121635132 Reluctance Motor

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Reluctance Motor

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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.

1

G.P.T.C, Muttom

Seminar Report on Reluctance Motor

2012-2013

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

Dept. Of Electrical & Electronics Engg.

2

G.P.T.C, Muttom

Seminar Report on Reluctance Motor

2012-2013

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

Dept. Of Electrical & Electronics Engg.

3

G.P.T.C, Muttom

Seminar Report on Reluctance Motor

2012-2013

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.

Dept. Of Electrical & Electronics Engg.

4

G.P.T.C, Muttom

Seminar Report on Reluctance Motor

2012-2013

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

Dept. Of Electrical & Electronics Engg.

5

G.P.T.C, Muttom

Seminar Report on Reluctance Motor

2012-2013

(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.

Dept. Of Electrical & Electronics Engg.

6

G.P.T.C, Muttom

Seminar Report on Reluctance Motor

2012-2013

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

Dept. Of Electrical & Electronics Engg.

7

G.P.T.C, Muttom

Seminar Report on Reluctance Motor

2012-2013

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

Dept. Of Electrical & Electronics Engg.

8

G.P.T.C, Muttom

Seminar Report on Reluctance Motor

2012-2013

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.

Dept. Of Electrical & Electronics Engg.

9

G.P.T.C, Muttom

Seminar Report on Reluctance Motor

2012-2013

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 roboticsDept. Of Electrical & Electronics Engg. 10 G.P.T.C, Muttom

Seminar Report on Reluctance Motor

2012-2013

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.

Dept. Of Electrical & Electronics Engg.

11

G.P.T.C, Muttom

Seminar Report on Reluctance Motor

2012-2013

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, whic