Induction motors are robust and inexpensive machines and are widely used for variable speed control because of recent development of electronic technique. In the case that loads of the motors are compressors, pumps, and so on, the constant V/F control of the induction motors is usually employed because it is difficult to install speed sensors and accurate speed control is not required. In such loads, the rotational speed of the motors is considerably fluctuated because the load torque is pulsated. When frequency of the torque pulsation is close to the resonant frequency of the mechanical system, large vibration and acoustic noise are produced especially in low frequency region. In this paper, authors propose a method to suppress the variation of the rotational speed of the V/F controlled induction motor with a fluctuated load by feedforward compensation using a timing sensor of 1 pulse/rev. considering that the load torque varies periodically. The feedforward data by a period for the compensation is obtained by the learning control based on the repetitive control in which the motor speed is controlled by periodically reflecting the past speed error on the present V/F input to the inverter. Effectiveness of the proposed method is confirmed by approximate analysis, simulations and experiments.
A feedback control seems to be useful to improve regulation and accuracy of an inverter. However, it has been pointed out that a feedback around a triangular wave carrier PWM inverter may cause a switching of output signal independent of the carrier frequency. An analysis of a voltage-output PWM inverter with an integral feedback loop suggests that, if a dead time element is incorporated in the pulse transfer path of the inverter and if the dead time is adjusted depending on the reference input value, then the output pulse can be synchronized to the carrier even at a higher loop gain. This was verified in a series of experiments conducted with a three-phase transistor inverter. As the dead time was adjusted properly, the three output pulse sequences were synchronized to the carrier signal when the reference input value was within 78% of full range. Outside this range, the output pulse frequency lowered. Sinusoidal disturbances up to half the carrier frequency were reduced by the increased-gain feedback control.
Rectifiers composed of diodes or thyristors have many harmonics in line currents. Recently, PWM converters have been actively studied to reduce the harmonics. In order to put them into practical use, not only PWM control method but also the following control methods must be established. (1) Method for reducing the capacity of dc link capacitor. (2) Restarting method from instantaneous power interruption. For reducing the capacity of do link capacitor, it is necessary to suppress the fluctuation of dc link voltage at a sudden change of load. In this paper, the load is inverter for variable speed induction motor drives. The load current of converter, that is the inverter input current, is detected without delay time and ripple by multiplying the inverter output currents or motor currents and the inverter PWM signals. By adding the detected signal to do link voltage control loop as a compensating component, the fluctuation of dc link voltage at a sudden load change can be suppressed. As a result, the capacity of the dc link capacitor or can be reduced to smaller than one-fifth of that in the conventional diode rectifier. For restarting from instantaneous power interruption, the very simple control method is proposed. When the dc link voltage becomes low limitation value setted at a little bit lower than that obtained in diode rectifier in the period of power interruption, the PWM signals to converter are blocked. After recovery from power interruption, when the dc link voltage reaches the low limitation value, the PWM control signals are supplied to the converter again and it is restarted the operation. From the experimental results, it was confirmed that the PWM converter can be restarted from the instantaneous power interruption very smoothly.
Current Source PWM Inverter has no delay time Td in principle, and supplies currents for a motorexactly. In addition, the induction motor has little magnetic noise, since the voltage wave forms of the motor are almost sinusoidal. Making good use of these characteristics of the current source inverker, the drive system havinghigher performance can be achieved without speed sensors. Above all, as the current and voltage are nearly sinusoidal wave forms, motor constants and parameters can be measured precisely under motor operation. If the motor parameters are given exactly, a vector-controlled system may be achieved theoreti-cally, where secondary resistance R2 is omitted in the control low for the inverter angular frequency ω1. Furthermore, it is possible to estimate the precise angular speed of a motor. In this paper, the principle and performance characteristics of this method are discussed. The simulation and experimental results of the tested machine show that the proposed method is very effective for improvement of the system characteristics, such as little speed regulation.
In development and design stage of electromagnetic actuator, two methods for magnetic field analysis are mainly used for the optimal design. One is a method based on magnetic equivalent circuit. But it is unsuitable for the accurate calculation because of leakage flux and nonlinearity in the magnetic circuit. On the other hand, nonlinear magnetic field analysis using Finite Element Method (FEM) has been widely used as a powerful and useful means in recent years. Since it requires a large sized computer for the calculation, it is mainly used for the large sized devices and hardly used for the small ones. In this paper, we use FEM to study the optimal design for the electromagnetic actuator, from the view of the generation of electromagnetic force. A plunger type solenoid which is widely used in practical use is analyzed to realize more compact shape and suitable characteristics of force. Consequently, it can be made clear how the shape of main air-gap, the saturation in yoke, and the lapping length between movable core and yoke affect the characteristics of the force. Further, it is demonstrated that each parameters can be used to get the optimal design.
The self-excited wound-rotor induction generator excited with condenser can be used for the small capacity wind power generator as independent source. Under constant exciting condenser capacity, the secondary circuit power of the induction generator is returned to its primary through a converter, an inverter and a transformer, and also its power is controlled at the inverter so that the generator voltage is maintained always a constant value in spite of the change of generator speed and its load. Under above mentioned conditions, generator basic characteristics are analyzed. The fundamental waves of both the main circuit and the feed back circuit are found. Using these results the equations of various portions of the generator are established, and solving simultaneous equations performance characteristics of the generator are analyzed and compared with experimental results.
It is desired from viewpoints of maintenances and constructions to make three-phase synchronous generators brushless and exciterless type. To meet this requirement, one of the authors devised a brushless and exciterless synchronous generators in which exciting currents flow simultaneously with load currents in armature windings. Thus, generators can be used both as the generators themselves and as the exciters. A generator system has already been reported by one of the authors. In this generator, the DC stator exciting current flows between the middle points per phase of the double-star armature winding through the rectifiers. However, the winding connection between the armature winding and the rectifiers are complicated. The three-sets of the rectifiers are needed by the stator excitation. The authors present a generator system by which the electric connection of between the stator armature winding and the rectifier can be simplified, and the number of the rectifiers can be reduced. In this system, the stator is provided with a double-star armature winding having two neutral points. In this paper, first, a circuit constitution and the principle for the brushless and exciterless three-phase synchronous generator are described, and secondary, the characteristics are made clear by being based on the characteristic analysis. Further, the results of the experiments for the generator carried on the test machine of 2.0kW are shown.
It is almost impossible to measure the starting performance of middle and large synchronous motors in a factory, because it needs a large capacity of equipments to test their performance. A new method is required to predict their performance by simple tests using a small capacity. This will make the motor's design and accessories more rationalized. Authors propose a new method which predicts the frequency response by using values of DC decay test in the armature winding at standstill. The key points of this method are summarized as follows: (1) DC voltage is applied to the armature winding (two terminals with the third one open) of a synchronous motor through a resistance. When two terminals are closed, the winding is shortcircuited and the current in the armature winding decays. The whole process of current decay is recorded. The impedance on motor terminals per phase are decided by Fourier transformation of the current decay. (2) This new method is quite useful. It is possible to predict easily and exactly a low frequency response below one Hz. The starting performance of a synchronous motor is predicted and agrees with actual data.