Reluctance motor

Seminar Report on Reluctance Motor

2012-2013

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.

Dept. Of Electrical & Electronics Engg.

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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 deenergized, 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


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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


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Variable reluctance Motor If an iron rotor with poles, but without any conductors, is fitted to a multiphase 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.


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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


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(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.


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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


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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


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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.


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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 Dept. Of Electrical & Electronics Engg.

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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.


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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


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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 3phase 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.


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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 deenergised 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 Dept. Of Electrical & Electronics Engg.

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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 deenergized 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.


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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


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implementing a soft start function in software form, in order to reduce the amount of hardware required.


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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.


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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.


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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.


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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


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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.


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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


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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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Reluctance motor

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