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SWITCHED RELUCTANCE MOTOR

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Introduction The reluctance motor is a type of synchronous machine. It

has wound field coils of a DC motor for its stator windingsand has no coils or magnets on its rotor.

Unlike a conventional synchronous motor, both the rotor and

stator of a switched reluctance motor have salient poles.

Hence, the machine is a doubly salient machine.

This doubly salient arrangement is very effective for

electromagnetic energy conversion.

Areluctance motor is a type ofelectric motor that induces non-

permanent magnetic poles on the ferromagnetic rotor. Torque is

generated through the phenomenon ofmagnetic reluctance.

http://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Ferromagnetichttp://en.wikipedia.org/wiki/Magnetic_reluctancehttp://en.wikipedia.org/wiki/Magnetic_reluctancehttp://en.wikipedia.org/wiki/Magnetic_reluctancehttp://en.wikipedia.org/wiki/Magnetic_reluctancehttp://en.wikipedia.org/wiki/Ferromagnetichttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Electric_motor

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SRMs are made up of laminated stator and rotor cores with

Ns=2mqpoles on the stator and Nr poles on the rotor. The

number of phases is m and each phase is made up of

concentrated coils placed on 2q stator poles. Most favored

configuration amongst many more options are 6/4 three

phase and 8/6 four phase SRMs as shown in the figure 1(a).

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Operation of SRM

The stator carries coils on each pole, the coils on opposite

poles being connected in series.

The rotor is aligned whenever the diametrically opposite

stator poles are excited.

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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 therotor and stator is very small. At this point the reluctance

is at a minimum.

Reluctance is the property of a magnetic circuit by virtue ofwhich it opposes the production of flux in the magnetic

circuit.

That is why it is called as switched reluctance motor.

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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.

Torque is produced as a result of the attraction between

the electromagnet and the iron rotor.

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consider that the rotor poles r1 and r1 and stator poles c

and c are aligned. Apply a current to phase a with the

current direction as shown in Fig.2a.

A flux is established through stator poles a and a and

rotor poles r2 and r2 which tends to pull the rotor poles r2

and r2 towards the stator poles aand a, respectively.

When they are aligned, the stator current of phase a is

turned off and the corresponding situation is shown in

Fig.2b. Now the stator winding b is excited, pulling r1 and

r1 toward band b, respectively, in a clockwise direction.

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Likewise, energizing phase cwinding results in the alignment

of r2 and r2 with c and c, respectively. Accordingly, by

switching the stator currents in such a sequence, the rotor is

rotated. i.e abc sequence for clock wise.

Similarly, the switching of current in the sequence ofacbwill

result in the reversal of rotor rotation. Since the movement

of the rotor, hence the production of torque and power,

involves a switching of currents into stator windings when

there is a variation of reluctance, this variable speed motor

is referred to as a switched reluctance motor (SRM).

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i.e The movement of the rotor, hence the production of

torque and power, involves switching of currents into stator

windings when there is a variation of reluctance;

therefore, this variable speed motor drive is referred to as a

switched reluctance motor drive.

Working animation:

http://www.youtube.com/watch?v=LXJUYumwh-k

Video file

http://www.youtube.com/watch?v=LXJUYumwh-khttp://www.youtube.com/watch?v=LXJUYumwh-khttp://www.youtube.com/watch?v=LXJUYumwh-khttp://www.youtube.com/watch?v=LXJUYumwh-khttp://www.youtube.com/watch?v=LXJUYumwh-khttp://www.youtube.com/watch?v=LXJUYumwh-khttp://www.youtube.com/watch?v=LXJUYumwh-khttp://www.youtube.com/watch?v=LXJUYumwh-khttp://www.youtube.com/watch?v=LXJUYumwh-khttp://www.youtube.com/watch?v=LXJUYumwh-k

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TYPES OF SRMS:

The initial classification is made on the basis of the nature ofthe motion (i.e., rotating or linear).

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The rotary machine-based SRMs are classified as

1. Radial field SRM.

2. Axial field SRM.

If the magnetic field path is perpendicular to the shaft, which

may also be seen as along the radius of the cylindrical stator

and rotor, the SRM is classified as Radial field SRM.

When the flux path is along the axial direction, the machine

is called an Axial field SRM.

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Radial field SRMs is most commonly used.

Radial field SRMs can be divided into shorter and longer

flux paths based on how a phase coil is placed.

The conventional one is the long flux path SRMs, in which

the phase coil is placed in the diametrically opposite slots.

In the shorter flux path SRMs, the phase coil is placed in

the slots adjacent to each other.

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The axial field configuration of a SRM

This type of SRMs is ideal for applications where the total

length may be constrained, such as in a ceiling fan or in a

propulsion application. These may be single-stack or

multi stack.

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SRM DRIVE SYSTEM AND CONTROL:

. It is made up of four basic components: power converter,control logic circuit, position sensor and the switched

reluctance motor.

i d i i f h

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Motoring and regeneration actions of the SRM

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ADVANTAGES AND DISADVANATGES OF SRM

Machine construction is simple and low-cost because of the

absence of rotor winding and permanent magnets. Bidirectional currents are not necessary, which facilitates

the reduction of the number of power switches.

The torquespeed characteristics of the motor can bemodified to the application requirement more easily during

the design stage than in the case of induction and PM

machines. The starting torque can be very high without the problem

of excessive in-rush current due to its higher self-

inductance.

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ADVANTAGES AND DISADVANATGES OF SRM conti..

The maximum permissible rotor temperature is higher,

since there are no permanent magnets.

Extremely high speeds with a wide constant power

region are possible.

High efficiency, high power.

Very robust because rotor has no windings and slip

rings.

Four quadrant operations are possible.

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Disadvantages of SRM

The torque ripple and acoustic noise are the most critical.

The doubly salient structure of the SRM also causes higher

acoustic noise compared with other machines.

Stator copper losses will be higher.

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APPLICATIONS OF SRM:

General purpose industrial drives.

Electric traction.

Application-specific drives: compressors, fans, pumps,

centrifuges.

Domestic drives: food processors, washing machines,

vacuum cleaners.

Electric vehicle application.

Aircraft applications.

Servo-drives.

Automotive applications such as power steering

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