Wenlong Qu

A New Torque Control Method for Torque Ripple Minimization of BLDC Motors With Un-Ideal Back EMF

In classical control of brushless dc (BLDC) motors, flux distribution is assumed trapezoidal and fed current is controlled rectangular to obtain a desired constant torque. However, in reality, this assumption may not always be correct, due to nonuniformity of magnetic material and design trade-offs. These factors, together with current controller limitation, can lead to an undesirable torque ripple. This paper proposes a new torque control method to attenuate torque ripple of BLDC motors with un-ideal back electromotive force (EMF) waveforms. In this method, the action time of pulses, which are used to control the corresponding switches, are calculated in the torque controller regarding actual back EMF waveforms in both normal conduction period and commutation period. Moreover, the influence of finite dc bus supply voltage is considered in the commutation period. Simulation and experimental results are shown that, compared with conventional rectangular current control, the proposed torque control method results in apparent reduction of the torque ripple.

A Novel PWM Technique With Two-Phase Modulation

In this paper, a new two-phase pulse width modulation (PWM) technique is proposed, in which the reference voltage is constructed by three space vectors. At any speed, the peak-to-peak value of the common-mode voltage (CMV) in one PWM cycle can be reduced to one third of the DC bus voltage, even despite of the dead-time effect. And in the low voltage region, not only the CMV but also the current ripple is restrained effectively. In the entire voltage range, the linear modulation region is not limited and the switching loss is reduced because of fewer switching actions. The validity of the proposed technique is verified through simulation and experimental results.

A Three-Phase Current Reconstruction Technique Using Single DC Current Sensor Based on TSPWM

Three-phase current reconstruction technique using dc current information in conventional two-level inverters can be used for the purpose of cost reduction and sensor fault tolerance. A novel phase current reconstruction scheme, with reduced immeasurable area and common mode voltage, is proposed in this paper. A tristate pulse-width modulation technique has been employed, in which three adjacent switching states are used to construct the reference voltage. The active switching states are arranged at the edge and the center of a PWM cycle. Fixed sampling and simultaneous three-phase currents can be easily achieved with very little hardware and software requirements. A detailed analysis of the effects of nonidealities leads to regional modifications of the switching sequence resulting in almost the whole hexagon as the feasible area. The usefulness of the proposed reconstruction algorithm has been verified by experimental results obtained from a 4-kW induction motor drive system. Smooth transitions between the redundant and fault-tolerant modes were observed.

An advanced synchronization and current-sharing method for paralleled DC/AC inverters without interconnection

In this paper, an advanced synchronization and current-sharing method for paralleled DC/AC inverters without interconnection is presented. In the synchronization method, the output voltage and current of individual inverter are detected to calculate the equivalent reactive power. As the equivalent reactive power can reflect the phase information, so the inverter can adjust its own phase to track the voltage of the output bus by controlling its value of equivalent reactive power. The current-sharing method uses the AC signal on the output line to realize current-sharing control. The frequency of the small signal is inversely proportional to the output active power of the inverter. By comparing RMS of the local small signal with that of the overall signals added to the bus, the controller then regulate its own reference voltage to track the minimum output active power of the inverter in the system. An experiment system was built to verify the proposed method, and the results indicate the precise power balance performance for parallel operation

Single current sensor operation with fixed sampling points based on TSPWM

The single current sensor operation (SCSO) is favorable for cost reduction and fault-tolerance purposes in three-phase inverter-driven systems. This paper addresses a SCSO method based on tri-state pulse-width modulation technique (TSPWM). It enables fixed sampling points with minimal hardware and software requirements. Simultaneous three phase currents can be easily computed due to symmetrical sampling. No change to TSPWM is needed for most working points, thus it maintains advantages in common-mode voltage (CMV), switching loss and DC current ripple reduction. Based on the detailed analysis of non-ideal factor effects, regional modifications enlarge the feasible voltage region to almost the whole hexagon. It works well in close-loop systems and enables seamless transition in fault-tolerance operation for current sensor. Experimental results on a 4-kW induction motor drive are presented to verify the above-mentioned conclusions.

Single current sensor operation with fixed sampling points using a common-mode voltage reduction PWM technique

A method is proposed for reconstructing AC currents with a single DC side current sensor in three-phase AC motor drive system. The method is based on a common-mode voltage (CMV) reduction PWM technique called LuPWM, which eliminates most of the invalid regions for current reconstruction both around zero voltage and near active-vectors. It realizes fixed-point sampling which is favorable for implementation. And the current prediction is also easier than traditional methods. Furthermore, it remains the good property of LuPWM that the peak-to-peak value of CMV, which leads to the premature deterioration of motor bearings, is reduced to 1/3 of the traditional three-phase PWM method in each sector. The additional computation load is minimal, and it can be widely applied. Experimental results are presented to fully support the above-mentioned claims.

A novel sensorless control method of IPMSM using dual PLL structure

Harmonics in back electromotive force (EMF) would greatly affect the accuracy of traditional sensorless control methods, and some of them may even fail. Phase locked loop (PLL) is a useful tool for phase detection in multiple-harmonic system, and is suitable for the control system of Interior Permanent Magnet Synchronous Motor (IPMSM) as it is often filled with harmonics when the motor is not carefully designed. In this paper, Dual-PLL structure is proposed to obtain the fundamental harmonic of the quantities of the motor, which are used in the speed and position estimation. The first PLL is the main path of the estimation and the phase of αβ-axes voltage is determined, while the second PLL focuses on speed estimation with low ripples. Extra measurement of the terminal voltage of the motor is not necessary and only the q-axis inductance is needed when the motor is run at id=0. The delay caused by the digital control and the IGBT-composed inverter is also considered. Experiments are finished on an inverter fed IPMSM control system, which demonstrates the correctness of both proposed strategies with good steady performance.

Equivalent neutral point voltage control strategies in neutral-point diode-clamped three-level converter

This paper shows the equivalent of different strategies in neutral point voltage control under different modulation methods --- CBPWM and SVPWM. With this concept we can go from one strategy in CBPWM to another in SVPWM freely, and also newly found strategies that will be used not only in CBPWM but also in SVPWM. A simple neutral current model is established to analyze the problem of unbalance of neutral point voltage. Two couples of strategies based on different modulation methods are compared to show the equivalent. Two new strategies that can be used in both CBPWM and VPWM are proposed. Simulation results are given to show the feasibility.

An accurate rotor time constant estimation method for self-commissioning of multi-scale induction motor drives

This paper presents a novel strategy for accurate rotor time constant estimation while the induction motor keeps standstill. Based on the ‘Γ’ equivalent motor circuit model with all leakage located in the stator, the inertial first-order delay occurs in the mutual current relative to the stator current. Moreover, the delay time constant is only decided by the rotor time constant. Thus, an observer to estimate the mutual current by sample signal can be constructed. By changing the stator current at special moment according to the estimated mutual current, a significant transient response of stator voltage can be observed if estimated rotor time constant incorrect. Subsequently, a fast and robust close-loop null regulator is built to self-tune the rotor time constant. This method is completely independent of motor parameters and immune to the influence of dead time effect and voltage drop of semiconductor device. Experimental results on two different IMs are presented to prove the validity of the proposed method.

EMF Feedback Control Strategy of Induction Motor for Wide Speed Range

Variation of winding resistance, magnetic saturation and iron loss in the induction motor result in field angle estimation error, which causes detuning problems. Especially, in cases when the iron loss and change of inductance cannot be neglected, ordinary resistance identification methods may lead to incorrect results. To solve this problem, a new method to directly revise the observed field position is proposed in this paper. The position angle of rotor flux observed can be continuously revised by adding a feedback compensator of electro-magnetic force (EMF), which guarantees the correct direction of rotor flux. The principle of compensation is introduced, and in order to verify the validity of this method, various tests have been done on a 60 kW induction machine. The simulation and experimental results indicate that the flux orientation of an induction motor field oriented control (FOC) system with EMF feedback control strategy is more accurate than an ordinary FOC system, especially in high speed range.