121635132 reluctance motor
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Reluctance MotorTRANSCRIPT
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 1 G.P.T.C, Muttom
INTRODUCTION
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. 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.
The phase windings in a SRM are electrically isolated from each other,
resulting in higher fault tolerance than inverter-driven AC induction motors. The
optimal drive waveform is not a pure sinusoid, due to the non-linear torque relative
to rotor displacement, and the highly position-dependent inductance of the stator
phase windings.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 2 G.P.T.C, Muttom
TYPES OF RELUCTANCE MOTOR
Synchronous reluctance motor
Variable reluctance motor
Switched reluctance motor
Variable reluctance stepping motor
Synchronous reluctance
If the rotating field of a large synchronous motor with salient poles is de-
energized, it will still develop 10 or 15% of synchronous torque. This is due to
variable reluctance throughout a rotor revolution. There is no practical application
for a large synchronous reluctance motor. However, it is practical in small sizes.
If slots are cut into the conductorless rotor of an induction motor,
corresponding to the stator slots, a synchronous reluctance motor results. It starts
like an induction motor but runs with a small amount of synchronous torque. The
synchronous torque is due to changes in reluctance of the magnetic path from the
stator through the rotor as the slots align. This motor is an inexpensive means of
developing a moderate synchronous torque. Low power factor, low pull-out torque,
and low efficiency are characteristics of the direct power line driven variable
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 3 G.P.T.C, Muttom
reluctance motor. Such was the status of the variable reluctance motor for a
century before the development of semiconductor power control.
The application of Syrm is Used where regulated speed control is required in
applications sue as metering pumps and industrial process'equipment.
Classification of syrm
Axially laminated
Radially laminated
Advantages of syrm over pm machine?
More reliable than PM machine
applications of syrm?
Synthetic fiber manufacturing equipment
Wrapping and folding machine
Auxiliary' time mechanism
Synchronized conveyors
Metering pumps
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 4 G.P.T.C, Muttom
Variable reluctance Motor
If an iron rotor with poles, but without any conductors, is fitted to a multi-
phase stator, a switched reluctance motor, capable of synchronizing with the stator
field results. When a stator coil pole pair is energized, the rotor will move to the
lowest magnetic reluctance path. (Figure below) A switched reluctance motor is
also known as a variable reluctance motor. The reluctance of the rotor to stator flux
path varies with the position of the rotor.
Reluctance is a function of rotor position in a variable reluctance motor.
Sequential switching (Figure below) of the stator phases moves the rotor
from one position to the next. The mangetic flux seeks the path of least reluctance,
the magnetic analog of electric resistance. This is an over simplified rotor and
waveforms to illustrate operation.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 5 G.P.T.C, Muttom
Variable reluctance motor, over-simplified operation.
If one end of each 3-phase winding of the switched reluctance motor is
brought out via a common lead wire, we can explain operation as if it were a
stepper motor. (Figure above) The other coil connections are successively pulled to
ground, one at a time, in a wave drive pattern. This attracts the rotor to the
clockwise rotating magnetic field in 60o increments.
Various waveforms may drive variable reluctance motors. (Figure below)
Wave drive (a) is simple, requiring only a single ended unipolar switch. That is,
one which only switches in one direction. More torque is provided by the bipolar
drive (b), but requires a bipolar switch. The power driver must pull alternately high
and low. Waveforms (a & b) are applicable to the stepper motor version of the
variable reluctance motor. For smooth vibration free operation the 6-step
approximation of a sine wave (c) is desirable and easy to generate. Sine wave drive
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 6 G.P.T.C, Muttom
(d) may be generated by a pulse width modulator (PWM), or drawn from the
power line.
Variable reluctance motor drive waveforms: (a) unipolar wave drive, (b) bipolar
full step (c) sinewave (d) bipolar 6-step.
Doubling the number of stator poles decreases the rotating speed and
increases torque. This might eliminate a gear reduction drive. A variable reluctance
motor intended to move in discrete steps, stop, and start is a variable reluctance
stepper motor, covered in another section. If smooth rotation is the goal, there is an
electronic driven version of the switched reluctance motor. Variable reluctance
motors or steppers actually use rotors like those in Figure below.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 7 G.P.T.C, Muttom
Switched Reluctance Motors
A switched reluctance or variable reluctance motor does not contain any
permanent magnets. The stator is similar to a brushless dc motor. However, the
rotor consists only of iron laminates. The iron rotor is attracted to the energized
stator pole. The polarity of the stator pole does not matter. Torque is produced as a
result of the attraction between the electromagnet and the iron rotor.
The rotor forms a magnetic circuit with the energized stator pole. The
reluctance of a magnetic circuit is the magnetic equivalent to the resistance of an
electric circuit. The reluctance of the magnetic circuit decreases as the rotor aligns
with the stator pole. When the rotor is inline with the stator the gap between the
rotor and stator is very small. At this point the reluctance is at a minimum. This is
where the name &147;Switched Reluctance&148; comes from.
The inductance of the energized winding also varies as the rotor rotates.
When the rotor is out of alignment, the inductance is very low, and the current will
increase rapidly. When the rotor is aligned with the stator, the inductance will be
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 8 G.P.T.C, Muttom
very large and the slope decreases. This is one of the difficulties in driving a
switched reluctance motor.
the advantages od SRM?
Construction is very simple
Rotor carries no winding
No brushes and requires less maintenance
The disadvantages of SRM?
It requires a position sensor
Stator phase winding shold be capable of carrying magnetizing
currents
The applications of SRM?
Washing machines
Fans
Robotic control applications
Vacuum cleaner
Future auto mobile applications
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 9 G.P.T.C, Muttom
Variable reluctance stepper :
The variable reluctance stepper has a toothed non-magnetic soft iron rotor.
When the stator coil is energized the rotor moves to have a minimum gap between
the stator and its teeth.
The teeth of the rotor are designed so that when they are aligned with one
stator they get misaligned with the next stator. Now when the next stator is
energized, the rotor moves to align its teeth with the next stator. This way
energizing stators in a fixed sequence completes the rotation of the step motor.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 10 G.P.T.C, Muttom
The resolution of a variable reluctance stepper can be increased by
increasing the number of teeth in the rotor and by increasing the number of phases.
applications of stepper motor
floppy disc drives
qurtz watch
camera shutter operation
dot matrix and line printers
small tool application
robotics
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 11 G.P.T.C, Muttom
The advantages and disadvantages of stepper motor?
Advantages:
it can be driven in open loop without feedback
it is mechanically simple
it requires little or no maintenance.
Disadvantages:
low efficiency
fixed step angle
limited power output.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 12 G.P.T.C, Muttom
DESIGN AND OPERATING FUNDAMENTALS
The stator consists of multiple projecting (salient) electromagnet poles,
similar to a wound field brushed DC motor. The rotor consists of soft magnetic
material, such as laminated silicon steel, which has multiple projections acting as
salient magnetic poles through magnetic reluctance. For switched reluctance
motors, the number of rotor poles is typically less than the number of stator poles,
which minimizes torque ripple and prevents the poles from all aligning
simultaneously—a position which can not generate torque.
When a rotor pole is equidistant from the two adjacent stator poles, the rotor
pole is said to be in the "fully unaligned position". This is the position of maximum
magnetic reluctance for the rotor pole. In the "aligned position", two (or more)
rotor poles are fully aligned with two (or more) stator poles, (which means the
rotor poles completely face the stator poles) and is a position of minimum
reluctance.
When a stator pole is energized, the rotor torque is in the direction that will
reduce reluctance. Thus the nearest rotor pole is pulled from the unaligned position
into alignment with the stator field (a position of less reluctance). (This is the same
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 13 G.P.T.C, Muttom
effect used by a solenoid, or when picking up ferromagnetic metal with a magnet.)
In order to sustain rotation, the stator field must rotate in advance of the rotor
poles, thus constantly "pulling" the rotor along. Some motor variants will run on 3-
phase AC power (see the synchronous reluctance variant below). Most modern
designs are of the switched reluctance type, because electronic commutation gives
significant control advantages for motor starting, speed control, and smooth
operation (low torque ripple).
Dual-rotor layouts provide more torque at lower price per volume or per
mass.
The inductance of each phase winding in the motor will vary with position,
because the reluctance also varies with position. This presents a control systems
challenge.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 14 G.P.T.C, Muttom
OPERATING PRINCIPLE
The SRM has wound field coils as in a DC motor for the stator windings.
The rotor however has no magnets or coils attached. It is made of soft magnetic
material (laminated-steel protuberances). When power is delivered to the stator
windings, the rotor's magnetic reluctance creates a force that attempts to align the
rotor with the powered windings. In order to maintain rotation, adjacent windings
are powered up in turn. As the stator does not turn, the switching of power from
winding to winding may be difficult to arrange in a fashion that is properly timed
to the movement of the rotor - brushes could be used, but this would eliminate
most of the advantages of the design. Instead, in modern designs a high-power
electronic switching system is used, which also offers advantages in terms of
control and power shaping.
Simple switching
If the poles A0 and A1 are energized then the rotor will align itself with
these poles. Once this has occurred it is possible for the stator poles to be de-
energised before the stator poles of B0 and B2 are energized. The rotor is now
positioned at the stator poles b. This sequence continues through c before arriving
back at the start. This sequence can also be reversed to achieve motion in the
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 15 G.P.T.C, Muttom
opposite direction. This sequence can be found to be unstable[clarification needed]
while
in operation.
Improved sequence
A much more stable system can be found by using the following
"quadrature" sequence. First, stator poles A0 and A1 are energized. Then stator
poles of B0 and B1 are energized which pulls the rotor so that it is aligned in
between the stator poles of A and B. Following this the stator poles of A are de-
energized and the rotor continues on to be aligned with the stator poles of B, this
sequence continues through BC, C and CA before a full rotation has occurred. This
sequence can also be reversed to achieve motion in the opposite direction.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 16 G.P.T.C, Muttom
In addition to more stable operation, this sequence provides a well-timed
sequence as the timings of the phase being both on and off are equal, rather than
being at a 1:2 ratio as in the simpler sequence.
Control
The control system is responsible for giving the required sequential pulses to
the power circuitry in order to activate the phases as required. While it is possible
to do this using electro-mechanical means such as commutators or simple analog
or digital timing circuits, more control is possible with more advanced methods.
Many controllers in use incorporate programmable logic controllers (PLCs)
rather than electromechanical components in their implementation. A
microcontroller is also ideal for this kind of application since it enables a very
precise control of the phase activation timings. It also gives the possibility of
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 17 G.P.T.C, Muttom
implementing a soft start function in software form, in order to reduce the amount
of hardware required.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 18 G.P.T.C, Muttom
POWER CIRCUITRY
Asymmetric Bridge Converter
The most common approach to the powering of a switched reluctance motor
is to use an asymmetric bridge converter.
There are 3 phases in an asymmetric bridge converter corresponding to the
phases of the switched reluctance motor. If both of the power switches on either
side of the phase are turned on, then that corresponding phase shall be actuated.
Once the current has risen above the set value, the switch shall turn off. The energy
now stored within the motor winding shall now maintain the current in the same
direction until that energy is depleted.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 19 G.P.T.C, Muttom
N+1 Switch And Diode
This basic circuitry may be altered so that fewer components are required
although the circuit shall perform the same action. This efficient circuit is known
as the (n+1) switch and diode configuration.
A capacitor, in either configuration, is used to suppress electrical and
acoustic noise by limiting fluctuations in the supply voltage.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 20 G.P.T.C, Muttom
ADVANTAGES
Simple construction- no brushes, commutator, or permanent magnets, no Cu
or Al in the rotor.
High efficiency and reliability compared to conventional AC or DC motors.
High starting torque.
Cost effective compared to bushless DC motor in high volumes.
Adaptable to very high ambient temperature.
Low cost accurate speed control possible if volume is high enough.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 21 G.P.T.C, Muttom
DISADVANTAGES
Current versus torque is highly nonlinear
Phase switching must be precise to minimize ripple torque
Phase current must be controlled to minimize ripple torque
Acoustic and electrical noise
Not applicable to low volumes due to complex control issues
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 22 G.P.T.C, Muttom
CONCLUSION
Reluctance motors can deliver very high power density at low cost, making
them ideal for many applications. A switched reluctance motor has a stator with a
first set of poles directed toward levitating a rotor horizontally within the stator. A
disc shaped portion of a hybrid rotor is affected by the change in flux relative to
the current provided at these levitation poles. A processor senses the position of the
rotor and changes the flux to move the rotor toward center of the stator. A second
set of poles of the stator are utilized to impart torque upon a second portion of the
rotor. These second set of poles are driven in a traditional switched reluctance
manner by the processor.
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 23 G.P.T.C, Muttom
REFERENCES
http://www.freepatentsonline.com
www.allaboutcircuits.com
T. J. E. Miller, Switched Reluctance Motors and Their Control, Magna
Physics Publishing and Clarendon press, Oxford, 1993
R. Krishnan, Switched Reluctance Motor Drives: Modelling, Simulation,
Analysis, Design, and Applications, CRC Press, 2001
Seminar Report on Reluctance Motor 2012-2013
Dept. Of Electrical & Electronics Engg. 24 G.P.T.C, Muttom
ABSTRACT
Reluctance motors can deliver very high power density at low cost, making
them ideal for many applications. A switched reluctance motor has a stator with a
first set of poles directed toward levitating a rotor horizontally within the stator. A
disc shaped portion of a hybrid rotor is affected by the change in flux relative to
the current provided at these levitation poles. A processor senses the position of the
rotor and changes the flux to move the rotor toward center of the stator. A second
set of poles of the stator are utilized to impart torque upon a second portion of the
rotor. These second set of poles are driven in a traditional switched reluctance
manner by the processor.