In the three phase system, the ripple components of the instantaneous active power and reactive power correspond to harmonic components of the current under the sinusoidal line voltage. So, the elimination of the harmonic current can be obtained by the ripple power reduction.
The control scheme of the proposed active filter is based on this principle. The ripple components of the instantaneous line power can be compensated by the three phase PWM converter. It is directly controlled by selecting a proper voltage vector not to flow any ripple power from the source line not by the direct control of the three phase current as the PWM switching technique.
The features of the active filter are simple construction, high accuracy, high efficiency and so on.
In this paper, we propose the new control scheme of the active filter, the principle of the PWM technique for the instantaneous power control and the construction of the system. After that, we will show some simulation and experimental results for the proposed system.

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This paper describes a vector control system of an induction machine using a neutral-pointclamped voltage source inverter (NPC-VSI) that is one of double series connected inverters.
The NPC-VSI is able to output 5 levels step-shaped line to line voltage without output transformers or reactors, and it may reduce harmonic currents corresponding to torque ripples. However, the NPC-VSI has a problem that excessive high voltage is applied to swiching devices when a neutral point of two DC capacitors of the NPC-VSI varies from the center of the DC link voltage, because the neutral point is floating.
This paper proposes a current controller to which the space vector theory is applied. This can reduce harmonic currents to 1/4 of those of a conventional voltage source inverter using 6 swiching devices, and regulate the neutral point potential within a preset range. To simplify the current controller, information about voltage space vectors is given from the vector controller to the current controller. Validity of the current controller is confirmed by a prototype using an induction machine of 2.2 kW.

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In general, diode rectifiers with electrolytic capacitors on the DC side have been used as DC power supplies for voltage source inverters. This type of rectifiers, however, causes many problems such as poor power factor and harmonics. Recently, voltage source PWM rectifier-inverters have been studied to provide the following advantages; (1) harmonic-free on both AC sides; ( 2) unity power factor on the input AC side; (3) power flow of either direction or power regeneration; (4) reduction of the DC capacitor. It is, however, difficult for a conventional one to regulate the capacitor voltage on transient states because of having only a voltage feed-back loop.
This paper describes a voltage source PWM rectifier-inverter with feed-forward control of instantaneous power. Based on the pq theory, the instantaneous power which is calculated in the control circuit of the inverter is fed forward to the control circuit of the rectifier. The feed-forward control of instantaneous power makes a great contribution to sufficiently suppress voltage fluctuation of the DC capacitor on transient states. Transient characteristics are discussed and some interesting experimental results of a laboratory model are shown.

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This paper describes a new scheme for the speed sensor-less control of the vector controlled induction motor drive.
Based on the model reference adaptive system (MRAS) theory, the rotor speed of the induction machine is estimated with a full order adaptive observer. The estimated speed is then used as the signal for the vector control and the speed control. In order to accurately estimate the speed at lower speed range, the value of the stator resistance is also identified at the same time and then is used to modify the resistance value set in the observer. The resulting system is verified to be hyperstable with Popov's criterion. As a result, this drive can operate stably in a wide range of the speed even at the zero speed. It is also shown theoretically that both speed and resistance of the rotor cannot be identified at the same time, because these two parameters are dependent on each other. Simulation results and experiments also show that the scheme is effective.

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The field oriented control method is widely used for induction motor drives. In these applications, a speed sensor such as a shaft encoder is used. However, a speed sensor cannot be mounted in some cases such as motor drives in a hostile environment and high speed motor drives. Several field oriented control methods without speed sensors have been proposed. Most of them can be applied to only the indirect field oriented control but the direct field oriented control. And a stability has not been explained clearly.
Therefore, a new method of estimating the induction motor speed based on the adaptive control theory is proposed. Characteristics of the direct field oriented control system with the proposed speed estimation scheme are verified by experiments. Further, the influence of the stator and rotor resistance variations on the speed estimation is analyzed, and its compensation method is also proposed.

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Permanent magnet field-type DC motors are used widely for many purposes because of such features as small size, light weight, high efficiency and the ease of the variable speed drive.
Permanent magnet field-type DC motors are small motor, so that there are few reports with respect to evaluation of performance for optimum design.
Therefore, this paper describes the evaluation of performance in regard to armature circuit inductance, demagnetization durability of permanent magnet and commutation characteristics.
The results are summerized as follows.
(1) The armature circuit inductance and the demagnetization durability of the permanent magnets fields can be calculated by magnetic field analysis using the finite element methods.
(2) The commutation characteristics of permanent magnet field-type DC motors without commutating poles can be calculated by commutating analysis using the flux distribution at the neutral zone.
(3) The armature circuit inductance can be increased by slotted-core armature and permanent magnet field with pole pieces.
(4) The demagnetization durability of the permanent magnets fields can be increased by slotted-core armature and stator with splitted-yokes.
It is proved that the measured results obtained by the trial machines have a good agreement with the analytical results.

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This paper presents approximate equations of transient currents and torque of synchronous machines during sudden three-phase short circuit phenomena and synchronizing phenomena. Although there are a lot of reports analyzing these problems, most of them study a relatively simple synchronous machine operated under special conditions which is easy to analyze; i.e., for example, three-phase short circuit for unloaded synchronous machines modeled to have one damper circuit both in d and q axis. However, recently more complicated models of synchronous machines with two or three damper circuits both in d and q axis are studied and used to analyze transient phenomena of synchronous machines.
The analytical technique presented in this paper is developed on the basis of a new approximate method. The method studies an armature transient neglected model at first and derives the solution taking no effects of armature transient into consideration. Next, the components arising from the effects of armature transient is solved. The complete solution is obtained by adding these two solutions. In addition, the derived solution has a general form for damper structure. Thus, it is easy to obtain a particular solution for the synchronous machines with any number of damper circuits. To confirm the accuracy of the derived solution, numerical simulations of above two problems are carried out for the synchronous machine with three rotor circuits both in d and q axis. As a result, good agreements are obtained between the approximate solution and the exact solution obtained by simulating the basic differential equations

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Rotating machinery has two major vibration problems caused by mass imbalance of a rigid rotor. One is whirling motion of the rotor. The other is vibration of the housing. Magnetic bearings can solve these problems by controlling the forces of electromagnets. As an example, the authors propose application of periodic learning control (PLC) with inverse transfer function compensation (ITFC), which can deal with periodic disturbances. When rotor displacement is treated as the output of the controlled magnetic bearing system, the PLC with the ITFC can suppress the whirling motion. When coil-currents are treated as the output of the controlled system, the PLC with the ITFC can reduce synchronous fluctuations of coil-currents. The reduction of the fluctuations of coil-currents results in the reduction of housing vibration. The PLC with the ITFC can carry out accurate compensation even when parameters of the controlled system are not accurately identified. The experimental results with a totally active magnetic bearing spindle are introduced.

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