148 Journal of Power Electronics, Vol. 11, No. 2, March 2011 JPE 11-2-5 Extending Switching Frequency for Torque Ripple Reduction Utilizing a Constant Frequency Torque Controller in DTC of Induction Motors Auzani Jidin † , Nik Rumzi Nik Idris ∗ , Abdul Halim Mohd Yatim ∗ , Tole Sutikno ∗∗ , and Malik E. Elbuluk ∗∗∗ † Dept. of Power Electronics and Drives, FKE, Universiti Teknikal Malaysia Melaka, Malaysia ∗ Dept. of Energy Conversion, FKE, Universiti Teknologi Malaysia, Malaysia ∗∗ Dept. of Electrical Engineering, Universitas Ahmad Dahlan, Indonesia ∗∗∗ Dept. of Electrical Engineering, University of Akron, USA Abstract Direct torque control (DTC) of induction machines is known to offer fast instantaneous torque and flux control with a simple control structure. However, this scheme has two major disadvantageous, namely, a variable inverter switching frequency and a high torque ripple. These problems occur due to the use of hysteresis comparators in conventional DTC schemes, particularly in controlling the output torque. This paper reviews the utilization of constant frequency torque controllers (CFTC) in DTC to solve these problems while retaining the simple control structure of DTC. Some extensions of the work in utilizing a CFTC will be carried out in this paper which can further reduce the torque ripple. This is particularly useful for a system which has a limited/low sampling frequency. The feasibility of a CFTC with an extended carrier frequency in minimizing the torque ripple is verified through experimental results. Key Words: Direct Torque Control, Induction Machine, Switching Frequency, Torque Ripple Reduction I. I NTRODUCTION The direct torque control (DTC) of induction motor drives has gained popularity in advanced motor drive applications since it offers fast instantaneous torque and flux control with simple implementation. This scheme is well known for its robustness in control as it is less dependent on machine parameters, does not require a complex field orientation block, a speed encoder and an inner current regulation loop. However, this scheme, which is based on hysteresis comparators [1], has major drawbacks namely a variable switching frequency, a large torque ripple and high sampling requirements for digital implementation. It makes sense that a reduction in output torque ripple can be achieved when a lower band of torque hysteresis is used in order to restrict the ripple within the band. However, this cannot be realized using a microprocessor or a digital signal processor, particularly when an extreme torque slope occurs with an inappropriate band level (which is too small). Ideally, Manuscript received Sep. 24, 2010; revised Jan. 30, 2011 † Corresponding Author: auzani@utem.edu.my Tel: +606-555-2345, Fax: +606-555-2226/2222, UTeM † Dept. of Power Electronics and Drives, FKE, Universiti Teknikal Malaysia Melaka (UTeM), Malaysia ∗ Dept. of Energy Conversion, FKE, Universiti Teknologi Malaysia (UTM), Malaysia ∗∗ Dept. of Electrical Engineering, Universitas Ahmad Dahlan (UAD), In- donesia ∗∗∗ Dept. of Electrical Engineering, University of Akron, USA hysteresis-based operation is suitable for a discrete system which has a fast processor such that the bang-bang control can be performed the same as in analog operation. Instead of lowering the hysteresis band with a fast processor, one can inject high-frequency triangular waveforms into the errors in torque and flux [2]. This method is called the dithering technique, and it is simple and effective in minimizing torque ripple. However, it still produces an unpredictable switching frequency since the torque slopes that determine the frequency of the torque controller vary depending on the operating conditions [3], [4]. Several methods have been proposed to overcome this problem (i.e. an unpredictable switching frequency) [5]-[15]. With consideration of the variations in torque slope, a constant switching frequency can be provided when the hysteresis bands themselves are adjusted according to operating condi- tions [5]. The adjustability of hysteresis bands is established based on a PI controller and a pulse counter for each of the torque and flux controllers. This, consequently, increases the complexity of the DTC drive. Moreover, this technique does not guarantee a reduction in torque ripple as it the case with hysteresis-based controllers. To eliminate the inherent problems of hysteresis-based controllers, it is possible to de- termine an optimal switching instant for each of the switching cycles that satisfies the minimum-torque ripple condition [7]- [9]. In this case, the term called a duty ratio is determined