Switched Reluctance Drives

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While the switched reluctance motor (SRM) has become an interesting concept of electro-mechanical energy conversion due to its robust, simple and cost effective construction, the research at the department Leistungselektronik und Elektrische Antriebstechnik (LEA) also addresses other areas of research  in this topic. The foci are to minimize the torque ripple and to minimize the effective phase voltage demand so to maximize the drives efficiency. To do so it is necessary to identify the flux linkage and the different inductances. These values are determined with the three-dimensional Finite-Element-Analysis (FEA) tool- Ansoft Maxwell 3D. With the results of the FE Analysis it is also possible to easily check the efforts of mechanical modifications like variation of the airgap or the back iron. Another way to increase the drives efficiency is to optimize the FPGA based control structure via self-optimization for specific operating points. The Lagrange method is used with respect to constraints like constant torque demand as well as maximum phase currents (see Literature [2]), which was developed in the course of the Collaborative Research Center 614 – Self-Optimizing Concepts and Structures in Mechanical Engineering. The results from this method are optimized phase current trajectories with tradeoff between variance of torque and overall efficiency η (see figure 3 below) compared to traditional control schemes.

Figure 1: Test stand in the LEA laboratory                           Figure 2: Finite Element Analysis of the SRM in Maxwell 3D
 

Figure 3: Torque and overall efficiency of different control structures (optimized control structure with Lagrange method, on-off control and indirect torque control
 

                    Figure 4: Realization of the control concept on the test stand with optimized current trajectories

Another researched topic is the energy transfer from the stator to the rotor. This implies on the one hand more effort in the rotor construction but offers on the other hand a contactless secondary power supply. Especially transportation systems like elevators or rail-bound vehicles with linear drives, which have a demand on energy supply for light or air conditioning on the car, are ideal applications to use the motor as a transformer. For common switched reluctance drives, the surplus energy Wret is undesired as the power electronic devices have to be sized for the entire electrical work Wel,max. The idea of the presented doubly-excited SRM is now to transfer this stored energy, at least a part of it, as Wtrans to the secondary for supply of auxiliaries.

Figure 5: Energy distribution of a complete stroke for a SRM

 

n= 100min-1

Wtrans

Wmech

Wlosses

λ

η

No transfer

0 Ws

45.457 Ws

56.15 Ws

0.4832

0.4474

Usec = 400V

-11.1 Ws

45.321 Ws

55.85 Ws

0.5338

0.5025

Usec = 300V

-31.292 Ws

45.13 Ws

55.84 Ws

0.6289

0.25778

n = 300min-1

 

 

 

 

 

No transfer

0 Ws

33.655 Ws

14.07 Ws

0.3606

0.7052

Usec = 400V

-1.532 Ws

34.847 Ws

14.56 Ws

0.385

0.7142

Usec = 300V

-8.887 Ws

33.51 Ws

14.04 Ws

0.4264

0.7512

Table 1: Overview of simulation results

As Table 1 shows, the overall losses of the motor are hardly influenced by the energy transfer. Thus the total efficiency of the drive with energy transfer can be increased to 57 % compared to the efficiency of 44 % of a pure mechanical drive without energy transfer at lower speeds. This also applies for the energy ratio, which can be increased to 63 % compared to the energy ratio of 48 % of a pure mechanical motor operation. The transferred electrical work can reach up to 70% of the mechanical work. It is verified that the torque generation is not disturbed while transferring energy, moreover, it is even possible to enhance the dynamics in certain operating points. These simulation results validate that the doubly-excited switched reluctance drive can be a viable alternative to energy transfer via conductor rails or catenaries.


References:


[1]
 
 
 
T. Schneider, C. Schulte, S. Mathapati and J. Böcker
Energy Transfer with Doubly-Excited Switched Reluctance Drive
Proc. International Symposium on Power Electronics, Electrical Drives, Automation and Motion (SPEEDAM), Pisa, Italy, 2010
[2]
 
 
K. Flaßkamp, J. Böcker, S. Ober-Blöbaum, M. Ringkamp, T. Schneider, C. Schulte
Berechnung optimaler Stromprofile für einen 6-phasigen geschalteten Reluktanzantrieb
Wissenschaftsforum Intelligente Technische Systeme, Heinz Nixdorf Forum, Paderborn, Germany, 05/2011

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