Smooth torque control of switched reluctance machines

The switched reluctance machine (SRM) has concentrated stator windings and a rotor which does not have windings nor permanent magnets, see Figure 1. This leads to a low-cost robust construction, which gives the motor the capability to be operated at ultra-high speeds and in harsh environments. Presently, switched reluctance motor drives are used for automotive applications, house-hold goods, electric vehicles, compressors, etc.

Figure 1:8/6 switched reluctance motor

An important drawback of the SRM is torque ripple. Conventional control strategies based an square-wave current excitation of the stator phases lead to a torque profile which exhibits large torque ripple. This makes the SRM not suitable for applications where high-dynamic and smooth torque control is required, as in the case of servo-drives.
The problem of torque ripple has been addressed in several papers in the past two decades. The present research presents a torque control strategy for SRMs based on the use of space vectors, in analogy to the use of space vector theory for induction and synchronous machines.

Figure 2 shows the geometry of a 6x4 SRM and the construction of a current space vector, built from individual vectors associated with each motor phase current.

By expressing the phase values of current and inductance in terms of a zero-sequence component and a space vector magnitude and angle, a relation can be derived which yields the reference currents that realize a desired torque. In the proposed space vector torque control, a space vector associated to the quadratic phase currents is aligned with a space vector associated with the spatial inductance derivatives.

Figure 2: Geometry of a 6x4 SRM and construction of a current space vector

Figure 3 shows the spatial inductance derivative profile for the motor depicted in figure 2. Figures 4 and 5 show simulation results for the reference currents and the torque. It can be seen that smooth torque control is possible by means of continuously varying phase currents.

Figure 3: Spatial inductance derivative profile: phase inductance derivatives and zero-sequence inductance derivative

Figure 4: quadratic phase currents

Figure 5: phase torque components and total torque

Relevant Publications

Kristof R. Geldhof, Thomas J. Vyncke, Frederik M.L.L. De Belie, Jan A.A. Melkebeek, Lieven Vandevelde, "A Space Vector Strategy for Smooth Torque Control of Switched Reluctance Machines," IEEE International Electric Machines and Drives Conference, IEMDC 2007, 3-5 May, Antalya, Turkey, proceedings on CD-rom.

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