Introduction. Introduction. Switched Reluctance Motors. Introduction

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1 UNIVERSITY OF TECHNOLOGY, SYDNEY FACULTY OF ENGINEERING Electrical Energy Technology Switched Reluctance Motors Topics to cover: 1. Introduction 2. Structures & Torque Production 3. Drive Circuits 4. Performance Introduction The reluctance motor is an electric motor in which torque is produced by the tendency of its moveable part to move to a position where the inductance of the excited winding is maximized. Basic Structure Introduction Introduction 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. 3 Phase 6/4 pole 4 Phase 8/6 pole It can be seen that both the stator and rotor have salient poles; hence, the machine is a doubly salient, singly excited machine.

Introduction The SR motor is designed for efficient power conversion at high speeds

Introduction The particular advantages are: (1) simple rotor structure, possibly with low

easy to cool; (4) higher permissible rotor temperature since there are no magnets; (5)

run at extremely high speeds; (8) easy adjustment of the torque/speed characteristic to

2 Introduction The SR motor is designed for efficient power conversion at high speeds comparable with those of the permanent magnet brushless DC motor. Introduction The particular advantages are: (1) simple rotor structure, possibly with low inertia; (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 - Cont. An Axial Field SRM A 8/6 poles short flux path SRM

SRM Configurations - Cont. SRM Configurations - Cont.

from the alignment, or to an appropriate position, so as to enable the generation of

A three phase linear SRM Operation Principles Operation Principles - Cont.

3 SRM Configurations - Cont. SRM Configurations - Cont. A Single phase SRM 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 electromagnetic torque. A three phase linear SRM Operation Principles Operation Principles - Cont. Cross sectional model of a three phase VR motor, winding arrangement, and equilibrium position with phase 1 excited How rotor rotates when excitation is switched from Ph1 to Ph2

Operation Principles - Cont. Operation Principles - Cont. Rotor rotation as switching sequence proceeds in a three phase VR motor Operation Principles - Cont.

Since the movement of the rotor, and 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 SR

4 Operation Principles - Cont. Operation Principles - Cont. Rotor rotation as switching sequence proceeds in a three phase VR motor Operation Principles - Cont. Therefore, the conduction angle for the phase current is controlled and synchronized with the rotor position, usually by means of a shaft position sensor. Since the movement of the rotor, and 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 SR motor is referred to as a switched reluctance motor (SRM). Torque Production The torque production in SRM can be explained using the elementary principle of electromechanical energy conversion. The incremental mechanical energy in terms of the electromagnetic torque and change in rotor position is: Δ W = TΔθ m e

5 T Torque Production - Cont. The electromagnetic torque is then: e ' ' 2 ΔW ΔWf W m f L( θ, i) i = = = = Δθ Δθ θ θ 2 where: L(θ, i) is the stator inductance at a particular position, and i the stator phase current. Torque Production - Cont. 1. The torque is proportional to the square of the current and hence, the current can be unipolar to produce unidirectional torque. 2. Since the torque is proportional to the square of the current, it has a good starting torque. 3. Because the stator inductance is nonlinear, a simple equivalent circuit development for SRM is not possible. Torque Production - Cont. The torque characteristics of SRM are dependent on the relationship between flux linkages and rotor position as a function of current. Torque Production - Cont. 0 ~ θ 1 : T e = 0 θ 1 ~ θ 2 : T e > 0 θ 2 ~ θ 3 : T e = 0 θ 3 ~ θ 4 : T e < 0 θ 4 ~ θ 5 : T e = 0

6 Torque Production - Cont. For rectangular currents, it can be seen that the motoring torque is produced for a short duration in pulsed form, resulting in a large torque ripple. Two ways can be applied to reduce the torque ripples: Optimal design of inductance profile Shape the phase current Electrical equation Equivalent Circuit di dl( θ, i) V = Ri s + L( θ, i) + ωmi dt dθ di = Ri s + L( θ, i) + e dt Equivalent Circuit - Cont. SRM Drive System DC Supply Variable Reluctance Motor Position Sensor Electronic Commutator Logic Circuit Switched Reluctance Motor = Variable Reluctance Motor + Electronic Commutator

7 Position Sensors Position Sensors - Cont. Commonly used position sensors are: Phototransistors and photodiodes Hall elements Magnetic sensors Pulse encoders Variable Differential Transformers Phototransistor position sensor Position Sensors - Cont. Power Converters for SRM 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

8 Asymmetric Bridge Converter Asymmetric Bridge Converter - Cont. Operation waveforms (n+1) switches topology - Cont. Bifilar Type Drive Circuit

switch can be very much higher than the source

The bifilar winding increases the complexity of

C-Dump converter has the advantage of minimum

9 Bifilar Type Drive Circuit - Cont. C-Dump Converter 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 - Cont. Control of SRM. 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

The average torque is controlled by adjusting the magnitude of winding current I p or by varying the dwell

10 Control of SRM - Cont.. An average torque will result due to the combined instantaneous values of torque pulses of all machine phases. The average torque is controlled by adjusting the magnitude of winding current I p or by varying the dwell angle θ d. Control of SRM - Cont.. Control of SRM - Cont.. Torque/Speed Characteristic Typical phase current waveforms at different speeds and advance angles. General torque/speed characteristic of SR motor

CHAPTER 2 MODELLING OF SWITCHED RELUCTANCE MOTORS

9 CHAPTER 2 MODELLING OF SWITCHED RELUCTANCE MOTORS 2.1 INTRODUCTION The Switched Reluctance Motor (SRM) has a simple design with a rotor without windings and a stator with windings located at the poles.