Unit 1

SynchronousReluctanceMotors
 UNIT 1

Synchronous Reluctance Motors

Constructional Features
Operating Principles
Types
Axial And Radial Flux Motors
Reluctance Torque
Torque Equation
Characteristics
Syncreldrivesystem
Phasordiagram
Applications.
UNIT 1

PART A
 PART A

Reluctance motor is a a) Induction type b) Synchronous type

1
c)Reactance type d) reflection type B
Synchroonus type motor types are 1.cageless 2. Line start 3.
2
Induction 4. reluctance a)1 only b)1& 2 c)2 only d)all B
Synchronous motor classifiede with respect to magnetization
3 a)radial type b)Axial type c) reluctane type d) Radial and axial
type d
Synchronous relucatnce motore advantage is a)Lower torque
4 ripple b)Medium torque ripple c)High torque ripple d)Ultra high
torque ripple a
Which motor has high speed capability a)Synchronous motor
5
b)Reluctance motor c)Induction motor d)Stepper motor b
What is the angle between stator direct axis and quadrature axis?
6
a)90° b)0° c)45° d)any of the mentioned a
Space angle, θr is measured between stator d-axis and _____
7 a)quadrature axis b)direct d-axis c)long rotor axis d)none of the
mentioned c
The reluctance offered to the stator flux by two very large air gaps
in series with high permeability iron, in reluctance machine is
8
maximum, when the space angle θr = ______ a)0° b)45° c)90°
d)180 c
The reluctance offered to the stator flux by two small air gaps in
series with high permeability iron, in reluctance machine is
9
minimum, when the space angle θr = ______ a) 0° b)45° c)90°
d)270° a
The variation of reluctance Rl with space angle θr depends on the
10 shape of __________ a)stator poles b)rotor poles c)stator or rotor
poles d)both stator and rotor poles d

Reluctance motor can produce torque at ________ a)any speed less

11 than synchronous speed b)synchronous speed only c)any speed
greater than synchronous speed d)any of the mentioned
b
For a reluctance motor , the maximum average torque occurs when
12
δ= __________ a)45° b)90° c)0° d)180° a
For a given reluctance motor, Rld and Rlq are ________ a)constant
13
b)varying c)zero d)any of the mentioned a

The single phase reluctance machine acts as a generator when angle

14
δ is _______ a)positive b)negative c)zero d)any of the mentioned
b
 Single phase reluctance motors are extensively used in ________
15 a)grinder applications b)Driving electric clocks and other timing
devices c) welding applications d)lifts/ elevators b

If the salient pole rotor in a single phase reluctance motor is

replaced by a cylindrical rotor, then(i) reluctance offered to stator
flux remains constant for all rotor positions (ii) no reluctance torque
16
will be (iii) reluctance torque will be developed (iv) reluctance
offered to stator flux changes for all rotor positions -Which of the
statment is true ? a) (i), (ii) b)(ii), (iii) c)(iii), (iv) d) (i), (iv)
a

Which of the following are applications of singly excited magnetic

17 systems ____________ a) electromagnets, relays b)moving-iron
instruments c)reluctance motors d)any of the mentioned
d
For which of the applications a reluctance motor is preferred
18 a)Electric shavers b)Refrigerators c)Signaling and timing devices
d)Lifts and hoists c
A reluctance motor a)Self-starting b)Is constant speed motor
19
c)Needs no D.C. excitation d)All of the above d
Reluctance motors are a)Singly excited b)Doubly excited
20
c)Either of the above d)None of the above a
UNIT 1
PART B
 PART B

1. Write the short notes on variable reluctance motor and the advantages

Variable Reluctance Stepper Motor

The principle of Variable Reluctance Stepper Motor is based on the
property of the flux lines which capture the low reluctance path. The stator and the
rotor of the motor are aligned in such a way that the magnetic reluctance is
minimum. There are two types of the Variable Reluctance Stepper Motor.

Working of a Variable Reluctance Stepper Motor

A four-phase or (4/2 pole), single stack variable reluctance stepper motor is

shown below. Here, (4/2 pole) means that the stator has four poles and the rotor
has two poles.

The four phases A, B, C, and D are connected to the DC source with the help
of a semiconductor, switches S A, SB, SC, and SD respectively, as shown in the above
figure. The phase windings of the stator are energized in the sequence A, B, C, D,
A. The rotor aligns itself with the axis of phase A as winding A is energized. The
rotor is stable in this position and cannot move until phase A is de-energized.

Now, phase B is excited and phase A is disconnected. The rotor moves 90

degrees in the clockwise direction to align with the resultant air-gap field which
lies along the axis of phase B. Similarly phase C is energized, and phase B is
disconnected, and the rotor moves again in 90 degrees to align itself with the axis
of the phase.

Thus, as the Phases are excited in the order as A, B, C, D, A, the rotor moves
90 degrees at each transition step in the clockwise direction. The rotor completes
one revolution in 4 steps. The direction of the rotation depends on the sequence of
switching the phase and does not depend on the direction of the current flowing
through the phase. Thus, the direction can be reversed by changing the phase
sequence like A, D, C, B, A.

The magnitude of the step angle of the variable reluctance motor is given as:

Where,

 α is the step angle

 ms is the number of stator phases
 Nr is the number of rotor teeth

The step angle is expressed as shown below:

Where, NS is the stator poles

The step angle can be reduced from 90 degrees to 45 degrees in a clockwise

direction by exciting the phase in the sequence A, A+B, B, B+C, C, C+ D, D,
D+A, A.

Similarly, if the sequence is reversed as A, A+D, D, D+C, C, C+B, B, B+A,

A, the rotor rotates at a step angle of 45 degrees in the anticlockwise direction.

Here, (A+B) means that the phase windings A and B both are energized
together. The resultant field is midway between the two poles. i.e. it makes an
angle of 45 degrees with the axis of the pole in the clockwise direction. This
method of shifting excitation from one phase to another is known
as Microstepping. By using Stepper Motor, lower values of the step angle can be
obtained with a number of poles on the stator.

Consider a 4 phase, (8/6 pole) single stack variable reluctance motor shown in the
figure below:

The opposite poles are connected in series forming 4 phases. The rotor has 6
poles. Here we have considered only phase A to make the connection simple.
When the coil AA’ is excited, the rotor teeth 1 and 4 are aligned along the axis of
the winding of phase A. Thus, the rotor occupies the position as shown in the
above figure (a).

Now, phase A is de-energized, and phase winding B is energized. The rotor

teeth 3 and 6 get aligned along the axis of phase B. The rotor moves a step of the
phase angle of 15 degrees in the clockwise direction. Further, phase B is de-
energized, and winding C is excited. The rotor moves again 15⁰ phase angle.

The sequence A, B, C, D, A is followed, and the four steps of rotation are

completed, and the rotor moves 60 degrees in a clockwise direction. For one
complete revolution of the rotor 24 steps are required. Thus, any desired step angle
can be obtained by choosing different combinations of the number of rotor teeth
and stator exciting coils.
2. Investigate the performance of the synchronous reluctance motor with neat
phasor diagram.
PHASER DIAGRAM OF SYNCHRONOUS RELUCTANCE MOTOR
The synchronous reluctance machine is considered as a balanced three phase
circuit, it is sufficient to draw the phasor diagram for only one phase. The basic
voltage equation neglecting the effect of resistance is

V = E – j IsdXsd – j Isq… ........... (1.1)

Where

V is the Supply Voltage

Is is the stator current

E is the excitation emf
Ȣ is the load angle

ɸ is the phase angle

Xsd and Xsq are the synchronous reactance of direct and quadrature axis

Isd and Isq are the direct and quadrature axis current

I = Isd + Isq…................ (1.2)

Isd is in phase quadrtur with E and Isq is in phase with E.

V = E – j IsdXsd – j IsqXsq
Equation (9) is the torque equation of synchronous reluctance motor.

Plotting the equation (9) as shown in fig indicates that the stability limit is reached
at Ȣ =± ᴨ /4

And by increasing g load angle torque also increases.

In synchronous reluctance motor, the excitation emf(E) is zero.
3. Explain the construction and operation of Axial and Radial flux machines

Axial Flux Motor

An axial flux motor is an electric motor design with a unique configuration

where the magnetic flux runs parallel to the motor's shaft. This layout allows for
compact, high-torque, and lightweight motor designs, making it suitable for
various applications, including electric vehicles and renewable energy systems.
Axial flux motors are known for their efficiency and adaptability in constrained
spaces.

Axial Flux Motor Construction

Axial flux motor construction consists of a stator with windings, and a rotor
with permanent magnets, positioned in a flat, disc-like arrangement. The various
parts of the Axial Flux Motor are:

o Disk-shaped Stator and Rotor

o Radial Magnets
o Winding Coils
o Compact Design
o Low Profile
o High Torque Density
o Direct Cooling
o Axial Air Gap

Disk-shaped Stator and Rotor

Axial flux motors feature a unique construction, with both the stator and
rotor designed as flat, circular disks. This design allows for a compact and space-
efficient motor, making it particularly well-suited for applications with limited
installation space.
Radial Magnets

The stator and rotor in an axial flux motor incorporate magnets arranged
radially, enabling an efficient and direct interaction between the magnetic fields.
This radial magnet arrangement enhances the motor's torque production, making it
suitable for various industrial and automotive applications.

Winding Coils
Copper wire coils are wound on the stator of the axial flux motor. These
winding coils are crucial in generating electromagnetic forces that drive the motor's
rotation. The precise arrangement and control of these coils are essential for the
motor's performance.
Working of Axial Flux Motor

Upon energising the coil, it transforms into an electromagnet within the axial
motor. The operation of the axial flux motor relies on the interplay between
permanent magnets and these newly formed electromagnets. In the design of axial
flux motors, fixed coils coexist with freely rotating permanent magnets.

Fig: Axial Flux Motor

When coil A is energised with a DC current, it triggers an attraction between

the S pole of the rotor and the opposite N pole of the stator. Simultaneously, like
poles repel each other. This interplay of tangential force components sets the rotor
in motion. However, when the rotor aligns precisely with coil A, the net force
acting on the permanent magnet reaches equilibrium.

The rotor's inertia carries it beyond the point of perfect alignment. During
this phase, the subsequent coil B is energised, thanks to the magnetic forces of
attraction and repulsion.
 Fig: Axial Flux Motor working

Following this, coil C receives its share of energy. In the subsequent half-
rotation, coil A is energised once more, but this time with reversed polarity,
achieved by altering the supply direction. This continuous repetition of the process
propels the rotor to maintain its rotation. However, a drawback surfaces in this
operation: two coils remain inactive, as exemplified by coil B's inactivity. These
dormant coils significantly diminish the motor's power output.

To resolve this issue, we introduce an additional coil pair by simply

running the opposite polarity current through the second coil. This arrangement
aligns two south poles together on the stator side. Once again, a net tangential
force is generated, as shown in the figure. This combined effect not only yields
more torque but also enhances the power output of the rotor. Remarkably, this
process ensures a consistent torque output from the motor.

Axial Flux Motor vs. Radial Flux Motor

Axial flux motors have magnetic flux along the motor's shaft, making them
more compact, while radial flux motors have flux radially, typically larger but with
higher torque. The table below draws a comparison between axial flux motor and
radial flux motor.

Characteristic Axial Flux Motor Radial Flux Motor

Magnetic Flux Path Parallel to the Axis of Rotation Radial, from Center to Edge
Motor Thickness Thin and Flat (Pancake Shape) Thicker, Cylindrical
Rotor-Stator Coaxial Discs (Parallel) Nested Cylinders
Configuration (Concentric)
Efficiency Typically Higher in Some Variable, Depending on
Applications Design
Cooling More Effective Due to Flat Cooling More Challenging
Design
Size and Weight Compact and Lightweight Bulkier and Heavier
Applications Electric Vehicles, Robotics, Generators, Industrial
Some Wind Turbines Drives, Appliances
Torque Handling Typically Lower Torque Higher Torque Capacity
Capacity
Manufacturing May Be Simpler in Some Cases More Complex in Some
Complexity Designs
Cooling and Heat Enhanced Heat Dissipation May Require Additional
Dissipation Cooling
Noise Level Generally Lower Noise Variable, Dependent on
Emission Design

Axial Flux Motor Advantages

The advantages of Axial Flow Motor are:

o Compact and lightweight design.

o High power-to-weight ratio.
o Effective cooling due to flat structure.
o Suitable for electric vehicles and robotics.
o Greater torque density in some cases.

Axial Flux Motor Disadvantages

The limitations include:

o Lower torque capacity compared to radial flux motors.

o Limited use in high-torque industrial applications.
o More complex manufacturing in certain designs.
o Heat dissipation challenges may arise.
o Noise levels can vary depending on design.

Applications of Axial Flux Motor

Following are the applications of Axial Flux Motor:

o Electric vehicles (EVs) propulsion.

o Robotics and automation systems.
o Wind turbines for renewable energy.
o Electric fans and blowers.
o Small-scale hydroelectric generators.
o Electric bicycles (e-bikes).
o Aircraft propulsion systems.
o Industrial machinery and conveyors.
o Water pumps for irrigation.
o Marine propulsion for boats and ships.

RADIAL FLUX MOTOR

Design and Operation of Radial Flux Motors

Most conventional electric motors used today are of the radial flux type. In
these motors, the stator (the stationary part of the motor) is designed around the
rotor (the rotating part of the motor). The magnetic field generated by the stator
windings travels radially across the air gap between the stator and rotor, thereby
inducing a torque that makes the rotor turn. This radial flow of magnetic field gives
these motors their name.

Advantages of Radial Flux Motors

1. Compactness: The radial design often allows for a more compact motor,
fitting into smaller spaces.
2. Efficiency: With a well-designed radial flux motor, higher efficiency can be
achieved, which can lead to significant energy savings over time.
3. Robustness: Thanks to their design, radial flux motors are generally more
robust and durable than other types of motors.
Applications of Radial Flux Motors

Radial flux motors are extensively used in numerous applications, owing to

their high power density and cost-effectiveness. These applications range from
home appliances such as washing machines and fans to electric vehicles and
industrial machinery. In fact, most electric vehicles on the road today use radial
flux motors due to their high torque capabilities and efficiency.

4. Differentiate between Axial and Radial airgap synchronous reluctance motors.

5.
Compare the performance of synchronous reluctance motor with switched
reluctance motor

Comparison of SRM vs SYNRM

Here are some of the differences between the switched reluctance motor vs
synchronous reluctance motor which further called SRM and SYNRM:

Working Principle
Stator of the SRM has concentrated windings and each phase produces the
salient pole. Whereas, the stator of the SYNRM has a distributed winding like
induction motor which produces the rotating magnetic field.
In the SRM, phase switching is done to change the location of the stator
poles with rotor position to produce the rotation of the rotor. In SYNRM, rotating
poles are generated due to the rotating magnetic field, and the rotor of the SYNRM
rotates with the same speed but slightly lag behind the rmf with some angle.
SRM required a position sensors to switch the phase energization with rotor
position to maintain the continuous rotation. While position sensors are required to
maintain the rotor angle to avoid loss of synchronism in SYNRM.

Rotor Design
The switched reluctance motor has a highly segmented rotor (Salient pole) that
is made up of magnetic poles and flux concentrators. In contrast, the synchronous
reluctance motor, the rotor has a smooth surface, and it is made up of laminations.

Control Mechanism
The synchronous reluctance motor requires a complex control mechanism,
including a feedback sensor, to operate effectively. On the other hand, the switched
reluctance motor can operate without a feedback sensor, making it less complex.

Efficiency
The synchronous reluctance motor is highly efficient, which can be designed
for an efficiency of up to 98%. In contrast, the switched reluctance motor has a
lower efficiency, typically between 80% and 90%.
Power Factor
The synchronous reluctance motor has a high power factor, which is
beneficial for the power system. The switched reluctance motor has a low power
factor, which may cause power quality issues.

Torque Ripple
The switched reluctance motor has a higher torque ripple compared to the
synchronous reluctance motor, making it less suitable for applications where a
smooth torque output is required.

Speed Control
The synchronous reluctance motor has better speed control, making it
suitable for applications where precise speed control is required. In contrast, the
switched reluctance motor has limited but simple and cost effective speed control
solutions
6. Summarize the design considerations of synchronous reluctance motor.

The idealized structure of reluctance motor is same as that of the salient pole
synchronous machine shown in fig. except that the rotor does not have any field
winding. The stator has a three phase symmetrical winding, which creates
sinusoidal rotating magnetic field in the air gap, and 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. Fig.
Idealized three phase two pole synchronous machine (Salient pole) The rotor of the
modern reluctance machine is designed with iron laminations in the axial direction
separated by nonmagnetic material as shown in fig., to increase the reluctance to
flux in the q-axis. Compared to the induction motor, it is slightly heavier and has a
lower power factor. With proper design, the performance of the reluctance motor
may approach that of induction machine. With a high saliency ratio (Lds/Lqs), a
power factor of 0.8 can be reached. The efficiency of a reluctance machine may be
higher than an induction motor because there is no rotor copper loss. Because of

inherent simplicity, robustness of construction and low cost, reluctance machines

have been popularly used in many low power applications such as fiber spinning
mills, where a number of motors operate synchronously with a common power
supply The synchronous reluctance motor has no synchronous starting torque and
runs up from stand still by induction action. There is an auxiliary starting winding.
Subsequent design modifications involved the introduction of a segmental rotor
construction of effort a flux barrier in each pole. This has increased the pull out
torque, the power factor and the efficiency. The simple applications where several
motors are required to rotate in close synchronism. Fig. Cross-section of
synchronous reluctance motor Synchronous reluctance motor is designed for high
power applications. It can broadly be classified into (a) Axially laminated and (b)
Radially laminated synchronous motors. These motors have the same stator
construction as the multiphase induction motor. Generally three types of rotors
used in synchronous reluctance motor. They are segmental, flux barrier (radially
laminated) and axially laminated structure. The ideal synchronous reluctance
machine is having a rotor whose structure such that the inductance of the stator
windings in the dq reference frame varies sinusoidally from a maximum value
Ld(direct inductance) to a minimum value Lq (quadrature inductance) as a function
of angular displacement of the rotor. Fig. Cross section of axially laminated SyRM
Fig. Cross section of radially laminated SyRM Rotor Design: Salient
rotor (Segmental): Salient rotor shape such that the quadrature air-gap is much
larger than the direct air gap. This yields reactively small Ld/Lq ratios in the range
of 2.3 Fig. Salient Rotor. Salient rotor design is shown in fig. The low Ld/Lq ratios
are largely the result of circulating flux in the pole faces of the rotor. However the
ruggedness and simplicity of the rotor structure has encouraged study of this
approach for high speed applications. Fig. Radially laminated rotor.Another
approach is to use laminations with "flux barriers" punched into the steel for a 4
pole machine as shown in fig. However these flux barriers and the central hole of
the lamination required for the shaft weaken the rotor structurally and thus makes
this approach a poor choice for high speed design. Axially laminated rotor: The fig.
shows the axially laminated rotor. Fig. Axially laminated rotor. This approach is to
laminate the rotor in the axial direction. For a two pole two phase axially laminated
rotor with an Ld/Lq ratio of 20, the maximum efficiency is 94% has been reported
in the literature. It is observed that torque ripple and iron losses are more in axially
laminated rotor than radially laminated rotor. Another rotor design is shown in fig.
In this case the rotor consists of alternating layers of ferromagnetic and non-
magnetic steel. If choose the thickness of the steel such that the pitch of the
ferromagnetic rotor segments matched the slot pitch of the stator. In this way the
ferromagnetic rotor segments always see a stator tooth pitch regardless of the angle
of rotation of the rotor. This is done to minimize flux variations and hence iron
losses in the rotor. Fig New rotor design. Special rotor laminations make it possible
to produce the same number of reluctance path as there are magnetic poles in the
stator. Synchronous speed is achieved as the salient poles lock in step with
magnetic poles of the
rotating stator field and cause the stator to run at the same speed as the rotating
field. The rotor is pressure cast with end rings similar to induction motor. Stator
windings are similar to squirrel cage induction motor Rotor Construction: To
construct the rotor, we are using a joining technique known as explosion bonding.
Explosion bonding uses explosive energy to force two or more metal sheets
together at high pressures. Conventionally the high pressure causes several atomic
layers on the surface of each sheet to behave as a fluid. The angle of collision
between the two metals forces this fluid to jet outward. Effectively cleaning the
metal surface, these ultra clean surfaces along with the high pressure forcing the
metal plates together provide the necessary condition for solid phase welding.
Experimental tests on a stainless steel/mild steel bond indicate that the tensile and
fatigue strengths of the bond are greater than those of either of the component
materials due to the shock hardening which occurs during the process. The bond
was also subjected to 10 cycles of temperature variation from 20°C - 70°C, with no
significant reduction in tensile strength. Explosion bonding technique is shown in
fig. 7.9, other joining techniques such as brazing, roll bonding, or diffusion
bonding may also appropriate for rotor construction. Fig. Explosion Bonding First
sheets of ferromagnetic and non-magnetic steel are bonded as shown in fig. The
bonded sheets are then cut into rectangular blocks which are machined into the
desired rotor. The rotor shaft can also be machined out of the same block as the
rotor. Working Of Synchronous Reluctance Motor:

In order to understand the working of synchronous reluctance motor, when a

piece of magnetic material is located in a magnetic field, a force acts on the
material tending to bring it into the densest portion of the field. The force tends to
align the specimen of the material in such a way that the reluctance of the magnetic
path that passes through the material will be minimum. When supply is given to
the stator winding, the revolving magnetic field will exert reluctance torque on the
unsymmetrical rotor tending to align the salient pole axis of the rotor with the axis
of the revolving magnetic field, because in this position, the reluctance of the
magnetic path would be minimum as shown in fig. 7.10. If the reluctance torque is
sufficient to start the motor and its load, the rotor will pull into step with the
revolving field and continue to run at the speed of the revolving field. Actually the
motor starts as an induction motor and after it has reached its maximum speed as
an induction motor, the reluctance torque pulls its rotor into step with the revolving
field, so that the motor now runs as synchronous motor by virtue of its saliency.

Fig. Rotor positions due to revolving magnetic field Reluctance motors have
approximately one-third the HP rating they would have as induction motors with
cylindrical rotors. Although the ratio may be increased to onehalf by proper design
of the field windings, power factor and efficiency are poorer than for the
equivalent induction motor Reluctance motors are subject to "cogging" since, the
locked rotor torque varies with the rotor position, but the effect may be minimized
by skewing the rotor bars and by not having the number of rotor slots exactly equal
to an exact multiple of the number of poles. Operating Principle Of Synchronous
Reluctance Motor To understand the working principle of synchronous reluctance
motor, let us keep in mind the following basic fact.

When a piece of magnetic material is located in a magnetic field, a force acts

on the material, tending to bring it into the most dense portion of the field. The
force tends to align the specimen of material in such a way that the reluctance of
the magnetic path lies through the material will be minimum. In a nutshell, when a
piece of magnetic material is free to move in a magnetic field, it will align itself
with the field to minimize the reluctance of the magnetic circuit. Fig. Synchronous
reluctance motor The Fig. shows the synchronous reluctance motor. The stator has
open slot and semi closed slot structures.

The rotor has two types of air gap viz., radial and axial. Here for simplicity,
the synchronous reluctance motor having the open slot stator and axial air gap rotor
structure is shown in Fig. All the configurations of synchronous reluctance motor
are having the same working principle. , symmetrical winding, which creates a
sinusoidal rotating field inThe stator has a 3 the air gap when excited. The rotor
has an unexcited ferromagnetic material with polar projections. When the supply is
given to the stator winding, the revolving magnetic field exerts reluctance torque
on the unsymmetrical rotor tending to align the salient pole axis of the rotor with
the axis of the revolving magnetic field. [It is the position, where the reluctance of
the magnetic path would be minimum].

So the reluctance torque is developed by the tendency of ferromagnetic

rotor to align itself with the magnetic field. The reluctance torque developed in this
type of motor can be expressed as. 2 Where, Number of polesP s Stator flux
linkage Lds Direct axis inductance with respect to synchronously rotating
frame Lqs Quadrature axis inductance with respect to synchronously rotating
frame Torque angle  If the reluctance torque is sufficient to start the motor
and its load, the rotor will pull into step with the revolving field and continue to
run at the speed of the revolving field. The motor starts as an induction motor and
after it has reached its maximum speed as an induction motor, the reluctance
torque pulls its rotor into step with the revolving field, so that the motor now runs
as synchronous motor by virtue of its saliency. Even though the rotor revolves
synchronously, its poles lag behind the stator poles by a certain angle known as
torque angle, [something similar to that in a synchronous motor]. The reluctance
torque increases with the increase in torque angle, attaining maximum value when
α = 45°. Reluctance motors are subjected to “cogging” since, the locked rotor
torque varies with the rotor position, but the effect may be minimized by skewing
the rotor bars and by not having the number of rotor slots exactly equal to an exact
multiple of the number of poles. The operation of motor at synchronism with
ideally zero rotor electrical losses will improve the efficiency. But the reluctance
motors have approximately one third the hp rating, when compared with the
condition that they would have operated as induction motors with cylindrical
rotors.

Although the ratio may be increased to onehalf by proper design of the field
windings, power factor and efficiency are poorer than for the equivalent induction
motor. Once the rotor of synchronous reluctance motor is synchronized, the cage
winding rotates synchronously with the stator field. Thus, the rotor winding plays
no part in the steady state synchronous operation of the motor. The machine
continues to operate synchronously, provided the pull-out torque of the motor is
not exceeded This is the load torque required to pull the rotor out of synchronism.
 The pull in torque is defined as the maximum load torque which the rotor
can pull into synchronism with a specified load inertia. The pull-in torque can be
increased at the expense of larger starting current, but it is always less than the
pullout torque. The reluctance motors have been widely used in adjustable-speed
multimotor drives requiring exact speed coordination between individual motors. If
all the motors in multimotor drive system are accelerated simultaneously from
standstill by increasing the supply frequency, the machines operate synchronously
at all times, and pull-in torque requirements.

The reluctance motor unfortunately exhibits a tendency towards instability at

lower supply frequencies, but it forms a low cost. robust and reliable synchronous
machine. The constant speed characteristics of the synchronous reluctance motor
makes it very suitable for the applications, such as, recording instruments, many
kinds of timers, signaling devices and phonograph.

7. Describe the constructional features and operation of variable reluctance

synchronous and reluctance motor

Definition: This is one kind of advanced motor which includes both the
stator and the rotor similar to a normal electric motor. These motors work with a
precise rotating magnetic field (RPM) by synchronizing the speed of the rotor
using the RMF of the stator. The power density delivered by these motors is high
at low cost to make them attractive in several applications. The working principle
of reluctance motor is, whenever a magnetic material is located within the
magnetic field, then it always brings into line in the less reluctance way.
The specifications of reluctance motor are a type of phase, pole ratio of the stator
to the rotor, rated power or torque, torque ripple, and constant torque speed range.
The power factor of reluctance motor is lagging PF and the machine efficiency
can range from 55 to 75%.
Construction of Reluctance Motor

The construction of this motor is shown below. The designing of this can be
done by removing the teeth in four locations to form a four-pole structure.

The rings at two ends are short-circuited. Once the stator of the motor is
aligned to a single-phase supply, the motor works like a single-phase induction
motor. Once the motor’s speed reaches the highest level of synchronous speed,
then a centrifugal switch will detach the auxiliary winding. The motor increases the
speed like a single-phase motor through the major winding in process.

Reluctance Motor
Construction

The torque of this motor can be generated because of the rotor tendency to connect
itself in the least reluctance position, once the motor speed is nearer to the
synchronous speed. Therefore, the rotor drags in synchronism. The inertia of load
must be in the limits for suitable effectiveness. At synchronization, the torque of
induction will disappear, except the rotor remains in synchronization because of
the torque in synchronous reluctance.
Working of Reluctance Motor

The essential parts of this motor are the stator and the rotor. These two are
stationary parts that are separated through an air gap. Based on the motor type, the
motor construction will be changed but the basic working principle will be the
same. The stationary part like stator includes salient pole-pairs which can be
formed through flowing current using a wire. The rotor can be formed with
ferromagnetic metal and it includes its own poles.

These poles follow the outlines of the magnetic field of the stator. Once the salient
pole of the rotor connects to the salient pole of the stator, then the rotor is in the
least reluctance position. So the magnetic resistance amount is less at this end.
When a stator pole connects to the slots or notches of the rotor, then the rotor will
be in the highest reluctance position. Because of energy protection, the rotor will
constantly move toward the least reluctance position. So when the rotor is not
aligned fully, then a reluctance torque can be generated. This torque will drag the
rotor toward the adjacent salient stator pole to cause rotation.

Reluctance Motor Torque Equation

Reluctance torque can occur once a ferromagnetic object is located within an

exterior magnetic field, then the object can be line up through the external
magnetic field. This will induce an inner magnetic field within the object because
of the generated torque.

This torque can be generated among the two fields which twirling the object in the
region of the line through the magnetic field. So, torque is used on the object to
provide less reluctance for the magnetic flux. This motor torque is also called
Saliency torque due to the saliency of the machine. This motor mainly depends on
the reluctance torque to operate. So this torque can be calculated by using the
following formula.

From the above equation, ‘V’ is applied voltage, ‘f’ is line frequency, 𝛿rel angle of
torque and ‘K’ is motor constant. The torque development can be done within the
motor because of the changing reluctance

Advantages

The advantages of reluctance motor include the following.

 It doesn’t require DC supply.
 Stable characteristics 
 Maintenance is less
 Less heat
 No magnets
 Speed control
Disadvantages

The disadvantages of reluctance motor include the following.

 Efficiency is less
 Power factor is poor
 Frequency control
 The capacity of these motors is less to drive the loads
 Less inertia rotor is required.
Applications

The applications of the reluctance motor include the following.

 Signaling Devices
 Control Devices
 Automatic regulators
 Recording Devices
 Clocks
 Tele printers
 Gramophones 
 Analog electric meters
 Electric vehicles
 Power tools like drill lathes, band saws & presses

8. Elaborate the construction and working of variable reluctance synchronous.

A variable reluctance stepper motor is electromechanical energy which

converts electrical energy to mechanical energy. The main difference between the
stepper motor and other DC motors is in stepper motors the motion of the rotor is
achieved in steps. Stepper motor also works on DC supply but the principle of
operation is not Lorentz force law. It works on the principle of reluctance. That is,
the magnetic flux always tries to follow in the least reluctance path. This principle
is also true for other reluctance motors such as switched reluctance motor and
synchronous reluctance motor. the working of variable reluctance stepper
motor depends on the principle of variable reluctance.

What is a Variable Reluctance Stepper Motor?

A variable reluctance stepper motor is an electromechanical energy

conversion device that converts electrical energy to mechanical energy. It works on
the principle of reluctance, according to which magnetic flux always flows through
a minimum reluctance path. The stator poles are excited with three-phase or single-
phase supply and the rotor is either made of permanent magnets or excited with
single-phase supply to obtain the rotor magnetic field.

Variable Reluctance Stepper Motor

There are two important features of variable reluctance motor as compared

to other reluctance motors. First is, in this case, the reluctance is not constant. It is
a variable reluctance. The variable reluctance means the magnitude of the magnetic
field is varied to obtain the variable reluctance. One more way of obtaining the
variable reluctance is by modifying the air gap between the stator rotor windings.
To obtain this, the outer surface of the rotor structure is made of tooth type slots
where the windings are placed. The second important difference is in this motor,
the rotor rotates not in continuous motion but in steps. The step angle is defined as

Step Angle = (360/m*Nr)

Where m is the number of stator phases and Nr is the number of rotor poles.

Variable Reluctance Stepper Motor Working Principle

Variable reluctance stepper motor works on the principle of variable

reluctance. When the stator winding is excited by three-phase or single-phase
supply, then it creates a rotating magnetic field around the stator windings. The
stator windings are excited by using a switching circuit. The switching circuit
consists of an inverter circuit, which converts the DC supply to three-phase AC
supply.

Switched Reluctance Motor Design

As shown in the circuit, the three-phase magnetic field is obtained by

connecting the switching circuit to the stator windings. The switching circuit
consists of the inverter circuit. An inverter circuit, six devices are connected to the
stator poles. The devices used are like MOSFET, IGBT, etc. the devices are
numbered as per the sequence of firing. The devices are classified into two groups.
Positive group and negative group. In the positive group, we have the devices
numbered as (1,3, and 5) and in the negative group, we have the devices numbered
as (2,4 and 6). The device, 1, and 2 are connected to one phase of stator windings,
let us say R phase. Similarly, device 3 and 4 are connected to phase Y, and 5 are
six are connected to B phase.

Now to obtain variable reluctance, the switching circuit is designed such

that, first R phase has maximum magnetic energy, then Y phase and then Y phase.
So when the R phase has maximum magnetic energy, the rotor will try to align
itself into the R phase. After a delay of some time, the maximum energy is shifted
to the Y phase. At this time, the rotor position is shifted to the Y phase through a
clockwise rotation. The rotation angle of the rotor is defined as the step angle given
by Step Angle = (360/m*Nr), where m is the number of phases and Nr is a number
of rotor poles. Interestingly, it may be noted that, in this motor, the number of
stator and rotor poles need not be the same.

Construction

In variable reluctance stepper motor construction, the stator poles are

made of silicon steel. Since the main role of stator poles is to produce a magnetic
field, they are made of good magnetic material. The windings are made of copper.
The stator windings in the case of three-phase machines are connected in delta or
star. The reason for connecting in star or delta is for producing a rotating magnetic
field. As per the theorem of rotating magnetic field, whenever three-phase
windings are excited with three-phase supply, a rotating magnetic field is created
which rotates at synchronous speed.

Construction

The rotor windings are made of ferromagnetic material. The ferromagnetic

material has the property of least reluctance. Whenever a ferromagnetic material is
placed under the influence of the magnetic field, then the magnetic field lines
completely pass through the material. No magnetic flux passes through the air. The
reason is air has high reluctance as compared to the reluctance of ferromagnetic
material. One more good property of ferromagnetic material is it produces the least
amount of eddy current and hysteresis losses. Due to the property of ferromagnetic
material, the number of magnetic cyclic variations are less due to which the
hysteresis losses are less. Similarly, to reduce the eddy current losses, the windings
of the rotor are well laminated.

Apart from the stator and rotor structure, the reluctance motor has switching
circuits to produce variable reluctance.

Variable Reluctance Stepper Motor Working

Variable reluctance stepper works fundamentally on the principle of variable

reluctance. To produce the variable reluctance, a switching circuit is used to excite
the stator windings at different phases. The switching circuit consists of an inverter
circuit which can produce a variable AC from a fixed DC supply. To obtain the
variable magnetic field, the triggering angle of the inverter circuit is varied
periodically such that each of the phases of the stator windings is excited with
maximum energy at one particular instant.

Generally in an inverter circuit, the delay angle triggering each device is 60

degrees. It can be varied periodically. One more method of obtaining the variable
reluctance is by designing the rotor and stator poles such that, air gap among them
is varied. Varied in the sense, at one instant, the air gap is maximum and at the
other instance, the air gap is minimum. This is obtained by using a toothed type of
rotor.
Motor Working

When one phase of the stator has maximum energy, the rotor is aligned to
that stator at that moment. As we discussed it follows the reluctance principle. As
per the reluctance principle, the magnetic flux flows through the minimum
reluctance path. Just like the current flows through the minimum resistance path.
The reluctance is the magnetic equivalent of resistance and flux is the magnetic
equivalent of current. So when one phase of the stator has maximum energy, the
rotor is aligned to it.

Once the energy is shifted, by using the switching circuit, the rotor also
shifts its position to the new phase where reluctance is minimum. The angle by
which it rotates is called a step angle. It is defined as Step Angle = (360/m*Nr),
where m is the number of phases and Nr is a number of rotor poles. For six poles, 4
phase the step angle would be approximately 15 degrees. It can be noted that the
step angle for a particular machine is fixed, as we can’t vary the number of phases
or number of rotor poles.

Advantages

The advantages of variable reluctance stepper motor include the following

 Its rotation is in stepped form. It is very useful for a particular operation

where we don’t need continuous motion.
 Losses are very less as most of the material is made of magnetic material.
 It can run with both AC and DC supply
 Due presence of electronic devices production of variable reluctance is very
easy.

Disadvantages of Variable reluctance stepper motor

  Speed control is not possible for this machine, as it rotates in steps.
 For a particular operation, the step size is fixed.
 Due to its step rotation, it has less number of applications
 The electronic devices cause some losses such as switching losses.

Applications

The applications of variable reluctance stepper motor include the following.

As mentioned, due to its stepped rotation it has less number of applications.

Mostly these types of motors are used in toys, tape recorders, advanced clocks,
printing machines, etc.

9 Derive the torque equation of synchronous reluctance motor.

Torque Equation of Synchronous Reluctance Motor

The working of both the permanent magnet synchronous motor and

synchronous reluctance motors are similar if the magnets are demagnetized
otherwise left. The torque equation for synchronous reluctance motor is shown
below. This equation includes two components; the first component is because of
the field. So this component must be left out to get the equation for torque. In the
following equation, the next component can be defined as reluctance torque.
So the reluctance motor’s developed torque can be expressed like the following.

Torque Equation of Synchronous Reluctance Motor

In the above equation, where

‘Te’ is the developed torque

‘P’ is the no. of poles

‘Ψ’ is the induced flux linkage through the field current

‘Lds’ is the direct axis inductance

‘Lqs’ is the Quadrature axis inductance.

‘δ’ is the Torque angle.

Synchronous reluctance motors are rugged, less cost & have high efficiency.
These motors operate at extremely high speeds. The conventional motors give poor
efficiency, low power factor & poor torque density because of the low saliency i.e,
low ratio of Ldm/Lqm.

But, the current development of these motors through anisotropic design has
a high Ldm/Lqm ratio, which has considerably enhanced power factor, efficiency
& torque density.

Phasor Diagram

The phasor diagram of synchronous reluctance motor includes the

following. The most important characteristic of this motor is its constant speed. In
the beginning, if the rotor fails to connect through the magnetic field of the stator,
then damper winding comes into the mind in that situation. These are also utilized
in synchronous motors. The arrangement of these windings can be done within
pole shoes that generate damping torque because of the disparity in the relative
speed among the rotor as well as the stator magnetic field.
Phasor Diagram

This occurs once the rotor not works to connect through the stator. The
damping torque generates depending on Lenz Law that seeks to resist the reason
for its construction, which is the speed disparity among the magnetic field of the
rotor as well as stator. Therefore, the damping torque moves the winding of the
rotor such that it is locked magnetically through the stator magnetic field. After
that, the rotor works at synchronous speed for the rest of the time.

The phase diagram of synchronous reluctance motor is illustrated above. The

two axes in the above diagram like d-axis & q-axis are defined depending on the
motor’s two-axis theory. Likewise, we can define Vd and Vq, which are the
voltage across the d and q axis. Here, gamma is the angle among ‘Is’ (stator
current) & d-axis. Like a rotor angle, this can also be defined and once the
synchronous torque is generated then it is a role of the rotor angle.

10. Elaborate the construction and working of hybrid synchronous reluctance

motor

A Hybrid Synchronous Reluctance Motor (HSRM) is a type of electric

motor that combines features of both synchronous reluctance motors and
 permanent magnet synchronous motors. It aims to leverage the advantages of both
technologies to achieve improved performance, efficiency, and power density.
Let's break down the construction and working of a Hybrid Synchronous
Reluctance Motor:

Construction:
1. Stator:
 Similar to other electric motors, the HSRM has a stationary part called the
stator.
 The stator typically consists of a laminated core made of magnetic steel
sheets with multiple slots to house the stator windings.
 Stator windings are usually made of copper or aluminum and are arranged in
a specific pattern to generate a rotating magnetic field when electric current
is applied.
2. Rotor:
 The rotor is the rotating part of the motor.
 In a synchronous reluctance motor, the rotor is a simple laminated structure
without any winding or magnets. It relies on the principle of reluctance to
align with the magnetic field.
 In an HSRM, however, the rotor is equipped with both salient poles and
permanent magnets. This combination enhances the motor's performance.
3. Permanent Magnets:
 Permanent magnets are strategically placed on the rotor to create a magnetic
flux that interacts with the stator's rotating magnetic field.
 The use of permanent magnets contributes to higher torque density and
efficiency compared to traditional reluctance motors.
 Working:
1. Synchronous Operation:
 Like synchronous reluctance motors, HSRMs operate in synchrony with the
supply frequency.
 The stator windings are energized with alternating current, creating a
rotating magnetic field.
 The rotor, which has both salient poles and permanent magnets, experiences
torque due to its tendency to align with the stator's magnetic field.
2. Reluctance Torque:
 The rotor's salient poles contribute to reluctance torque, which is generated
based on the magnetic reluctance of the rotor material.
 As the rotor turns, it seeks alignment with the magnetic field to minimize
reluctance, resulting in torque production.
3. Permanent Magnet Torque:
 The presence of permanent magnets on the rotor enhances the motor's
performance by providing additional torque.
 The magnetic attraction and repulsion between the stator field and the
permanent magnets contribute to the overall torque output.
4. Control and Efficiency:
 HSRMs are often controlled using advanced motor control techniques, such
as field-oriented control (FOC) or direct torque control (DTC), to optimize
performance.
 The combination of reluctance torque and permanent magnet torque allows
for better control over the motor's speed and torque characteristics.

In summary, a Hybrid Synchronous Reluctance Motor combines features of

synchronous reluctance motors and permanent magnet synchronous motors to
achieve a balance between cost-effectiveness and performance. The integration of
permanent magnets enhances torque density and efficiency, making HSRMs
suitable for various applications where a high level of performance is required.
UNIT 1
PART C
 Part C

1.Briefly explain about the constructional features of synchronous reluctance

motor.
A synchronous reluctance motor (SynRM) is an electric motor that operates based
on the principle of magnetic reluctance. It typically consists of the following key
constructional features:

Stator:
 The stator is the stationary part of the motor and is responsible for
generating a rotating magnetic field when supplied with electric current.
 It usually comprises a laminated core made of magnetic steel sheets to
minimize energy losses due to eddy currents.
 Stator windings, typically made of copper or aluminum, are placed in slots

on the stator core. These windings are connected to the power supply to
produce the magnetic field.

Rotor:

 The rotor is the rotating part of the motor and is designed to move in
synchrony with the rotating magnetic field produced by the stator.
 In a synchronous reluctance motor, the rotor is a simple laminated structure
without any winding or permanent magnets

 The rotor may have a salient pole design, where certain portions are shapedto
enhance the reluctance effect
 .
.
Salient Poles:
 The rotor often features salient poles, which are regions of the rotor that
protrude outward. These poles help create a variation in magnetic
reluctance as the rotor turns.
Constructional Symmetry:
 The stator and rotor are designed with symmetry to ensure smooth and
balanced operation.
 Symmetry in the construction helps in achieving a uniform distribution of
magnetic flux and facilitates efficient energy conversion.
Bearings:
 Bearings are employed to support the rotor and allow it to rotate smoothly.
Common types of bearings used include ball bearings or roller bearings.
Enclosure:
 The motor is usually enclosed in a housing or casing to protect the internal
components from environmental factors and ensure safety.

Working Principle:
Magnetic Reluctance:
 The synchronous reluctance motor operates on the principle of magnetic
reluctance, which is the opposition to the establishment of magnetic flux.
The rotor tends to align itself with the magnetic field produced by the stator
due to the lower reluctance path.
Synchronous Operation:
 The motor is designed to operate at synchronous speed, where the rotor
turns at the same speed as the rotating magnetic field produced by the
stator.
Torque Generation:
 Torque is generated as the rotor attempts to align with the changing
magnetic field produced by the stator. The reluctance torque is a result of
the rotor's tendency to minimize the magnetic reluctance.

Synchronous reluctance motors are known for their simplicity, robustness,

and efficiency. They are often used in applications where precise speed control
and high efficiency are essential.

.
2.Drive the voltage equation for synchronous reluctance motor.
3.A three – phase, 230 V, 60Hz, 4 pole, star – connected reluctance motor has
Xsd = 22.5 Ω and Xsq = 3.5 Ω. The armature resistance is negligible. The
load torque is TL = 12.5N-m. The voltage to frequency ratio is maintained
constant at rated value. If the supply frequency is 60 Hz, Determine (a) torque
angle (b) the line current (c) The input power factor.

Recommend a suitable type of synchronous reluctance motor for rewinding

mill.
 PF≈0.8444

4 Explain briefly with neat sketch about

(i) Axial flux motor
(ii) Radial flux motor

Axial Flux Motor

An axial flux motor is an electric motor design with a unique

configuration where the magnetic flux runs parallel to the motor's shaft.
This layout allows for compact, high-torque, and lightweight motor
designs, making it suitable for various applications, including electric
vehicles and renewable energy systems.
Axial flux motors are known for their efficiency and adaptability in
constrained spaces.

Axial Flux Motor Construction

Axial flux motor construction consists of a stator with windings, and

a rotor with permanent magnets, positioned in a flat, disc-like
arrangement. The various parts of the Axial Flux Motor are:

o Disk-shaped Stator and Rotor

o Radial Magnets
o Winding Coils
o Compact Design
o Low Profile
o High Torque Density
o Direct Cooling
o Axial Air Gap
Disk-shaped Stator and Rotor

Axial flux motors feature a unique construction, with both the

stator and rotor designed as flat, circular disks. This design allows for
a compact and space- efficient motor, making it particularly well-suited
for applications with limited installation space
Radial Magnets

The stator and rotor in an axial flux motor incorporate magnets

arranged radially, enabling an efficient and direct interaction between
the magnetic fields. This radial magnet arrangement enhances the
motor's torque production, making it suitable for various industrial and
automotive applications

Winding Coils
Copper wire coils are wound on the stator of the axial flux motor. These
winding coils are crucial in generating electromagnetic forces that drive
the motor's rotation. The precise arrangement and control of these coils are
essential for the motor's performance

Working of Axial Flux Motor

Upon energising the coil, it transforms into an electromagnet within the

axial motor. The operation of the axial flux motor relies on the interplay
between permanent magnets and these newly formed electromagnets. In
the design of axial flux motors, fixed coils coexist with freely rotating
permanent magnets
 .

Fig: Axial Flux Motor

When coil A is energised with a DC current, it triggers an attraction

between the S pole of the rotor and the opposite N pole of the stator.
Simultaneously, like poles repel each other. This interplay of tangential
force components sets the rotor in motion. However, when the rotor
aligns precisely with coil A, the net force acting on the permanent
magnet reaches equilibrium.

The rotor's inertia carries it beyond the point of perfect alignment.

Duringthis phase, the subsequent coil B is energised, thanks to the
magnetic forces of attraction and repulsion.
 Fig: Axial Flux Motor working

Following this, coil C receives its share of energy. In the subsequent half-
rotation, coil A is energised once more, but this time with reversed polarity,
achieved by altering the supply direction. This continuous repetition of the
process propels the rotor to maintain its rotation.

However, a drawback surfaces in this operation: two coils remain

inactive, as exemplified by coil B's inactivity. These dormant coils
significantly diminish the motor's power output.

To resolve this issue, we introduce an additional coil pair by simply

running the opposite polarity current through the second coil. This
arrangement aligns two south poles together on the stator side.
 Once again, a net tangential force is generated, as shown in the
figure. This combined effect not only yields more torque but also
enhances the power output of the rotor. Remarkably, this process
ensures a consistent torque output from the motor.

Axial Flux Motor vs. Radial Flux Motor

Axial flux motors have magnetic flux along the motor's shaft, making
them more compact, while radial flux motors have flux radially, typically
larger but with higher torque. The table below draws a comparison between
axial flux motor and radial flux motor.

Characteristic Axial Flux Motor Radial Flux Motor

Magnetic Flux Path Parallel to the Axis of Radial, from Center to
Rotation Edge
Motor Thickness Thin and Flat (Pancake Thicker, Cylindrical
Shape)
Rotor-Stator Coaxial Discs (Parallel) Nested Cylinders
Configuration (Concentric)
Efficiency Typically Higher in Variable, Depending
Some Applications on Design
Cooling More Effective Due to Cooling More
Flat Design Challenging
Size and Weight Compact and Lightweight Bulkier and Heavier
Applications Electric Vehicles, Generators,
Robotics, Some Wind Industrial Drives,
Turbines Appliances
Torque Handling Typically Lower Higher Torque Capacity
Torque Capacity
Manufacturing May Be Simpler in Some More Complex in
Complexity Cases Some Designs
Cooling and Enhanced Heat Dissipation May Require
Heat AdditionalCooling
Dissipation
Noise Level Generally Lower Variable, Dependent
Noise Emission on Design

Axial Flux Motor Advantages

The advantages of Axial Flow Motor are:

o Compact and lightweight design.

o High power-to-weight ratio.
 o Effective cooling due to flat structure.
o Suitable for electric vehicles and robotics.
o Greater torque density in some cases.

Axial Flux Motor Disadvantages

The limitations include:

o Lower torque capacity compared to radial flux motors.

o Limited use in high-torque industrial applications.
o More complex manufacturing in certain designs.
o Heat dissipation challenges may arise.
o Noise levels can vary depending on design.
Applications: Following are the applications of Axial Flux Motor:

o Electric vehicles (EVs) propulsion.

o Robotics and automation systems.
o Wind turbines for renewable energy.
o Electric fans and blowers.
o Small-scale hydroelectric generators.
o Electric bicycles (e-bikes).
o Aircraft propulsion systems.
o Industrial machinery and conveyors.
o Water pumps for irrigation.
o Marine propulsion for boats and ships.
RADIAL FLUX MOTOR

Design and Operation of Radial Flux Motors

Most conventional electric motors used today are of the radial

flux type. In these motors, the stator (the stationary part of the motor)
is designed around the rotor (the rotating part of the motor). The
magnetic field generated by the stator windings travels radially across
the air gap between the stator and rotor, thereby
inducing a torque that makes the rotor turn. This radial flow of magnetic
field gives these motors their name.

Advantages of Radial Flux Motors

5. Compactness: The radial design often allows for a more

compact motor, fitting into smaller spaces.
6. Efficiency: With a well-designed radial flux motor, higher
efficiency can be achieved, which can lead to significant energy
savings over time.
7. Robustness: Thanks to their design, radial flux motors are
generally more robust and durable than other types of motors.
Applications of Radial Flux Motors

Radial flux motors are extensively used in numerous applications,

owing to their high power density and cost-effectiveness. These
applications range from home appliances such as washing machines and
fans to electric vehicles and industrial machinery. In fact, most electric
vehicles on the road today use radial flux motors due to their high torque
capabilities and efficiency.
5. Explain briefly about synchronous reluctance motor drive with
necessary diagrams.
What is a Synchronous Reluctance Motor?

Synchronous reluctance motor is an electromechanical energy conversion

device, which converts electrical energy to mechanical energy. The motor
always runs at synchronous speed due to magnetic locking between the rotor
magnetic field and the stator magnetic field. In a DC motor, the torque is
produced due to interactions between stator and rotor magnetic fields, which is
also knowns as Lorentz force law.

Synchronous Reluctance Motor

But in synchronous reluctance motor, it follows the reluctance principle.

Constant reluctance torque is produced in the rotor which causes the motor to
run. It may be noted that in switched reluctance motor, a variable reluctance
torque is produced, which is produced because of switching circuits.
Switching circuits produce a rotating magnetic field at the stator. But in
synchronous switching circuits are not required as we are not going to vary the
speed of the motor.
Synchronous Reluctance Motor Principle

The fundamental concept behind the operation of this motor is the reluctance
principle. When a ferromagnetic material is placed under the influence of the
magnetic field, the magnetic lines flow through the material avoiding the path
outside or surrounding the material. Since in the surrounding, the air is present
and air has high reluctance as compared or reluctance of the materials.
Reluctance is equivalent to resistance in the magnetic field. As we know that,
the current always try to flow in the least resistance path, similarly, the flow of
magnetic lines which is also called flux lines, try to flow in the least reluctance
path.

When the stator is supplied with 3 phase supply, the stator windings which are
placed in star or delta configuration, produce a rotating magnetic field. The
reason for the production of rotating magnetic field is as per theorem of
rotating magnetic field according to which, whenever a three-phase winding is
supplied excited with a three-phase supply then it produces a rotating
magnetic field which rotates at a speed of synchronous speed given by
(120*f/P), where f is the frequency and P is the number of poles. For a six-
pole machine, the magnetic field rotates at 1000 RPM.

The rotating magnetic field, when crosses the air gap and interacts with rotor
windings, the rotor winding tends to attract the stator magnetic field.It must be
noted that the rotor is mostly of squirrel cage type like in an induction motor.
But whereas in a synchronous motor, the rotor is of a salient pole of the
cylindrical structure. When the rotor winding is cut by the stator magnetic
field, they too produce a rotating magnetic field and based on reluctance
principle, try to align with the stator magnetic field. Since the stator magnetic
field is rotating at synchronous speed, the rotor also starts rotating at
synchronous speed due to magnetic locking between the stator and rotor
magnetic field.

Please refer to this link to know more about Synchronous Motor MCQs

Phasor Diagram

One important characteristic of synchronous reluctance motor is its constant

speed. At the start, if the rotor fails to align with the stator magnetic field, in
that case, damper winding comes into the picture. They are also used in
synchronous motors. The damper windings which are placed in pole shoes,
produce damping torque due to relative speed difference between the rotor
magnetic field, and the stator magnetic field.

Phasor Diagram

This happens when the rotor fails to align with the stator. The damping torque
produced, according to Lenz Law tries to oppose the reason for its production,
which is the speed difference between rotor and stator magnetic field. Hence
the damping torque pushes the rotor winding such that it gets magnetically
locked with stator magnetic field. And afterward, the rotor runs on
synchronous speed for the remaining time. The synchronous reluctance
motor phasor diagram is shown above. The q axis and d axis are defined
based on the two-axis theory of synchronous machine. Similarly, we can
define Vd and Vq, which are the voltage across the d and q axis. Gamma is the
angle between d-axis and stator current Is. This is also defined as a rotor angle.
The synchronous torque produced is a function of the rotor angle.

Construction

Constructional features mainly include stator and rotor windings. The stator
winding is of three phases in nature. Which means that they are connected in
star or delta. The reason for this, when they are excited with a three-phase
supply, they must produce a rotating magnetic field. The stator winding is
made of silicon steel stampings.

Synchronous Motor Construction

The rotor windings are made up of ferromagnetic material. The reason for this,
since the rotor, has to align with stator magnetic material as per reluctance
principle, therefore the reluctance of the rotor windings must be least. As
shown in the figure, the armature windings are placed on the stator and the
field winding is placed on the rotor. It is just the opposite of the DC generator.
The reason is mainly due to insulation problems, and since the armature
windings carry the armature currents, it is better to keep them static. The stator
winding is excited with a three-phase supply as shown in the figure. In low
rating machines, permanent magnets are used for rotor windings. They don’t
need any separate excitation.

Synchronous Reluctance Motor Working

When the stator windings are excited with a three-phase supply, they produce
a rotating magnetic field in the stator windings. The magnetic field rotates at a
synchronous speed based on the number of poles and frequency. The
fundamental concept behind the reluctance motor is the reluctance principle.
The rotor windings are of squirrel cage in shape just like in induction motor.
When the rotor windings are excited with DC supply, they produce a magnetic
field at rotor windings. Now we have two magnetic fields, one is the stator
magnetic field and the other one is the rotor magnetic field. The stator
magnetic field is rotating at a speed of synchronous speed.

Now the rotor windings are constructed in such a manner that, when the stator
magnetic field tries to align with the rotor magnetic field, it forms a minimum
reluctance path. For that minimum reluctance path, the rotor tries to align itself
with the stator magnetic field, and in that process, it gets magnetically locked
with a stator magnetic field. If the stator magnetic field is rotating at
synchronous speed, the rotor also rotates are synchronous speed.
In case of any overloads, the rotor comes out of magnetic locking. The rotor
axis falls out of synchronism. Then immediately, there is the production of
damping torque in the damper windings. The damper windings try to bring the
rotor back to magnetic locking. This phenomenon is called hunting. Which is
more dominant in a synchronous motor. The machine tries to hunt the
synchronous state. Once the rotor is back to the magnetic locking, the damping
torque or synchronizing torque disappears. The same can be concluded for
underloads.

Advantages and Disadvantages

Due to its robust nature, the advantages and disadvantages of synchronous

reluctance motor include the following.

The advantages are

 Due to its magnetic locking concept, the machine is able to produce

constant speed under all circumstances. Any change in loads like
underload or overloads will be overcome by synchronizing torque. The
speed will be maintained in all respects.
 Due to the reluctance principle, it does not need any starting method like
in a synchronous motor that needs starting methods and not self-starting.
 The self-starting aspect of the machine makes it more robust.
 Due to a less complicated structure, it requires less maintenance.
 There are no ripple torques like in a switched reluctance motor.

The disadvantages are

  The biggest disadvantage of the motor is that, due to its constant speed
application, no speed control is possible. Since the speed cannot be
varied.
 Due to the need for both stator and rotor magnetic fields, the machine is
also known as a doubly excited machine. We need two excitations, one
for stator and the other for the rotor.
 Due to the presence of three-phase windings, copper losses are more, as
compared to DC motor.

Applications

The applications of synchronous reluctance motor include the following.

Mostly the motors are used in industry due to their property of speed control.
But since the synchronous reluctance motor has a constant speed and speed
control is not possible, it has very few applications. Like in conveyor belts,
rice mills, paper mills, where constant speed is required.

6.Recommend a suitable type of synchronous reluctance motor for

rewinding mill.

For a rewinding mill application, you'd typically want a synchronous

reluctance motor that can provide high torque and efficiency, as well as
operate reliably under varying load conditions. Here are some suitable
options:
1. High Torque Synchronous Reluctance Motor (HT-SynRM): These

motors are designed to deliver high torque density, making them ideal
for applications like rewinding mills where high torque is required
during start-up and operation.

2. Variable Speed Synchronous Reluctance Motor: A motor with

variable speed capabilities can offer precise control over the rewinding
process, allowing for optimal performance and energy efficiency.

3. Closed-loop Control Synchronous Reluctance Motor: Motors

equipped with closed-loop control systems can adjust their operation

in real-time based on load changes, ensuring consistent performance
and reliability in the rewinding mill.

4. Customized Synchronous Reluctance Motor: Depending on the

specific requirements of your rewinding mill, you may need a

customized motor solution tailored to your application's unique
demands, such as specific torque-speed characteristics or integration
with existing control systems.