Accurate flux estimation and control of stator flux by the flux control loop is the determining factor in effective implementation of DTC algorithm. In this paper a comparison of voltage-model-based flux estimation techniques for flux response improvement is carried out. The effectiveness of these methods is judged on the basis of Root Mean Square Flux Error (RMSFE), Total Harmonic Distortion (THD) of stator current, and dynamic flux response. The theoretical aspects of these methods are discussed and a comparative analysis is provided with emphasis on digital signal processor (DSP) based controller implementation. The effectiveness of the proposed flux estimation algorithm is investigated through simulation and experimentally validated on a test drive. 1. Introduction In a DTC induction motor drive, a decoupled control of torque and flux can be achieved by two independent control loops [1–3]. The steady state as well as the dynamic performance of the drive is closely related to the efficient implementation of these two control algorithm. There are few well-known methods to estimate these parameters. Most of them are voltage model based [3], where the flux and torques are estimated by sensing stator voltage and current. The methods based on voltage models are most preferable for sensorless drives since these methods are less sensitive to the parameter variations and does not require motor speed or rotor position signals. However, the estimation of stator voltage when the machine is operating at low speed introduces error in flux estimation which also affects the estimation of torque and speed in case of sensorless drive [4–11]. In a conventional DTC drive the basic-voltage-model-based flux estimation is carried out by integrating the back emf of the machine. A pure integrator has the following limitations.(1)Any transduction error in measured stator current due to offset introduces DC component and hence results in integrator saturation.(2)Integration error due to incorrect initial values. A commonly employed solution is to replace a pure integrator with a low-pass filter [12, 13], however it is achieved at the expense of deteriorated low-speed operation of the drive, when the operating frequency of the drive is lower than the cutoff frequency of the low-pass filter. Flux estimation based on the current model is most suitable for low-speed operation [14, 15], however it is a parameter dependent method, which require rotor speed or position. Thus parameter independent operation, which makes a DTC drive more robust and reliable compared to a FOC drive,
References
[1]
I. Takahashi and T. Noguchi, “A new quick-response and high efficiency control strategy of Induction Motor,” IEEE Transactions on Industry Applications, vol. 22, no. 5, pp. 820–827, 1986.
[2]
M. Depenbrock, “Direct self-control (DSC) of inverter-fed induction machine,” IEEE Transactions on Power Electronics, vol. 3, no. 4, pp. 420–429, 1988.
[3]
G. S. Buja and M. P. Kazmierkowski, “Direct torque control of PWM inverter-fed AC motors—a survey,” IEEE Transactions on Industrial Electronics, vol. 51, no. 4, pp. 744–757, 2004.
[4]
G. Buja and R. Menis, “Steady-state performance degradation of a DTC IM drive under parameter and transduction errors,” IEEE Transactions on Industrial Electronics, vol. 55, no. 4, pp. 1749–1760, 2008.
[5]
M. H. Shin, D. S. Hyun, S. B. Cho, and S. Y. Choe, “An improved stator flux estimation for speed sensorless stator flux orientation control of induction motors,” IEEE Transactions on Power Electronics, vol. 15, no. 2, pp. 312–318, 2000.
[6]
E. D. Mitronikas and A. N. Safacas, “An improved sensorless vector-control method for an induction motor drive,” IEEE Transactions on Industrial Electronics, vol. 52, no. 6, pp. 1660–1668, 2005.
[7]
J. Holtz, “Sensorless position control of induction motors—an emerging technology,” IEEE Transactions on Industrial Electronics, vol. 45, no. 6, pp. 840–852, 1998.
[8]
J. Holtz and J. Quan, “Drift- and parameter-compensated flux estimator for persistent zero-stator-frequency operation of sensorless-controlled induction motors,” IEEE Transactions on Industry Applications, vol. 39, no. 4, pp. 1052–1060, 2003.
[9]
K. D. Hurst, T. G. Habetler, G. Griva, and F. Profumo, “Zero-speed tacholess im torque control: simply a matter of stator voltage integration,” IEEE Transactions on Industry Applications, vol. 34, no. 4, pp. 790–795, 1998.
[10]
B. K. Bose and N. R. Patel, “A programmable cascaded low-pass filter-based flux synthesis for a stator flux-oriented vector-controlled induction motor drive,” IEEE Transactions on Industrial Electronics, vol. 44, no. 1, pp. 140–143, 1997.
[11]
J. Holtz and J. Quan, “Sensorless vector control of induction motors at very low speed using a nonlinear inverter model and parameter identification,” in Proceedings of the 36th IAS Annual Meeting, vol. 4, pp. 2614–2621, October 2001.
[12]
J. Hu and B. Wu, “New integration algorithms for estimating motor flux over a wide speed range,” IEEE Transactions on Power Electronics, vol. 13, no. 5, pp. 969–977, 1998.
[13]
M. Hinkkanen and J. Luomi, “Modified integrator for voltage model flux estimation of induction motors,” IEEE Transactions on Industrial Electronics, vol. 50, no. 4, pp. 818–820, 2003.
[14]
M. Bertoluzzo, G. Buja, and R. Menis, “A direct torque control scheme for induction motor drives using the current model flux estimation,” in Proceedings of the IEEE International Symposium on Diagnostics for Electric Machines, Power Electronics and Drives (SDEMPED '07), pp. 185–190, September 2007.
[15]
H. U. Rehman, A. Derdiyok, M. K. Guven, and L. Xu, “A new current model flux observer for wide speed range sensorless control of an induction machine,” IEEE Transactions on Power Electronics, vol. 17, no. 6, pp. 1041–1048, 2002.
