In this paper, the position sensorless direct torque and indirect flux control (DTIFC) of BLDC motor with nonsinusoidal (non-ideal trapezoidal) back-EMF has been extensively investigated using three-phase conduction scheme with six-switch inverter. In the literature, several methods have been proposed to eliminate the low-frequency torque pulsations for BLDC motor drives such as Fourier series analysis of current waveforms and either iterative or least-mean-square minimization techniques. Most methods do not consider the stator flux linkage control, therefore possible high-speed operations are not feasible. In this work, a novel and simple approach to achieve a low-frequency torque ripple-free direct torque control with maximum efficiency based on dq reference frame similar to permanent magnet synchronous motor (PMSM) drives is presented. The electrical rotor position is estimated using winding inductance, and the stationary reference frame stator flux linkages and currents. The proposed sensorless DTC method controls the torque directly and stator flux amplitude indirectly using d–axis current. Since stator flux is controllable, flux-weakening operation is possible. Moreover, this method also permits to regulate the varying signals. Simple voltage vector selection look-up table is designed to obtain fast torque and flux control. Furthermore, to eliminate the low-frequency torque oscillations, two actual and easily available line-to-line back- EMF constants (kba and kca) according to electrical rotor position are obtained offline and converted to the dq frame equivalents using the new Line-to-Line Park Transformation. Then, they are set up in the look-up table for torque estimation. The validity and practical applications of the proposed three-phase conduction DTC of BLDC motor drive scheme are verified through simulations and experimental results.
- Brushless dc (BLDC) motor
- Position sensorless control
- Direct torque control (DTC)
- Stator flux control
- Fast torque response
- Non-sinusoidal back-EMF
- Low frequency torque ripples
Fig. 1. Overall block diagram of the position sensorless direct torque and indirect flux control (DTIFC) of BLDC motor drive using three-phase conduction mode.
EXPECTED SIMULATION RESULTS:
Fig. 2. Simulated indirectly controlled stator flux linkage trajectory under the sensorless three-phase conduction DTC of a BLDC motor drive when is changed from 0 A to -5 A under 0.5 N·m load torque.
Fig. 3. Actual q– and d–axis rotor reference frame back-EMF constants versus electrical rotor position and
Fig.4. Steady-state and transient behavior of the experimental (a) q–axis stator current, (b) d–axis stator current, (c) estimated electromagnetic torque and (d) ba–ca frame currents when under 0.5 N·m load torque.
Fig. 5. Experimental indirectly controlled stator flux linkage trajectory under the sensorless three-phase conduction DTC of a BLDC motor drive when at 0.5 N·m load torque.
Fig. 6. Steady-state and transient behavior of the actual and estimated electrical rotor positions from top to bottom, respectively under 0.5 N·m load torque.
This study has successfully demonstrated application of the proposed position sensorless three-phase conduction direct torque control (DTC) scheme for BLDC motor drives. It is shown that the BLDC motor could also operate in the field weakening field weakening region by properly selecting the d–axis current reference in the proposed DTC scheme. First, practically available actual two line-to-line back-EMF constants (%”# and %$#) versus electrical rotor position are obtained using generator test and converted to the dq frame equivalents usingthe new Line-to-Line Park Transformation in which only two input variables are required. Then, they are used in the torque estimation algorithm. Electrical rotor position required in the torque estimation is obtained using winding inductance, stationary reference frame currents and stator flux linkages. Since the actual back-EMF waveforms are used in the torque estimation, low-frequency torque oscillations can be reduced convincingly compared to the one with the ideal trapezoidal waveforms having 120 electrical degree flat top. A look-up table for the three-phase voltage vector selection is designed similar to a DTC of PMSM drive to provide fast torque and flux control. Because the actual rotor flux linkage is not sinusoidal, stator flux control with constant reference is not viable anymore. Therefore, indirect stator flux control is performed by controlling the flux related d–axis current using bang-bang (hysteresis) control which provides acceptable control of time-varying signals (reference and/or feedback) quite well. Since the proposed DTC scheme does not involve any PWM strategies, PI controllers as well as inverse Park and Clarke Transformations to drive the motor, much simpler overall control is achieved.
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