121635132 Reluctance Motor

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.

http://en.wikipedia.org/wiki/Electric_motor

http://en.wikipedia.org/wiki/Ferromagnetic

http://en.wikipedia.org/wiki/Magnetic_reluctance

http://en.wikipedia.org/wiki/Stepper_motor

http://en.wikipedia.org/wiki/Electrical_equipment_in_hazardous_areas

http://en.wikipedia.org/wiki/Fault_tolerance

http://en.wikipedia.org/wiki/Sinusoid



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

http://en.wikipedia.org/wiki/Switched_reluctance_motor



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



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.



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



(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.



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



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



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.



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



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.



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

http://en.wikipedia.org/wiki/Stator

http://en.wikipedia.org/wiki/Electromagnet

http://en.wikipedia.org/wiki/Silicon_steel





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.

http://en.wikipedia.org/wiki/Solenoid

http://en.wikipedia.org/wiki/Ferromagnetic

http://en.wikipedia.org/wiki/Metal

http://en.wikipedia.org/wiki/Magnet

http://en.wikipedia.org/wiki/3-phase_AC

http://en.wikipedia.org/wiki/3-phase_AC

http://en.wikipedia.org/wiki/Electric_power

http://en.wikipedia.org/wiki/Commutator_%28electric%29

http://en.wikipedia.org/wiki/Inductance

http://en.wikipedia.org/wiki/Control_system



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

http://en.wikipedia.org/wiki/DC_motor




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.

http://en.wikipedia.org/wiki/Wikipedia:Please_clarify

http://en.wikipedia.org/wiki/File:SRM_Simple_Sequence.jpg



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

http://en.wikipedia.org/wiki/File:SRM_Final_Sequence.jpg



implementing a soft start function in software form, in order to reduce the amount

of hardware required.

http://en.wikipedia.org/wiki/Soft_starter



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.

http://en.wikipedia.org/wiki/File:Asymmetric_Bridge_Converter.jpg






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.

http://en.wikipedia.org/wiki/File:N+1_switch_and_diode.jpg






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.



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



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.



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

http://www.freepatentsonline.com/

http://www.allaboutcircuits.com/



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.

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121635132 Reluctance Motor

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