Srm

Switched Reluctance Machine

SWITCHED RELUCTANCE MACHINE

Schematic diagram of 8/6 SRM

Stator

Excited Stator pole

Salient pole Rotor Torque

SWITCHED RELUCTANCE MACHINE

SWITCHED RELUCTANCE MACHINE

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A reluctance machine is one in which torque is produced by the tendency of its movable part to move to a position where the inductance of the excited winding is maximized.

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SRM - salient poles on both the rotor and stator.

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Excitation is provided to a concentrated winding on each stator pole

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The excitation of diametrically opposite stator poles when the rotor poles are nearby, creates a torque tending to align the stator and rotor poles.

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The synchronization of the turn on of the excitation with rotor position can be accomplished with simple rotor position feed back.

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The inductance L reaches maximum of L max at aligned position and a minimum of L min at unaligned position.

ADVANTAGES OF SRM: The rotor is simple and requires relatively few manufacturing steps, it also tends to have a low inertia. The stator is simple to wind, the end-turns are short and robust and have no phase-phase crossovers. In most applications the bulk of the losses appear on the stator, which is relatively easy to cool. The torque is independent of the polarity of phase current, for certain applications this permits a reduction in the number of power semiconductor switches needed in the controller. Most converter circuits used with SR motors are immune from shoot- through faults, unlike the inverters used with a.c. and brushless d.c. drives. Extremely high speeds are possible. The torque/speed characteristic can be 'tailored' to the application

DEMERITS Torque Ripple. Acoustic Noise. SRM & Stepper Motor SRM-Conduction angle for Phase Currents is controlled and synchronized with the rotor position. SRM – Efficient Power Conversion at high speeds.

Flux lines in aligned position

Flux lines in unaligned position Unaligned Position:Conjunction of any rotor interpolar axis with the axis of stator poles

Design Constraints ε = 2∏/qNr

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Step Angle

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Fundamental Switching Frequency = n Nr;

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To get largest possible variation of inductance 2∏ Nr

n=rev/s

βr > βs

To get largest possible variation of phase inductance interpolar arc of rotor must exceed stator pole arc

FEASIBLE POLE ARCS

SRM CHARACTERISTICS

The flux-current characteristics in the unaligned position is approximately linear because the magnetic path is dominated by large air gap and flux densities are small. In the aligned position the air gap reluctance is small and flux density is high, which causes high saturation at higher currents.

SRM TORQUE SPEED CHARACTERISTICS

CONSTANT TORQUE REGION • Current, and hence torque, kept constant by PWM or chopping. • At low speeds current rises instantaneously due to small back-emf. • At medium speeds, phase advancing is necessary. Phase turn-off is also advanced so that current decays to zero before rotor passes alignment. PWM or chopping is still possible. CONSTANT POWER REGION • High back-emf forces current to decrease once pole overlap begins. • PWM or chopping no longer possible. • Conduction angle is increased in proportion to speed, primarily through phase advancing.