Reluctance Motor - Principle Of Operation

• A reluctance motor is a type of electric motor that induces non-permanent
magnetic poles on the ferromagnetic rotor.
• The rotor does not have any windings. It generates torque through magnetic
reluctance.
• There are various types of reluctance motors
❖Synchronous reluctance
❖Variable reluctance
❖Switched reluctance
❖Variable reluctance stepping

1. RELUCTANCE MOTOR -PRINCIPLE
OF OPERATION

2. Reluctance motor
•Areluctance motoris a type ofelectric motorthat induces non-permanent
magnetic poles on theferromagnetic rotor.
•The rotor does not have any windings. It generates torque throughmagnetic
reluctance.
•There are various types of reluctance motors
❖Synchronous reluctance
❖Variable reluctance
❖Switched reluctance
❖Variable reluctance stepping

3. •Reluctance motors can deliver very highpower densityat low cost, making them
ideal for many applications.
•Disadvantages include hightorque ripple(the difference between maximum and
minimum torque during one revolution) when operated at low speed, and noise due
to torque ripple.

4. Construction
Stator
•The stator of the reluctance motor is similar to that stator of thesingle phase
induction motor.
•It consists of starting and running winding in the stator slots.
•This type of motor also called assplit phasereluctance motor.

5. Rotor
•The rotor of the reluctance motor is of salient or projecting poles.
•Let us consider that the rotor of the squirrel cage induction motor consists of 24
copper bars.
•If the rotor bar 5, 6, 11, 12,17,18,23 and 24 are cut, it is similar to 4 salient poles.

6. Principle of operation
•The stator has three phase symmetrical winding, which creates sinusoidal
rotating magnetic field in the air gap.
•The reluctance torque is developed because the induced magnetic field in the
rotor, has a tendency to cause the rotor to align with the stator field at a
minimum reluctance position.

7. Working
•Whenasinglephasesupplyisgiventothestatorwinding,
arotatingmagneticfieldisproducedinthestatorwinding.
•Whenasalientpolesrotorcutthismagneticfield,rotoralignsin
theminimumreluctancepathduetoreluctancetorque.
•Thereluctancedependsuponairgapbetweenstatorandrotor.
•FigureAshows4polesalientpolerotorinwhichdirectionoffour
highPermeanceandfourlowPermeanceisshown.
•HighPermeancemeanshighermagneticconductivityandhigher
inductance.SimilarlylowPermeancemeanslowermagnetic
conductivityandlowerinductance.

9. •The reluctance is inverse of Permeance. Low reluctance means higher inductance and
vice versa.
L α N
2
/ S
Where , L = Inductance
S = Reluctance of magnetic path
•Low air gap means low reluctance and vice versa
S = L / μ
0μ
ra
Where,
L = Length of air gap
μ
0= Absolute permeability = 4π ×10
–7
Henry /meter
μ
r= Relative permeability
a = Area

10. •Thereislowreluctancepathbetweenstatorandsalientpolesdue
tosmallairgapwhereashighreluctancepathbetweenstatorand
interpolaraxisduetolargeairgap.
•Thereluctancemotorstartsasaninductionmotor.
•Whentherotorrotatesatitsmaximumspeed,italignswiththe
statorsynchronousmagneticfieldduetoreluctancetorque.
•Theanglebetweenstatorpolesandrotorpolesofoppositepolarity
iscalledastorqueangle.
•Asthetorqueangleincreases,thereluctancetorquealsoincreases.
•Themaximumreluctancetorqueattainsattorqueangleof45
0
.
•Theloadtakenbythereluctancemotorisonlyfractionoftheload
takenbythethreephaseinductancemotor.

11. Advantages
•Low maintenance
•DC supply not necessary
•Simple construction
•Constant speed characteristic

12. Disadvantages
•Low efficiency
•Low power factor
•Only fraction of load taken as compared to three phase induction motor

13. Applications
•Automatic regulator
•Signaling devices
•Recording instruments
•Tele printer
•Timer circuits
•Gramophone

14. Synchronous Reluctance
Motor(SyRM)

15. CONSTRUCTION

16. Stator:
•Armature or stator core is made of ferromagnetic material and
laminated
Why?
•To reduce the hysteresis and eddy current losses
•Stator core is attachedto the stator frame
•Slots for housing the armature winding are provided along the
inner periphery of stator core
•Semi-closed slots are used
•Stator carries three-phase winding and arranged for required
number of poles
•Distributed windings are used

17. Rotor:
•Rotor is constructed in such a way that the armature inductance varies
sinusoidally
•Inductance should be maximum along direct axis & minimum along
quadrature axis
•Difference between direct axis & quadrature axis inductance should be as
large as possible
❑Why?
•To generate the maximum torque
❖Different type of Rotor Construction:
•Segmental rotor
•Radially laminated rotor
•Axially laminated rotor

20. Axially Laminated Rotor:
•Flux barriers are introduced in the quadrature flux path
•Flux barriers are made of thin sheets of non-magnetic material
•Brass or aluminumis used as flux barriers
•Direct axis inductance is not affected much by flux barriers as their
thickness is very small
Radially Laminated Rotor:
•Compared to axially laminated rotor they are cheaper & easier
•Since circular laminations can be used
•Motor has self starting capability

21. WORKING PRINCIPLE
•Balanced3-phasesinusoidalsupplyvoltageisgiveninthestator
•Producesarotatingmagneticfieldintheairgapwhichrotatesatsynchronous
speed
•Rotoracceleratestowardssynchronousspeedwiththehelpofdamperwindingor
cagewindingprovided
•Rotatingmagneticfieldexertsreluctancetorqueontherotortoalignitsprojecting
polesord-axistohaveaminimumreluctancetorque
•Synchronousreluctancestartsasaninductionmotorandatrunningcondition
reluctancetorquepullstherotorinsynchronismwithstatorfield

22. •Rotor accelerates towards synchronous speed
•At a “critical” speed, the low-reluctance paths
provided by the salient poles will cause them to
“snap” into synchronism with the rotating flux.

24. •When the rotor synchronizes, slip is equal to zero
•Rotor pulled around by “reluctance torque”
•Figure at right shows the rotor synchronized at
no load .

25. •A“step”increaseinloadslowstherotordown,
andtherotorpoles“lag”thestatorpoles.
•Theangleoflag,δ,iscalledthe“torqueangle”.
•Themaximumtorqueangle,δmax=45°.

26. OPERATION AT MAXIMUM LOAD
•Maximumloadiswhenδ=45°.
•Ifloadincreasessothatδ>45°,
•thefluxpathis“overstretched”
andtherotorfallsoutofsynchronism.
•Motorrunsatslipspeed

30. SWITCHED RELUCTANCE
MOTOR

31. INTRODUCTION
•Switched reluctance motor (SRM) is similar to a variable reluctance stepper
motor in closed loop operation.
•SRM drive is better alternative to conventional DC series motor and variable
speed induction motor drives.
•The performance of SRM drives depends on control system used.
•At preset many industries are using SRM with its full potential.

32. CONSTRUCTION
•The stator of the SRM is built by stacking suitably punched silicon laminations to
the appropriate length.
•It has salient poles and carries concentric windings.
•The rotor contains no winding or permanent magnet.
•It is built up of steel laminations.
•It is due to this mechanical simplicity that the cost of SRM is promisingly low.

33. •By selecting the number of phases,
the number of stator poles and the
number of rotor teeth, many
configurations of SRM can be
obtained

34. WORKING PRINCIPLE
•Consider an SRM with 8 stator poles and 6 rotor teeth.
•It has 4 phases, A-A', B-B', C-C', D-D'.
•These phases can be excited by DC supply through switches S1, S2, S3 and S4.
•Let A-A' be energised for a significant time so that the rotor rests in the
equilibrium position.
•Phase A-A' is de-energised by turning switch S1 OFF and excite B-B' by turning
switch S2 ON.
•The rotor moves by 15
0
in CCW direction and attains stable position.
•By operatig the switches in sequence S1, S2, S3, S4, S1....., we can make the rotor
to rotate in the CCW with a step angle of 15
0
.

36. Conditions for the succesful operation of SRM are:
•No.of rotor teeth and stator poles must be even and not equal.
•Stator phase is energised when the inductance of that phase is low or increasing
•Sensor for rotor position is required. The rotor position sensing is essential for
switching operations at correct instants.
•The flux density is changing in magnitude and direction in the magnetic circuit
when rotor moves. this results in iron loss. To reduce iron loss, laminated rotor
and stator structures should be used.

37. ADVANTAGES OF SRM
•High efficiency
•Good performance in terms of torque to inertia ratio
•Maximum operating speed and simple construction
•Available in various sizes, power and speed ranges.

41. POWER CONVERTERS

42. REQUIREMENTS
a)EachphaseofSRMshouldbeexcitedindependently.
b)AnexcitedpoleshouldbedemagnetisedbeforetheentryofSRMinto
generatingzone.
c)Freewheelingshouldbeachievedduringthechoppingperiod.
d)Thedemagnetisationenergyshouldbefedbacktothesourceforusingitinthe
subsequentconductingzone.
e)Thecircuitshouldbecost-effective.

43. TWO SWITCHING DEVICES PER PHASE
Power converter circuit for three phase SRM

44. •PhasewindingsareexcitedbyturningONthestaticswitches
•ThesedevicesgetturnedOFFbydeenergisingthephase.
•Thestoredenergyisfedbacktothesupplythroughthefreewheelingdiodes.
•Theperiodofconductioniscontrolledbyasuitablecontrolcircuit.

45. (n+1)SWITCHING DEVICES AND (n+1)DIODES

46. •For energisingphase A, T and T
1are turned ON.
•When these devices are turned OFF, the stored energy will fedbackto the supply
through diodes D and D
1
•T and T
1 are used for switching phase B.
•T and T
3are turned ON for exciting phase C.

47. ADVANTAGES
•Cost is reduced as less number of switching devices are used.
•Free wheeling is possible.
DISADVANTAGES
•Higher torque ripple.
•Higher switching stress for T as it is conducting current while other phases are
energised.

48. CONVERTER CIRCUIT FOR SRM WITH BIFILAR
WINDING

49. •EachphaseofSRMhastwoidenticalandmagneticallycoupledcoils.
•WhenT
1isturnedON,currentflowsthroughwindingA.
•WhenitisturnedOFF,thestoredenergygoesbacktothesupplythroughA’and
D
1.
DEMERITS
•Doublenumberofconnections
•Poorutilisationofcopperandvoltagespikesdueto
Imperfectcoupling.

50. SPLIT-LINK CIRCUIT

51. •Themainpowersupplyisdividedintotwohalves.
•Duringconduction,energyissuppliedtothephasebyonehalfofthepower
supply.
•Duringcommutationperiod,thestoredenergyisfedbacktotheotherhalfofthe
supply.
•Forexample,whenTisturnedON,phaseAisenergisedbytheupperhalfofthe
supply.
•WhenT
1isturnedOFF,thestoredenergyisfedbackthroughD
2tothelowerhalf.

52. C-DUMP CIRCUIT
This circuit uses (n+1) additional devices to feed the stored energy from the dump Capacitor C
back to the supply through step down chopper circuit.