Presentation of special Electrical Machines

1. SPECIAL ELECTRICAL MACHINES
INTRODUCTION

2. ELECTRICAL MACHINES- IN GENERAL
 DC MACHINES- MOTORS
 AC MACHINES- GENERATOR
 INDUCTION MOTORS- MOTOR

3. LIMITATIONS
 DC Motor
 Armature and field voltage control
 IM Motor
 Frequency and Stator voltage control
 Alternator
 Designed for low synchronous speed

4. STATIC DEVICES- AFTER 1960’S
 Innovation of Static Devices such as SCRs
etc.
 DC motor- Variable DC supply
 IM Motor- Variable frequency drive
 Later years development of Power
transistors, MOFET, IGBT etc. increases the
ease control

5. DEVELOPMENT OF TECHNOLOGY
 Computer peripheral equipments
 Robotics
 Computer Numeric Control (CNC) Machines
 Electrical vehicles

6. OVERALL COST
 high material price, such as permanent
magnet, copper, and iron
 To reduce the cost
 Need to improve torque density

7. NON CONVENTIONAL MACHINES
 Stepper Motor- position Control
 Reluctance Motor- medium traction
 Brushless DC motor- High Torque
Applications
 Linear IM- High Speed transportation

8. ADVANTAGES
 Specific applications
 High torque density
 Low cost
 Better performance
 Easy to implement digital control

9. SYLLABUS-COVERAGE
 Reluctance Motor- 2nd Module
 Permanent Magnet Synchronous Motor- 4th Module

10. RELUCTANCE MOTOR
MODULE 2

11. ELECTRICAL MACHINES- CLASSIFICATION
 broadly classified into two categories on the
basis of how they produce torque
 electromagnetically or
 by variable reluctance.

12. TORQUE- ELECTROMAGNETICALLY
 Motion is produced by the interaction of two
magnetic fields, one generated by the stator
and the other by the rotor.
 Two magnetic fields, mutually coupled,
produce an electromagnetic torque tending
to bring the fields into alignment.
 The same phenomenon causes opposite
poles of bar magnets to attract and like poles
to repel.

13. TORQUE- RELUCTANCE
 In the second category, motion is produced as a
result of the variable reluctance in the air gap
between the rotor and the stator.
 When a stator winding is energized, producing
a single magnetic field, reluctance torque is
produced by the tendency of the rotor to move
to its minimum reluctance position.
 This phenomenon is analogous to the force that
attracts iron or steel to permanent magnets.
 In those cases, reluctance is minimized when
the magnet and metal come into physical
contact.

14. RELUCTANCE
 Magnetic reluctance, or magnetic resistance, is
a concept used in the analysis of magnetic
circuits.
 It is analogous to resistance in an electrical
circuits, but rather than dissipating electric
energy it stores magnetic energy.
 In likeness to the way an electric field causes
an electric current to follow the path of least
resistance, a magnetic field causes magnetic
flux to follow the path of least magnetic
reluctance

16. RELUCTANCE MOTOR-DEFINITION
 A reluctance motor is a type of electric
motor that induces non-permanent magnetic
poles on the ferromagnetic rotor. Torque is
generated through the phenomenon
of magnetic reluctance.

17. ADVANTAGES
 Reluctance motors can deliver very high
power density at low cost, making them ideal
for many applications.
 Washing machines.
 Control rod drive mechanisms of nuclear
reactors.

18. DISADVANTAGES
 Disadvantages are high Torque Ripple (the
difference between maximum and minimum
torque during one revolution) when operated
at low speed, and
 Noise caused by torque ripple

19. CLASSIFICATION
 Synchronous reluctance motor
 Variable reluctance motor or Switched
Reluctance Motor

20. SYNCHRONOUS RELUCTANCE MOTOR
 Synchronous reluctance motors have an
equal number of stator and rotor poles.
 As the rotor is operating at synchronous
speed and there are no current-conducting
parts in the rotor, rotor losses are minimal
compared to those of an induction motor
 Once started at synchronous speed, the
motor can operate with sinusoidal voltage.
Speed control requires a variable frequency
drive.

21. VARIABLE RELUCTANCE MOTOR
 The switched reluctance motor (SRM) is a form
of stepper motor that uses fewer poles. The
SRM has the lowest construction cost of any
industrial electric motor because of its simple
structure.
 Common uses for an SRM include applications
where the rotor must be held stationary for long
periods, and in potentially explosive
environments such as mining because it does
not have a mechanical commutator.

22. SUMMARY
 Conventional Electrical Machines are mainly
used bulk energy conversions.
 Special Electrical Machines- invented for
specific control applications.
 Reluctance motors
 Synchronous reluctance Motor
 Switched reluctance Motor

23. SWITCHED RELUCTANCE MOTOR
 The Switched reluctance motor is an electric
motor in which torque is produced by the
tendency of its moveable part to move to a
position where the inductance of the excited
winding is maximized.

24. CONSTRUCTION

25. CONTINUED
 It has wound field coils of a DC motor for its
stator windings and has no coils or magnets
on its rotor.
 It can be seen that both the stator and rotor
have salient poles; hence, the machine is a
doubly salient machine.

26. STATOR
 Built by stacking suitably punched silicon
laminations to appropriate length.
 Has salient poles.
 Poles carry concentric windings.
 The coils on the opposite poles are
connected in series to form phases.

28. ROTOR
 The rotor contains no windings or permenant
magnet.
 Build up of steel laminations and laminations
are stacked to the shaft.

29. DIFFERENT CONFIGURATIONS

30. WORKING PRINCIPLE
 The rotor is aligned whenever the
diametrically opposite stator poles are
excited.
 In a magnetic circuit, the rotating part
prefers to come to the minimum reluctance
position at the instance of excitation. While
two rotor poles are aligned to the two stator
poles, another set of rotor poles is out of
alignment with respect to a different set of
stator poles.

31. CONTD…

32. CONTD…
 the movement of the rotor, hence the
production of torque and power, involves a
switching of currents into stator windings
when there is a variation of reluctance, this
variable speed motor is referred to as a
switched reluctance motor (SRM).

33. PRINCIPLE OF OPERATION

34. DESIGN ASPECTS OF STATOR AND ROTOR POLE ARC
 s = Stator Pole Arc
 s = Stator Slot Arc
 r = Rotor Pole Arc
 r = Rotor Slot Arc

35.  s + s = 360 / Ns
 r + r = 360/ Nr
 Ns, Nr - Stator projections and Rotor projections respectively.
 Step Angle = (1/ Nr – 1/ Ns ) * 360

36. AIR GAP INDUCTANCE
 Tc= No of turns
 R= radius of the rotor at air gap
 g= gap length; = overlap angle

37. AIR GAP INDUCTANCE
Lphase= 2 Lcoil

38. SPEED EQUATION
 Speed (rpm)= f * step angle in deg. * no of stator phases *
60
360
f- switching frequency

39. LAWRENSON ANALYSIS
 Method to select the tooth and slot
dimensions of the stator and the rotor so as
to obtain feasible and optimised values for
Lmax and Lmin.
 Torque = dL/dt * I2

41. CONTD…
 To allow a quick build up of the current from a
voltage source, it is desirable that the winding
be switched when the inductance is low and
fairly constant.
 This is possible only when the stator pole arc is
less than the rotor slot width.

42.  s Should be less than 2/Ns
 s < r or s > r ??
 s < αs ; Larger stator slot width allow more
ampere-conductors.
 so s < r

43.  s > Step angle, ε

46. R POHL. THEORY OF PULSATING MACHINES
 rotor tooth arc be chosen as appox. 40% of
the rotor slot pitch for maximizing the
difference Lmax - Lmin.

47. FOR 8:6 SRM
 Rotor slot pitch= 360/Nr= 60
 so r = 24; r = 36
 Now s < 24 and > step angle 15
 s + s = 360 / Ns ie 45; So s > 21
 assume
 so s = 21 so s = 21

48. DWELL ANGLE
 So dwell angle,

52. TUTORIAL
 A four phase eight pole switched reluctance
motor has six rotor teeth. Find the step angle
and commutation frequency.
 A three-phase SRM has six stator poles and
four rotor teeth. Draw the feasible zone for
stator and rotor pole arcs. Design the pole
arc and rotor tooth arc. Sketch the L-θ
profile.

53. SRM DRIVE SYSTEM

54. COMPONENTS
 Converter Topology
 Position Sensors
 Control Circuitry

55. POWER CONVERTER FOR SRM

56. ASYMMETRIC BRIDGE CONVERTER

57.  When T1 and T2 are ON Va1a2= V
 When T1 and T2 are OFF Va1a2= -V
Ie D1 and D2 become forward biased and
send power back to the dc bus.
 When T1 or T2 is OFF Va1a2= 0 V, current
free wheels during this period.

58. N+1 SWITCHING DEVICES AND N+1 DIODES
 Higher torque ripple
 Higher switching stress for T

59. BIFILAR TYPE CONVERTER TOPOLOGY
 Poor copper utilization
 Voltage spikes due to imperfect coupling

60. C-DUMP CIRCUIT

61. POWER CONVERTER
 A three-phase, 6/4-pole reluctance machine,
in which i is the current of a single phase

62. POSITION SENSORS
 In the SRM drives, rotor position is essential
for the stator phase commutation and
advanced angle control. The rotor position is
usually acquired by the position sensors.
 The commonly used position sensors are
phototransistors and photodiodes, Hall
elements, magnetic sensors, pulse encoders
and variable differential transformers.