Introduction Introduction UNIVERSITY OF TECHNOLOGY, SYDNEY UNIVERSITY OF TECHNOLOGY, SYDNEY FACULTY OF ENGINEERING FACULTY OF ENGINEERING 48550 Electrical Energy Technology 48550 Electrical Energy Technology The reluctance motor is an electric motor in which torque is produced by the tendency Switched Reluctance Motors Switched Reluctance Motors of its moveable part to move to a position Topics to cover: where the inductance of the excited winding 1. Introduction is maximized. 2. Structures & Torque Production 3. Drive Circuits 4. Performance Introduction Introduction Introduction Introduction • Basic Structure • The reluctance motor is a type of synchronous machine. It has wound field coils of a DC motor for its stator windings and has no coils or magnets on its rotor. • It can be seen that both the stator and rotor have salient poles; hence, the machine is a doubly salient, singly excited machine. 3 Phase 6/4 pole 4 Phase 8/6 pole
Introduction Introduction Introduction Introduction The particular advantages are: The SR motor is designed for efficient power conversion at (1) simple rotor structure, possibly with low inertia; high speeds comparable with those of the permanent magnet brushless DC motor. (2) simple and reliable stator windings; (3) major loss on the stator, which is easy to cool; (4) higher permissible rotor temperature since there are no magnets; (5) simple unipolar drive; (6) high starting torque without inrush currents; (7) ability to run at extremely high speeds; (8) easy adjustment of the torque/speed characteristic to meet the application requirements. SRM Configurations SRM Configurations - SRM Configurations SRM Configurations - Cont. Cont. An Axial Field SRM A 8/6 poles short flux path SRM
SRM Configurations - SRM Configurations - SRM Configurations SRM Configurations - Cont. Cont. - Cont. Cont. A permanent magnet on the stator is used to pull the rotor away from the alignment, or to an appropriate position, so as to enable the generation of maximum electro- magnetic torque . A Single phase SRM A three phase linear SRM Operation Principles Operation Principles - Operation Principles Operation Principles - Cont. Cont. Cross sectional model of a three phase VR motor, winding How rotor rotates when excitation is switched from Ph1 to Ph2 arrangement, and equilibrium position with phase 1 excited
Operation Principles - Operation Principles - Operation Principles Operation Principles - Cont. Cont. - Cont. Cont. Rotor rotation as switching sequence proceeds in a three phase VR motor Operation Principles - Torque Production Operation Principles Torque Production - Cont. Cont. Therefore, the conduction angle for the phase The torque production in SRM can be explained current is controlled and synchronized with the rotor using the elementary principle of electro- position, usually by means of a shaft position sensor. mechanical energy conversion. The incremental mechanical energy in terms of Since the movement of the rotor, and hence the the electromagnetic torque and change in rotor production of torque and power, involves a switching position is: of currents into stator windings when there is a Δ = Δ θ W T variation of reluctance, this variable speed SR motor m e is referred to as a switched reluctance motor (SRM).
Torque Production - Torque Production - Torque Production Torque Production - Cont. Cont. - Cont. Cont. 1. The torque is proportional to the square of the current The electromagnetic torque is then: and hence, the current can be unipolar to produce unidirectional torque. Δ ∂ Δ ∂ θ ' ' W W 2 W L ( , ) i i = = = = ⋅ 2. Since the torque is proportional to the square of the f f m T Δ θ Δ θ ∂ θ ∂ θ current, it has a good starting torque. e 2 3. Because the stator inductance is nonlinear, a simple where: L ( θ , i ) is the stator inductance at a particular equivalent circuit development for SRM is not position, and i the stator phase current. possible. Torque Production - Torque Production - Torque Production Torque Production - Cont. Cont. - Cont. Cont. • 0 ~ θ 1 : T e = 0 • The torque characteristics of SRM are • θ 1 ~ θ 2 : T e > 0 dependent on the relationship between • θ 2 ~ θ 3 : T e = 0 flux linkages and rotor position as a • θ 3 ~ θ 4 : T e < 0 function of current. • θ 4 ~ θ 5 : T e = 0
Torque Production - Equivalent Circuit Torque Production Equivalent Circuit - Cont. Cont. • For rectangular currents, it can be seen that the Electrical equation motoring torque is produced for a short duration in pulsed form, resulting in a large torque ripple. θ di dL ( , ) i = + θ + ω V R i L ( , ) i i θ s m dt d • Two ways can be applied to reduce the torque di ripples: = + θ + R i L ( , ) i e s –Optimal design of inductance profile dt –Shape the phase current Equivalent Circuit - SRM Drive System Equivalent Circuit SRM Drive System - Cont. Cont. Variable Reluctance DC Motor Supply Position Sensor Logic Circuit Electronic Commutator Switched Reluctance Motor = Variable Reluctance Motor + Electronic Commutator
Position Sensors Position Sensors - Position Sensors Position Sensors - Cont. Cont. Commonly used position sensors are: Phototransistors and photodiodes – – Hall elements – Magnetic sensors – Pulse encoders – Variable Differential Transformers Phototransistor position sensor Position Sensors - Power Converters for SRM Position Sensors Power Converters for SRM - Cont. Cont. • Since the torque in SRM drives is independent of the excitation current polarity, the SRM drives require only one power switch per phase winding. Hall element position sensor
Asymmetric Bridge Converter Asymmetric Bridge Converter Asymmetric Bridge Converter - Asymmetric Bridge Converter - Cont. Cont. Operation waveforms (n+1) switches topology n+1) switches topology - Bifilar Type Drive Circuit ( Bifilar Type Drive Circuit - Cont. Cont.
Bifilar Type Drive Circuit - C- -Dump Converter Dump Converter Bifilar Type Drive Circuit C - Cont. Cont. • The voltage across the power switch can be very much higher than the source voltage. • The bifilar winding increases the complexity of the motor. C- -Dump Converter Dump Converter - Control of SRM . C Control of SRM - Cont. Cont. . C-Dump converter has the advantage of minimum power switches allowing independent phase current control. The torque produced in one phase for motoring and regeneration
Control of SRM - Control of SRM - Control of SRM Control of SRM - Cont.. Cont.. - Cont.. Cont.. An average torque will result due to the combined The average torque instantaneous values of torque pulses of all machine is controlled by phases. adjusting the magnitude of winding current I p or by varying the dwell angle θ d . Control of SRM - Torque/Speed Characteristic Control of SRM Torque/Speed Characteristic - Cont.. Cont.. Typical phase current waveforms at different speeds and advance angles. General torque/speed characteristic of SR motor
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