EE-463-SECTION 1.pptx Special machines slides

Special machines

1. 1 EE 463-SPECIAL ELECTRICAL MACHINES January 2024 D. Kwegyir Department Electrical & Electronic Engineering College of Engineering

2. 2 Section One General Classification of AC Motors and Single-Phase Motor Types

3. 3 Learning Objectives Understand why three-phase motors are self-starting, but single-phase motors are not self-starting. Know the different starting methods employed to make single-phase motors self-starting.

4. 4 Section Outline Introduction – AC Motors Classification of AC Motors Why Three Phase Motors Are Self-Starting Why Single-Phase Motors Are Not Self-Starting Making Single-Phase Motors Self-Starting Types of Single-Phase Motors

5. 5 Section 1.1: Introduction-AC Motors Motors are the commonest and best known of electrical machines. Motors are classified into two groups, depending on whether they are suitable for use on DC or AC systems. More AC motors are in use than DC motors, for two reasons. Firstly, the majority of our supply systems are AC. Secondly, the AC. motor is simpler and cheaper than its DC counterpart.

6. 6

7. 7 Types of Electric Motors

8. 8 Section 1.1: Introduction-AC Motors Where variable speed over a wide range is required such as lift cranes, locomotives, etc, use is made of the DC motor, since its speed can easily be varied .

9. 9 Section 1.2: Classification of AC Motors AC motors may be classified as follows, according to: Type of current Speed requirement Principle of Operation Type of Current Single phase three phase

10. 10 Section 1.2: Classification of AC Motors Speed constant speed Variable/ adjustable speed Principle of operation Synchronous Asynchronous (induction)

11. 11 Section 1.3: Why Three Phase Motors Are Self-Starting Upon connection of the stator (armature) windings to a three-phase system, three separate alternating AC fluxes ensue, and interact. The interaction gives rise to a resultant revolving or rotating flux called synchronous flux in the field windings, and rotates at synchronous speed .

12. 12 Section 1.3: Why Three Phase Motors Are Self-Starting The synchronous flux induces voltage and hence current and hence induced fluxes in the rotor (armature) windings. The inducing fluxes in the field and the induced fluxes in the armature interact/react to produce an electromagnetic torque , which causes the rotor to rotate, though at a speed less than the synchronous speed.

13. 13 Section 1.4: Single-Phase Motors The number of machines operating from single-phase supplies is greater than all other types taken in total. They are widely used in homes, offices, factories, workshops and business establishments. For the most part, however, they are only used in the smaller sizes, less than 5 kW and mostly in fractional horsepower range.

14. 14 Section 1.4: Single-Phase Motors Applications such as space vehicles, aircrafts, power tools, etc. are possible due to advances in manufacturing of single-phase motors. They operate at lower power factors and are relatively inefficient when compared with three-phase motors . Since the performance requirements of the various single-phase motor applications differ so widely, many different types of single-phase motors have been developed.

15. 15 Section 1.4.1: Why Single-Phase Motors Are Not Self-Starting When fed from a single-phase supply, the stator winding of a single-phase motor produces an alternating or pulsating flux (or field). It is not a synchronously revolving or rotating flux , as in the case of a two or three-phase stator winding fed from a 2-phase or 3-phase supply.

16. 16 An alternating or pulsating flux acting on a stationary rotor cannot produce rotation. Only a revolving flux can. That explains why single-phase motors are not self-starting. This peculiar behavior of the single-phase motor can be explained by two-field or double-field theory . Section 1.4.1: Why Single-Phase Motors Are Not Self-Starting

17. 17 Section 1.5: Double Field Theory State that an alternating uni-axial quantity can be represented by two oppositely-rotating vectors of half magnitude. Therefore, an alternating sinusoidal flux can be represented by two revolving fluxes, each equal to half the value of the alternating flux and each rotating synchronously at (120f/p) in opposite directions.

18. 18 Section 1.5: Double Field Theory Consider the figure on the left: In Fig.a , the alternating flux has a maximum value of . Its component fluxes A and B based on double field theory will be each revolving in anticlockwise and clockwise directions respectively.  

19. 19 Section 1.5: Double Field Theory After some time in Fig. b , A and B rotate through + and - with a resultant of After a quarter cycle of rotation, fluxes A and B will be oppositely-directed as shown in Fig. c. This causes their resultant to be zero.  

20. 20 Section 1.5: Double Field Theory After half a cycle in Fig. d , fluxes A and B will have a resultant of . Finally the resultant becomes zero after three-quarters of a cycle. This continues.  

21. 21 Section 1.5: Double Field Theory Given that the slip of the rotor is with respect to the forward rotating flux ( i.e flux in the same direction as rotor) then its slip with respect to the backward rotating flux is Proven below; Rotor slip or Keeping in mind that the backward flux rotate opposite to the rotor, rotor slip with respect this flux is;  

22. 22 Section 1.5: Double Field Theory Rotor slip with respect to this flux is The torque produced by each of the two fluxes are given as;  

23. 23 Section 1.5: Double Field Theory: Torque Characteristics The total torque is At starting, the two torques are equal but opposite in direction hence single-phase motors do not start. Since the slips are and  

24. 24 Section 1.6: Making Single-Phase Motors Self Starting To make the motor self-starting, some means must be provided to create an initial (starting) torque . But the starting torque is only possible, if a rotating or revolving flux is created in the stator.

25. 25 There are many methods by which the necessary phase difference between two stator fluxes (or winding currents) can be created, and so make the single-phase motors self-starting for the most satisfactory performance. The various methods employed for starting explain the different types of single-phase motors and constructional features. Section 1.6: Making Single-Phase Motors Self Starting

26. 26 It is known that a rotating flux is produced when there is a difference of 90 between the currents of two stationary coils. In other words , if the stator possesses two fluxes having a large phase difference , the result is a rotating flux. Section 1.6: Making Single-Phase Motors Self Starting

27. 27 Section 1.7: Types of Single Phase Motors Single-phase motors may be classified as follows, depending upon their construction and method of starting: Single Phase Induction Motors Single Phase Commutator Motors Single Phase Synchronous Motors

28. 28 Section 1.7.1: Single Phase Induction Motors Split-Phase Motors Inductor-Start Motor Capacitor-Start Motor Permanent-Split (Single-Value) Capacitor Motor Two-Value Capacitor Motor Shaded-Pole Motors Reluctance-Start Motor Repulsion-Start Motor

29. 29 Section 1.7.2: Single Phase Commutator Motors Repulsion Motors Repulsion-Induction Motors AC Series Motors Universal Motors

30. 30 Section 1.7.3: Single Phase Synchronous Motors Reluctance Motors Hysteresis Motors Sub-synchronous Motors

31. 31 Section 1: Assignment 1 Derive the resultant flux of the fluxes produced by three phase motors.

32. End, Thank you !!!