EE2403- SPECIAL ELECTRICAL MACHINES
UNIT I SYNCHRONOUS RELUCTANCE MOTOR
Synchronous reluctance motor
Construction:
1. Structure of reluctance motor is same as that of the salient pole synchronous machine
shown in fig.1.1 except that the rotor does not have any field winding.
2. The stator has a three phase symmetrical winding, which creates sinusoidal rotating
magnetic field in the air gap, and the reluctance torque is developed.
3. 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
4. The rotor of the modern reluctance machine is designed with iron laminations in the
axial direction separated by, nonmagnetic material as shown in fig.1.2 to increase the
reluctance to flux in the q-axis
5. Compared to the induction motor, it is slightly heavier and has a lower power factor.
6. 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.
7. 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.
8. The synchronous reluctance motor has no synchronous starting torque and runs up
from stand still by induction action.
9. There is an auxiliary starting winding. This has increased the pull out torque, the
power factor and the efficiency.
10. Synchronous reluctance motor is designed for high power applications. It can be
broadly be classified into
a) Axially laminated
b) Radially laminated
11. Three types of rotors used in synchronous reluctance motor. 1. Segmental 2. flux
barrier (radially laminated) 3. axially laminated structure.
12. Inductance of the stator windings in the dq reference 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.
Rotor design:
Rotor design:
Salient rotor (Segmental):
1. 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.
2. Salient rotor design is shown in fig.1.3 . The low Ld/Lq ratios are largely the result of
circulating flux in the pole faces of the rotor.
3. The ruggedness and simplicity of the rotor structure has encouraged study of this approach
for high speed applications.
Radially Laminated Rotor (Flux barrier):
1. Another approach is to use laminations with "flux barriers" punched into the steel for
a 4 pole machine as shown in fig.1.4.
2. 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.1.5 shows the axially laminated rotor.
1. This approach is to laminate the rotor in the axial direction.
2. 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.
3. It is observed that torque ripple and iron losses are more in axially laminated rotor
than radially laminated rotor. 4. Another rotor design is shown in fig.1.6 . In this case the rotor consists of alternating
layers of ferromagnetic and non-magnetic steel.
5. If choose the thickness of the steel such that the pitch of the ferromagnetic rotor
segments matched the slot pitch of the stator.
6. 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.
7. Special rotor laminations make it possible to produce the same number of reluctance
path as there are magnetic poles in the stator.
8. The rotor is pressure cast with end rings similar to induction motor. Stator windings
are similar to squirrel cage induction motor.
Rotor construction:
1. To construct the rotor, we are using a joining technique known as explosion bonding.
2. Explosion bonding uses explosive energy to force two or more metal sheets together
at high pressures.
3. The angle of collision between the two metals forces this fluid to jet outward.
4. 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.
5. 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.
6. The bond was also subjected to 10 cycles of temperature variation from 20C - 70C,
with no significant reduction in tensile strength.
7. Explosion bonding technique is shown in fig.1.7, other joining techniques such as
brazing, roll bonding, or diffusion bonding may also appropriate" for rotor
construction.
8. 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:
1. 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.
2. 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.
3. 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.1.8 .
4. 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.
5. 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.
6. Reluctance motors have approximately one-third the HP rating they would have as
induction motors with cylindrical rotors.
7. Although the ratio may be increased to one-half by proper design of the field
windings, power'. factor and efficiency are poorer than for the equivalent induction
motor.
8. 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
Primary Design Considerations:
1.
2.
3.
4.
5.
High output power capability .
Ability of the rotor to withstand high speeds
Negligible zero-torque spinning losses.
High reliability.
High efficiency.
6. Low cost
Torque and speed characteristics:
1. The motor starts at anywhere from 300 to 400 percent of its full load torque.
2. The magnetic rotating field created by a starting and running winding displaced 900 in
both space and time.
3. At about 3/4th of the synchronous speed a centrifugal switch opens the starting
winding and the motor continues to develope a single phase torque produced by its
running winding only.
4. it approaches synchronous speed, the reluctance torque (developed as a synchronous
motor) is sufficient to pull the rotor into synchronism with the pulsating single phase
field.
5. The motor operates at constant speed upto a little over 200% of its full load torque.
6. If it is loaded beyond the value of pull out torque, it will continue to operate as a
single phase induction motor upto 500% of its rated output.
Phasor diagram of and torque equation 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
where
V is the, supply voltage.
Is is the stator current
E is the excitation .emf
is the load angle
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.
From the voltage equation the phasor diagram neglecting resistance can
ADVANTAGES AND DISADVANTAGES OF SYNCHRONOUS RELUCTANCE MOTOR:
Advantages:
1. There is no concern with demagnetization, hence synchronous reluctance machines are
inherently more reliable than PM machines.
2. There need not be any excitation field as torque is zero, thus eliminating electromagnetic
spinning losses.
3. Synchronous reluctance machine rotors can be constructed entirely from high-strength, low
cost materials.
Disadvantages:
1. Compared to induction motor it is slightly heavier and has low power factor. But
increasing the saliency ratio Lds/Lqs , the power factor can be improved.
2. High cost than induction motor.
3. Need speed synchronization to inverter output frequency by using rotor position sensor and
sensor less control.
