Kunio Koga

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>>

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>>

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>>

Evaluations on operating performances of three typical V/f control schemes in PWM inverter drive induction motor system

The authors consider the evaluation of a PWM (pulse-width-modulated) inverter induction motor system driven by general-purpose V/f control from the point of view of operating performance. For this purpose, three basic V/f control schemes are under study. They are classified according to the control policy to be adopted as follows; (i) the ratio of stator voltage to frequency is kept constant (Type I); (ii) the magnitude of stator flux is kept constant (Type II), and (iii) the magnitude of rotor flux is kept constant (Type III). Operating performances become better in the orders Type I, II, and III. In particular, Type III gives an ideal aspect for V/f control and gives an ultimate control style to the induction motor.<<ETX>>

A current sensor of high accuracy fit for precise and exact motion control

A current sensor of high-accuracy fit for precise and exact motion control, particularly appropriate for the control of an AC-servo drive system, is presented. The proposed sensor, named for the sensing principle it uses, is called the magnetic-field-controlled type and has the ability to fulfil the following specifications: (i) the sensor is a noncontacting type; (ii) the operating frequency ranges from DC to several hundred hertz; (iii) the dynamic range can be extended from several milliamperes to 100 A; (iv) a detection accuracy of 0.01% full-scale is guaranteed over the whole range; and (v) a sensor of the same detection characteristics is easily obtainable and is applicable to each phase current sensing for three phases.<<ETX>>

Uniqueness for constant control of torque-transfer function in induction motor

The uniqueness of the constant control for the torque-transfer function (CCTTF) is discussed. The CCTTF has been developed for high-performance induction motor drives with a current controlled source. Two regions are discussed: the constant field region and the field weakening region. In the constant field region, two control schemes, vector control (VC) and a field acceleration method, have already been presented. However, in order to realize CCTTF, only VC is possible. In the field weakening region, the generation of the transient torque which disturbs CCTTF is inevitable. It is proved that there exists a unique way of minimizing the transient component, and such a control is proposed.<<ETX>>