load that can be carried by V - V bank is only 57.7% of it. it can be proved as follows. LECTURE 3:- C onnections (CONTINUED)

22. DTEL 22 LECTURE 3:- C onnections (CONTINUED)

23. DTEL 23 It can be seen from the Fig. 2(a) ∆ - ∆ capacity = √3 V L I L = √3 V L (√3 I ph ) ∆ - ∆ capacity = 3 V L I ph............(i) It can also be noted from the Fig. 2(b) that the secondary line current I L is equal to the phase current I ph. V- V capacity = √3 V L I L = √3 V L I ph...............(ii) Dividing equation (ii) by equation (i) Thus the three phase load that can be carried without exceeding the ratings of the transformers is 57.5 percent of the original load. Hence it is not 66.7 % which was expected otherwise. The reduction in the rating can be calculated as {(66.67 - 57.735)/(57.735)}x 100 = 15.476 LECTURE 3:- C onnections (CONTINUED)

24. DTEL 24 Key point : This overload can be carried temporarily if provision is made to reduce the load otherwise overheating and breakdown of the remaining tow transformers would take place. Suppose that we consider three transformers connected in ∆ - ∆ fashion and supplying their rated load. Now one transformer is removed then each of the remaining tow transformers will be overloaded. The overload on each transformer will be given as, LECTURE 3:- C onnections (CONTINUED)

25. DTEL 25 LECTURE 3 THANK YOU

26. LECTURE 4 : PARALLEL OPERATION OF TRANSFORMERSDTEL 26

27. DTEL 27 By parallel operation we mean two or more transformers are connected to the same supply bars on the primary side and to a common bus bar/load on the secondary side. Such requirement is frequently encountered in practice. The reasons that necessitate parallel operation are as follows:  Non-availability of a single large transformer to meet the total load requirement.  The power demand might have increased over a time necessitating augmentation of the capacity. More transformers connected in parallel will then be pressed into service.  To ensure improved reliability. Even if one of the transformers gets into fault or is taken out for maintenance/repair the load can continued to be serviced.  To reduce the spare capacity. LECTURE 4 : PARALLEL OPERATION OF TRANSFORMERS

28. DTEL 28 Conditions for Parallel operation of transformers  The voltage ratio must be the same.  The per unit impedance of each machine on its own base must be the same.  The polarity must be the same, so that there is no circulating current between the transformers.  The phase sequence must be the same and no phase difference must exist between the voltages of the two transformers. LECTURE 4 : PARALLEL OPERATION OF TRANSFORMERS

29. DTEL 29 LECTURE 4 THANK YOU

30. DTEL 30 LECTURE 5:- THREE PHASE INDUCTION MOTOR  3-phase induction motors are simple, rugged, low- cost, and easy to maintain.  They run at essentially constant speed from zero-to-full load.  Therefore, they are the motors most frequently encountered in industry. Induction Motor

31. LECTURE 5:- INDUCTION MOTOR COMPONENTSDTEL 31 A 3-phase induction motor has two main parts:  A stator – consisting of a steel frame that supports a hollow, cylindrical core of stacked laminations. Slots on the internal circumference of the stator house the stator winding.  A rotor – also composed of punched laminations, with rotor slots for the rotor winding.

32. LECTURE 5:- INDUCTION MOTOR COMPONENTSDTEL 32  Squirrel cage rotor consists of copper bars, slightly longer than the rotor, which are pushed into the slots.  The ends are welded to copper end rings, so that all the bars are short circuited.  In small motors, the bars and end-rings are die- cast in aluminium to form an integral block Rotor

33. LECTURE 5:- INDUCTION MOTOR COMPONENTSDTEL 33  A wound rotor has a 3-phase winding, similar to the stator winding.  The rotor winding terminals are connected to three slip rings which turn with the rotor. The slip rings/brushes allow external resistors to be connected in series with the winding.  The External resistors are mainly used during start-up – under normal running conditions the windings short- circuited externally.

34. LECTURE 5:- Induction motor Operating PrincipleDTEL 34  Operation of 3-phase induction motors is based upon the application of Faraday’s Law and the Lorentz Force on a conductor.  When current carrying conductor placed in a magnetic field it experiences a force. It is given by Force F = B.I. L B – Magnetic Flux Density I – Current carrying L – length of Conductor

35. LECTURE 5DTEL 35 THANK YOU

36. LECTURE 6 :- STARTER  When induction motor is directly switched on, it takes five to seven times full – load current. This excessive current produce large line – voltage drop, it will affect the operation of other electrical equipment which are connected on the same supply line.  The reduced voltage starting has reduced starting current current. It produces objectionable reduction in starting torque.  There are three types of starters commonly used for Three phase induction Motor a) DOL Starter b) Autotransformer Starter c) star-Delta starter DTEL 36 NECESSITY OF STARTER

37. DTEL 37 LECTURE 6:- STARTER Direct on line Starter (DOL starter) If the supply is of sufficient power capacity and the low- power factor starting current Surges do not cause objectionable voltage drop in the supply line voltage. At that time direct-on-line starter is used.

38. DTEL 38 LECTURE 6:- STARTER Autotransformer Starter The auto-transformer is connected across the stator terminals. with the help of autotransformer a reduced voltage is applied across the stator windings. It reduces starting current. After the motor has gained 70 % to 80 % of its normal speed the auto – transformer is cut-out, thus applying rated voltage across stator windings.

39. DTEL 39 LECTURE 6:- STARTER Autotransformer Starter

40. DTEL 40 LECTURE 6:- STARTER Star-Delta Starter  This method is used in the case of motors which are built to run normally with a delta- connected stator windings.  It consists of a two way switch which connects motor in star for starting and then in delta for normal running.  When star connected,the applied voltage over each motor phase is reduced by factor of 1/√3 and hence the torque developed becomes 1/ 3 rd of that which would have been developed if the motor were directly connected in delta.  The line current reduced to 1/3.Hence during starting period when motor is star connected it takes 1/√3 of starting current and develops 1/ 3 rd of torque it would have develop had it been directly connected in delta

41. DTEL 41 LECTURE 6 THANK YOU

42. DTEL 42 LECTURE 7:- SPEED CONTROL OF THREE-PHASE INDUCTION MOTORS Unlike D.C. Motors, A.C. Induction Motors are not suitable for variable speeds. Their speed control and regulation is comparatively difficult when compared with D.C. Motors. These are some of the methods which are commonly used for the speed control of squirrel cage induction motors: 1. Changing Applied Voltage 2. Changing Applied Frequency 3. Changing Number Of Stator Poles The above three methods are most commonly used for the speed control of squirrel cage Induction motors.

43. DTEL 43 Speed control by changing applied voltage The Induction motor speed can be controlled by changing the applied voltages the stator, Because in induction motor the out put torque is Directly Proportional to the Squire of the voltage and inversely proportional to the resistance of the rotor, while S= Slip. T∞ V 2 /Rr S Thus the motor speed controlled with out changing the supply frequency, for example, if the supply voltage value is decreases to its half, the motor torque is decreases ¼ times; the torque is directly proportional to the speed of the motor. In stator voltage control method, the stator voltage is controlled by a SCRs, these SCRs are connected with three phase supply (With each Phase) in anti parallel conduction, the out put voltage of the SCR is controlled by the firing angle of the SCR, Increasing the firing angle, decreases the out put voltage and this way the speed of the induction motor is decreases. By decreases the firing angle, increasing the out put voltages and the speed of the motor is increases. LECTURE 7:- SPEED CONTROL OF THREE-PHASE INDUCTION MOTORS

44. DTEL 44 Speed control by Frequency Changing or Variable Frequency control method. The frequency method is very important through which the speed of the induction motor is controlled. Ns = 120f/P Form the above formula, it is clear that the speed of the induction motor directional proportional to the frequency. It means that the frequency of the supply voltage changes, the speed of the indication motor also changes, increases the in put frequency of the supply increases the speed of the induction motor also increase and vice versa. In the frequency controlled method, the three phase supply is controlled through a controlled rectifier. The voltage is applied on the forced Commutated Bridge rectifier, which produce a variable frequency for control of the induction motor. LECTURE 7:- SPEED CONTROL OF THREE-PHASE INDUCTION MOTORS

45. DTEL 45 Speed control by Frequency Changing or Variable Frequency control method. In the give circuit an induction motor is shown, with each phase of the motor the SCRs are connected in anti parallel conduction. In this circuit two SCR are conduct at the same time, this way a very large current flow through the Stator, therefore this method is used for small or medium size motors. LECTURE 7:- SPEED CONTROL OF THREE-PHASE INDUCTION MOTORS

46. DTEL 46 THANK YOU LECTURE 7