Showing papers by "Seung-Ki Sul published in 2000"

A new PWM strategy for common mode voltage reduction in neutral-point clamped inverter-fed AC motor drives

Abstract: In this paper, the relationship between common-mode voltage and switching states in a neutral-point-clamped (NPC) inverter is clarified, and a new pulsewidth-modulation (PWM) strategy for reducing the common-mode voltage in an NPC inverter-fed AC motor drives is proposed. Among a total of 27 switching states in the NPC inverter, the proposed PWM uses only 19 switching states that generate common-mode voltage of which the magnitude is the same or less than one-sixth of DC-bus voltage. Moreover, the proposed PWM strategy satisfied the constraint that the output voltage vector should be changed by only one switching action. Not requiring extra hardware, the NPC inverter with the proposed PWM results in a remarkable reduction in peak-to-peak of common-mode voltage, compared to a two-level inverter and an NPC inverter controlled by conventional PWM. The proposed PWM strategy can be easily implemented in software without degradation of control performance in AC motor drives.

AC/AC power conversion based on matrix converter topology with unidirectional switches

Abstract: In this paper, a new type of AC/AC power converter is proposed. The proposed converter is capable of direct AC/AC power conversion and, except for a few small snubber elements, it does not require the use of any input inductors or a DC-link capacitor. In contrast to the matrix converter, which requires bidirectional switches, the proposed converter consists of only unidirectional switches such as insulated gate bipolar transistors. The converter has a unity input displacement power factor, and its input line current waveform is similar to that of a diode rectifier with a DC-link inductor. The proposed converter has been validated experimentally using an induction motor driven with field-oriented control.

Torque control strategy for a parallel-hybrid vehicle using fuzzy logic

Abstract: In this article, we introduce a fuzzy logic controller to the driving strategy of a hybrid electric vehicle (HEV). The decision making of this fuzzy logic controller is useful to nonlinear and uncertain systems such as electric vehicle applications, and the fuzzy logic controller is immune to various vehicle load and road conditions. To construct the proper rule base of the fuzzy logic controller, the torque-producing, pollutant-emission, and fuel-consumption characteristics of the diesel engine and the hybrid system are clarified through dynamo testing. Then, the driving patterns of the driver and vehicle load on the service course are investigated. These results are also reflected in constructing the rule bases of the proposed fuzzy logic controller. To prove the usefulness of the proposed fuzzy logic controller, actual road tests of the parallel HEV are carried out. Note that the battery maintains its nominal voltage through 20 days of running without extra charging process.

Advanced gear-shifting and clutching strategy for a parallel-hybrid vehicle

Abstract: In parallel-hybrid electric vehicles (HEVs), both the electric drive and the internal combustion engine provide driving torque to the wheels either separately or together. The electric drive also can be used as a generator to recharge the batteries when the engine produces more power than is needed to propel the vehicle. As a result of the battery peak power density requirement, the vehicle needs a multispeed transmission between the electric drive and the main differential. In addition, due to the load sharing between the two independent driving sources, a clutch to separate the internal combustion engine and electric drive from the drive shaft is also required. Because the shift quality is directly related to the driving comfort of the vehicle, it is important to reduce both shift shock and time. To enhance the shift quality, we have proposed the advanced gear-shifting and clutching strategy for a parallel-hybrid drive train with an automated manual transmission (AMT). By using the electronically controlled AMT, users can achieve the optimal gear shifting, with regard to the efficiency of the hybrid drive train. Owing to the speed control of the induction machine and the diesel engine at gear shifting, the synchronization is always guaranteed and it reduces the shift shock and shortens the shift time. The dynamo-based experiments have been carried out for the purpose of proving the validity of the proposed transmission and clutch control in parallel-hybrid drive trains.

Current measurements in digitally controlled AC drives

Abstract: In this article, the current-sampling error has been investigated in the field-oriented control of an AC machine by a PWM inverter. The expression of the harmonic current due to a low-pass filter has been studied analytically from the reference voltage vector in the SVPWM strategy. The fundamental components of the current-sampling error causes improper torque regulation while the high-frequency ripple of the error introduces permissible gain of the current regulators with a resultant impairment in dynamic behavior and limited stability range of the control systems. It has been proposed that the magnitude and phase error from the filter can be avoided with delayed sampling. The effectiveness of the delayed sampling was verified through simulations and experimental results including steady state and dynamic responses.

A common mode voltage reduction in boost rectifier/inverter system by shifting active voltage vector in a control period

Abstract: This paper proposes a new space-vector pulse width modulation (SVPWM) strategy that can reduce the number of common mode voltage pulses in a three-phase boost rectifier/inverter system using a synchronized switching sequence. In the proposed SVPWM strategy, it is possible to eliminate one common mode voltage pulse in every control period by shifting the active voltage vectors of the inverter to align to those of the boost rectifier. Thus, a reduction in the total number of common mode voltage pulses and RMS motor leakage current can be obtained without extra hardware. Since the proposed SVPWM strategy can be simply implemented in software, it is widely applicable regardless of the power capacity of the converter and results in no increment of converter volume, weight and price. Moreover, because the proposed SVPWM strategy maintains the magnitude of the active voltage vector required for motor control and simply changes the distribution of the zero, voltage vector, it does not effect the control performance of the power converter.

Physical understanding of high frequency injection method to sensorless drives of an induction machine

Abstract: This paper presents a physical understanding of a high frequency signal injection method to sensorless drives of an induction machine and proposes a sensorless algorithm based on it for zero or low stator frequency operation. The fluctuating high frequency signal at a synchronously rotating reference frame with the fundamental stator frequency generates an asymmetry of the spatial impedances in an induction machine. The difference of impedances between the flux axis and the quadrature axis at high frequency signal injection on the rotor flux angle is explained by the equivalent circuit equation of the induction machine. And the physical phenomenon due to the injected signal is analyzed and examined by finite element method under various operating conditions. The difference is also verified experimentally on the test motors. An algorithm to estimate the rotor flux angle and the rotor speed utilizing the physical phenomena without any speed transducer is proposed. It contains a scheme extracting the high frequency impedance components related to the rotor flux position. The experimental results clarify the satisfactory operation of the algorithm with heavy load torque at zero stator frequency or zero speed regardless of operation mode-generating or motoring.

Vertical-vibration control of elevator using estimated car acceleration feedback compensation

Abstract: In this paper, a vibration suppression strategy is proposed for improving the riding comfort of an elevator, using car acceleration feedback compensation. The vertical vibration of a lift car is mainly caused by the resonance of elastic ropes between the car and the sheave, and the resonant frequency of the system is dependent upon both passenger load and lift position. To suppress the vibration of a lift car, the car velocity or acceleration is needed, but only a sheave velocity is measurable in a practical situation. The proposed method applies an extended full-order observer for the simultaneous estimation of car acceleration and the identification of mechanical parameters. Acceleration feedback compensation is used for the vibration suppression control. Experimental evaluation has been performed with a 30 kVA insulated gate bipolar transistor inverter and a medium-speed elevator system in an elevator test tower. Computer simulated and experimental results prove the feasibility of the proposed vertical-vibration controller.

A novel ride-through system for adjustable speed drives using common-mode voltage

Abstract: This paper proposes a new ride-through system for adjustable-speed drives (ASDs) using common-mode voltage. In the proposed ride-through system, the common-mode voltage inherent in a pulsewidth-modulation boost rectifier-inverter system is used for charging the energy storage device at normal conditions of the source power. That is, controlling the common-mode voltage with a small reactor while driving motors can regulate charging current. Using simple additional circuits and the existing rectifier, at the whole range of voltage disturbances from balanced or unbalanced shallow voltage sag to full outage, ride-through is successively available with the proposed system. Various energy storage devices can be selected according to the applications and two kinds of charging topology-using inverter-side or rectifier-side common-mode voltage-are also available in the proposed system.

A harmonic reference frame based current controller for active filter

Abstract: This paper presents a new method of current regulation for active filters based on the idea of compensating each harmonic current at its own rotating reference frame. Because the harmonic current to be compensated for is DC in the steady-state within a rotating reference frame that is rotating in the same direction and at the speed of the harmonic, the steady state error of each harmonic compensated for can be eliminated. Eventually, the harmonics in the line current can be cancelled out. In real situations, there are many unbalanced utility voltage conditions, so the active filters can compensate properly under these conditions too. The proposed current controller can work under unbalanced voltage conditions. The experimental results illustrate the effectiveness of the proposed method.

A new sensorless thrust control of linear induction motor

Abstract: This paper presents the field-oriented thrust control of a linear induction motor without flux or speed sensor. The induction motor is structurally symmetrical but it has the difference of the reluctances between the flux axis and the quadrature axis due to the main flux saturation. This electrical saliency can be utilized to estimate an instantaneous flux angle by high frequency signal injection. In the case of a rotary induction motor, it has been verified that this high frequency signal injection method gives reasonable torque controllability at zero or low stator frequency even under a heavily loaded condition. In this paper, it is shown that the electrical saliency also exists in the linear induction motor and the comparison with the rotary induction motor is described. From this result, the sensorless field-oriented thrust control of a linear induction motor in low operating frequency region is presented and the sensorless control in the overall frequency range is implemented by the combination with an adaptive observer.

Design and control of multispan tension simulator

Abstract: In this paper, the design and control aspects of a multispan tension simulator are presented. The simulator consists of four driven rolls, including unwinder, winder, and two bridle rolls. Some relationships between design parameters and characteristics of the simulator are explained as a design guide of a multispan system. A new control algorithm of continuous load balance control is proposed for the control of speed and tension in the multispan system. The strip tension is controlled by the continuous balancing of the load torque of rolls without a tension sensor. The simulation and experimental results reveal conspicuous improvement of tension control performance by the proposed algorithm.

A development of an advanced shift control algorithm for a hybrid vehicles with automated manual transmission

Abstract: In this study, an advanced shift control algorithm for a parallel hybrid drivetrain system with automated manual transmission (AMT) is proposed. The AMT can be easily realised by mounting pneumatic actuators and sensors on the clutch and on the shift levers of the conventional manual transmission (MT). By using the electronic-controlled AMT, the engine load and the induction machine, it is possible to achieve the integrated control of the overall system regarding the efficiency and the performance of the vehicle. Performing the speed control of the induction machine and the engine, the synchronisation at gear shifting and the smooth engagement of the clutch are guaranteed. It also enables the shift shock to be reduced and the shift time to be shortened. Hence, the shift quality and the driving comfort of the vehicle are improved. Various dynamometer-based experiments are carried out to prove the validity of the proposed shift control algorithm.

A new discontinuous PWM strategy of neutral-point clamped inverter

Abstract: In-this paper, a new carrier-based discontinuous PWM strategy for neutral-point-clamped (NPC) PWM inverter is proposed. The proposed discontinuous PWM scheme takes advantage of the special properties available in NPC inverter to minimize switching loss of the inverter simultaneously with neutral-point voltage control. This switching loss minimization is derived from the following two factors: less switching frequency ratio and the absence of switching in the vicinity of peak load current. The proposed PWM strategy differs from the conventional carrier-based PWMs in that the proposed one distributes nonswitching regions variably according to switching energy loss and variation of neutral-point voltage estimated from sampled phase current. In addition, it can produce the highest available output voltage because it is based on the concept of offset (zero sequence) voltage. The effectiveness of the proposed PWM strategy is verified by experiments.

Improvement of low speed characteristics of railway vehicle by sensorless control using high frequency injection

Abstract: This paper presents a way to improve the torque control characteristics of railway vehicle at low speed region using the high frequency injection method. In traction drives, the demanded robustness, which can only be reached with expensive, specially constructed speed sensors, limits the speed quantization (often only 50-100 pulses per revolution) so that the speed information is inaccurate and is not available instantaneously. Due to this reason precise torque control with FOC (field oriented control) is degraded at the standstill and low speed region. The sensorless control with high frequency signal injection method has been proposed and it is known that this high frequency injection method gives the feasibility of FOC at the standstill and the robustness against parameter variation. In this study, the rotor flux angle is estimated by the high frequency injection method at low speed region and when the speed exceeds a threshold, the control is transitioned to the indirect vector control mode. The proposed strategy makes the precise torque control so that the slip between wheel and rail on the starting can be eliminated and electrical braking can be extended to the standstill. The strategy has been validated by simulation study, experiments with 7.5 kW laboratory setup, and experiments with 1.2 MVA IGBT traction drive system with four 210 kW induction motors.

Controller Design of Sensorless Induction Machine by High Frequency Voltage Injection

Abstract: This paper describes a new sensorless induction motor drive scheme by injecting high frequency voltage signal. The signal is not a rotating one, but a fluctuating one at a synchronous rotating reference frame with the fundamental stator frequency. The difference of impedance between the flux axis and the quadrature axis at high frequency signal injection on the rotor flux angle is used to track the rotor flux angle. Since the injection signal is independent on the stator frequency, the monitoring of the differences of impedance can be done even at stand still condition of motor. Speed and current controller suitable for high frequency voltage injection are designed and verified by experiments.

A new space vector pulsewidth modulation strategy for reducing ground to stator-neutral voltage in inverter-fed AC motor drives

Abstract: This paper proposes a new two-phase PWM scheme that can reduce the ground to stator neutral voltage in three-phase diode rectifier/inverter system with corner grounded delta-connected input source. According to the sign of voltage difference between ground and DC-bus midpoint voltage, the proposed PWM use only one zero space vector in one control period. Thus, the peak-to-peak value of ground to stator neutral voltage can be reduced to 2/3 compared with conventional three-phase SVPWM. Without any degradation of control performance of inverter, the proposed PWM scheme can be easily implemented in software and requires no extra hardware except a simple neutral voltage measurement circuit.

On-line compensation of friction loss for continuous strip processing line

Abstract: This paper proposes a tension control to compensate friction loss using an on-line friction torque observer for a continuous strip processing line. Friction loss of roller results in significant deviation of strip tension, accordingly it has an influence on the operation of other adjacent rolls. To avoid tension variation of the strip, a friction torque observer is designed in an adjacent roll, which operates in speed control mode. The observed torque is added to the torque limit reference of the pay-off reel for on-line compensation of both friction loss and acceleration/deceleration torque at the same time. The simulation and experimental results show improvement of tension control performance by the proposed friction compensation method.

Suppression of vibration for elevator system using dead-time compensation with current prediction

Abstract: In this paper, a vibration suppression scheme is proposed for the riding comfort of elevator using dead-time compensation with an accurate current information. To obtain an accurate current information at switching instants, a current prediction is used. The prediction current at switching instant for dead-time compensation is calculated from the sampled current, which greatly reduces the polarity error of the current at switching instants and increases the compensation accuracy. Using the proposed dead-time compensation scheme, the torque ripple of 6 times harmonics and the corresponding elevator vibration are reduced. Experimental evaluation has been performed with an 11 kW induction machine and a medium-speed elevator system. The result obtained proves the feasibility of the proposed method.