Toshikatsu Sonoda

Stability analysis in induction motor driven by V/f controlled general-purpose inverter

For an induction motor driven by a sinusoidal voltage source of adjustable amplitude and frequency, such as a pulse-width-modulation (PWM) inverter, the occurrence of sustained oscillations has often been observed when the drive is underloaded. In order to solve this problem, a linearized model is considered. The conditions under which the oscillations originate and how to determine the oscillating range in the stator voltage versus frequency are clarified. A physical explanation is also given for the analytical results. It is found that the motor becomes unstable when the rotor circuit time constant becomes larger than the mechanical time constant in rotor motion: however, the existence of transient inductance works so as to suppress the instability and stabilize the motor. There exist two asymptotes which restrict the oscillating range in the plane composed of the axis of the equivalent magnetizating current magnitude and that of stator frequency.<<ETX>>

Experimental results and their simplified analysis on instability problems in PWM inverter induction motor drives

The instability problems often experienced in PWM (pulsewidth modulation) inverter induction motor drive systems are discussed. Experimental results are presented on how the unstable regions and oscillating frequencies are dependent on dead time. It is noted that the experimental results can clarify the physical description of sources of the instability phenomena. It is shown that a switching-type inverter necessarily includes regenerative intervals. Its existence is the main source of unstable phenomena. It is also shown that a stable region can be determined by applying analytical results to a self-excited induction generator. >

Constitution of V/f control for reducing the steady-state speed error to zero in induction motor drive system

In order to extend the applicability of a voltage/frequency (V/f) controlled induction motor drive system, the problem of reducing to zero the steady-state speed error caused by load changes without using a rotor speed sensor is treated. The theoretical basis for this is given, and a scheme for constituting the control system is presented. The frequency of the stator voltage is controlled so as to compensate for the error, and this strategy is called the frequency compensation control (FCC). Transforming an induction motor into the corresponding idealized induction motor (IIM), which is defined as a motor having no stator resistance and leakage inductances, plays an essential role in realizing the FCC. The V/f control can exhibit excellent abilities when and only when it is applied to IIM, and is of the open-loop type in terms of the rotor speed. A well-known distinctive feature of Vf control, that it can generate a relatively large torque in the low-speed range to some extent, can be made use of with the maximum of ability. The proposed drive system is applicable to a part of the area in which the usual vector control equipped with a speed sensor has been utilized until now. In addition, as it has a detecting function for electromagnetic torque, torque monitoring is feasible.<<ETX>>

Stability problem in induction motor drive system

Stability problems associated with the interactions between electric transients and rotor dynamics are examined. The concept of an inherently stable idealized induction motor is introduced. It is shown that the destabilizing factors are stator winding resistance and leakage inductance in both stator and rotor windings. The importance of stator voltage and current control as a phasor is pointed out.<<ETX>>

Thrust force distribution on the surfaces of stator and rotot poles of switched reluctance motor

An effort is made to establish an effective means of determining the optimum geometrical configuration for such a motor. It is necessary to know the thrust force distribution as a function of the relative displacement between stator and rotor pole faces. The authors discuss the problem of the force distribution for a linear switched-reluctance motor whose structure is very simple. Taking the narrowness of the air gap into account, it is possible to establish a method for measuring the thrust force by introducing the concept of partial magnetic flux. >

Estimation of the radius of the inner core and the angle of the easy axis of its magnetization for as-prepared amorphous magnetostrictive wire

The estimation of the domain structure of an as-prepared amorphous magnetostrictive wire (Fe/sub 77.5/Si/sub 7.5/B/sub 15/, 125- mu m diameter) is considered. Through this investigation, it is clarified that the second-harmonic component of the voltage induced in a pickup coil wound about the wire is generated essentially from the inner core only when a sufficiently small sinusoidal field is applied in the direction of the wire axis. Using this property, the angle of the easy axis of magnetization in the inner core is estimated. The radius of the inner core is also calculated from the estimated value. In the estimation, it is assumed that the angle of the easy axis is proportional to the distance from the center line of the wire. Based on this assumption, the rectangularity and the magnitude of the pulse voltage are calculated. The validity of the estimation is confirmed by comparing the magnitude of the pulse voltage obtained when the estimated values are substituted into the expression for the pulse voltage with that measured by the Sixtus-Tonks experiment. >

A new type switched reluctance motor

A linear-type switched-reluctance motor (SRM) of simple structure is presented. It is quite useful for investigations on force characteristics, magnetic flux, and force distributions. It is shown that (1) the SRM is quite effective as a generating force; (2) each external shape of the stator and rotor poles has to be specified based on the flux distribution, and that the edge effect particularly has to be effectively taken into account; and (3) for ease of experimental testing, the magnetic flux can be measured on the stator surface and it is appropriate for estimating the force distribution.<<ETX>>

An AC and DC current sensor of high accuracy

A current sensor that can detect AC and DC currents with high accuracy of the same degree is presented. The detection characteristics are not degraded even when the sensor is exposed to large temperature variations and/or to external magnetic fields. The proposed sensor, which is named for the sensing principle it employs, will be referred to as the magnetic field controlled type or the zero magnetic field type of field and has the ability to fulfil the following specifications: (1) the sensor is a noncontacting type; (2) the operating frequency range covers from DC to several hundred hertz; (3) the dynamic range can be extended from several milliamperes to 100 A; (4) a detection accuracy of 0.01% full scale (FS) is guaranteed over the sensors whole range; and (5) several sets of current sensor having the same detection characteristics are easily obtainable even if the B-H of the core to be used for each sensor is very different. >

Achievement of high performances for general purpose inverter drive induction motor system

An induction motor control idea derived on the basis of the concept of the idealized induction motor, which is defined as a motor having neither stator resistance nor leakage inductance, is presented. The proposed control is to compensate for the effects of these parameters and to try to keep the magnetic flux which interlinks with the rotor circuit at a constant state where it is a rotating phasor. A concept for analyzing transient phenomena in the induction motor proves that the instability is essentially caused by the parameters. The proposed control does not give rise to any electrical transient phenomena and realizes absolutely stable dynamic performance. Computer simulation results and experimental investigations are also presented.<<ETX>>

Field and force sensors using amorphous ribbons

Amorphous ribbon is sensitive to magnetic field and has a large tensile strength. By making use of these distinctive features, excellent sensors can be constituted. This paper presents two types of sensors: a magnetic field and a tensile force sensors. On the magnetic sensor, zero flux method is introduced for detection of field level in a ribbon magnetized with external field to be detected. High sensitivity and accuracy is achieved. These performances are not affected with demagnetization effects, sizes and shapes of the ribbon utilized. Applying this technique to dc leak current sensing, the order of 0.0002% of main dc current which flows into a pair of one-turn conductors can be detected. On the tension sensor, the lowest order frequency mode of transverse vibrations in a ribbon is available. The frequency is proportional to a square root of the force. When the ribbon is excited with magnetic force of the same frequency, any position on the ribbon oscillates in-phase with the exciting force. Using this in-phase relation, an adaptive sensing is performed like a PLL (phase locked loop).