Direct Torque Control of a 6-Phase Switched Reluctance Motor

Transcription

1 Direct Torque Control of a 6-Phase Switched Reluctance Motor Xu Deng, Prof. Barrie Mecrow and Dr. Shady Gadoue Electrical Power Research Group School of Electrical and Electronic Engineering Newcastle University

2 Outline Background Conventional SRM Control Method Direct Torque Control Method for a 6-phase SRM Simulation Conclusion

3 Background rotor of a six phase SRM stator of a six phase SRM Rotor: no winding or permanent magnet. Stator: concentric windings. Advantages: Simplest structure of all electrical motors with high fault-tolerant. Applications: aerospace, electric vehicles, high-speed drives, small automotive applications, cooling fans and pumps

4 Background Disadvantages: Doubly salient structure Highly unlinear magnetization characteristics Ununiform torque output 16 Torque Output(Nm) relationship between rotor position, current and torque relationship between rotor position, flux and current Time(s) Large torque ripple: vibration of stator main source of noise application limited 3%

5 Conventional SRM Control Method A Voltage Current of of single single phase/(v) phase(a) Current of single phase(a) A 14 A A A Torque sharing function (TSF) Torque command Current or voltage reference profile -5 Torque Torque output of single phase()nm 15 Torque output output of of single single phase(nm) phase()nm Time(s) Time(s) Single phase torque output driven by Current Chopping Control Voltage of single phase/(v) 5-5 Torque output of single phase(nm) Time(s) Single phase torque output driven by Angle Position Control

6 Conventional SRM Control Method Q1: If we can control instantaneous torque directly, rather than by controlling current or voltage, can we achieve a better torque performance? Q2: How can we control the torque directly? Current Torque

7 Basic Principle of DTC in SRMs Torque equation of SRMs on steady sate flux linkage discrete expression >, T>, flux acceleration <, T<, flux deceleration the change of stator flux linkage is be controlled by selecting an suitable voltage vector Current in SRM is unipolar, i is positive. Basic principle of DTC in SRMs (a) Stator flux linkage vector of the motor is kept at a constant by selecting an suitable voltage vector. (b) Torque can be controlled by accelerating or decelerating the stator flux vector relative to the rotor movement.

8 DTC Method for a 6-phase SRMs U6(ϕ d ) U7 U8(ϕ e ) U5 N5 N6 N7 U9 U4(ϕ c ) y N4 U3 N3 N2 N1 N8 N9 N1 N12 N11 U1(ϕ f ) U11 Flux and voltage vector arrangement x ϕ s U2(ϕ b ) U1 U12(ϕ a ) Space Voltage Vectors U1 (+1,+1,-1,-1,-1,-1) U3 (-1,+1,+1,-1,-1,-1) U5 (-1,-1,+1,+1,-1,-1) U7 (-1,-1,-1,+1,+1,-1) U9 (-1,-1,-1,-1,+1,+1) U11 (+1,-1,-1,-1,-1,+1) Asymmetric Half Bridge Converter (AHB) +1 = +Vdc -1 =-Vdc U2 (+1,+1,+1,-1,-1,-1) U4 (-1,+1,+1,+1,-1,-1) U6 (-1,-1,+1,+1,+1,-1) U8 (-1,-1,-1,+1,+1,+1) U1 (+1,-1,-1,-1,+1,+1) U12 (+1,+1,-1,-1,-1,+1) Zone1~Zone12 are defined to locate stator flux

9 DTC Method for a 6-phase SRMs U5 U4(ϕ c ) U3 ϕ s Assume: U6(ϕ d ) U7 N5 N6 N7 N4 N3 N2 N1 U2(ϕ b ) ϕ s U1 1. Stator flux rotates along anticlockwise and locates in Zone1. 2. Both flux and torque feedbacks are lower than reference value. U8(ϕ e ) N8 N9 N1 N12 N11 U12(ϕ a ) How can we choose voltage vectors? Step1: Select voltage vector to increase stator flux U9 U1(ϕ f ) Operation example U11 U11, U12, U1, U2, U3 Step2: Select Voltage vectors to increase torque U2, U3

10 Simulation Current Chopping Control(CCC) N=1rpm Vdc=56V I_ref + - Current Hysteresis Controller AHB Converter M Torque reference=13 N m Current reference=12a I_real Control diagram of current chopping control T_real Direct Torque Control(DTC) N=1rpm Vdc=56V Torque reference=13 N m Flux reference=.52 Wb T_ref + Flux_ref Torque Hysteresis Controller Flux_real Flux Hysteresis Controller AHB Converter Voltage vector controller Flux Position Control diagram of direct torque control M

11 Simulation Current Chopping Control Direct Torque Control.6 Flux Linkage(Wb).4 Flux Linkage(Wb) Current(A) 15 Current(A) Torque(N m) Time(s) Torque(N m) Time(s)

12 Simulation Torque output comparison 33.3% 11.5% Current Chopping Control Direct Torque Control Y Axis(Wb) Stator flux linkage vector comparison X Y Plot Direct Torque Control Current Chopping Control Time(s) Same average torque output =13.1Nm Torque ripple sharply reduced X Axis(Wb) X-Y stationary frame Highly linear trajectory Relatively rounded

13 Conclusion DTC method has been used on a 6-phase SRM By using DTC method, producing a same average torque, the torque ripple of DTC method is sharply reduced. The challenges of DTC method is to select reasonable voltage vectors and chose flux and torque reference value properly to ensure the efficiency.

14