摘要:

Steep-fronted voltage transients, generated by prestriking conditions in circuit-breakers and contactors when closing, produce severe inter-turn insulation stresses in machine windings. There is a need to calculate the nature and severity of these transients at the system design stage. A computer program simulating the production of prestriking transients is described. The simulation is based on a full three-phase system representation, including busbars, switching device, cable and motor winding. The complex interaction between the system and the switching device, and also the interaction between the three poles of the switching device, are fully taken into account. A solution method based on the Fourier transform and using a combination of voltage and current generators to simulate switching actions is used in the calculations. Validation of the accuracy of the simulation is made by a comparison of on-site test results with the computer simulation. It is shown that system conditions can be simulated with a reasonable degree of accuracy. Illustrations of the wide range of system variables, current and voltage, that can be determined are presented. Finally, to indicate the utility of the program two studies are made for a simple system controlled by a vacuum switching device. The first of these studies examines the effect of mechanical pole closure scatter. The results clearly indicate the large effect that this parameter has on the magnitude and severity of prestriking transients. The second study gives a comparison of series inductor and shunt resistor-capacitor schemes for motor winding prestriking transient protection. The results of this study indicate that although both schemes are useful in reducing the severity of prestriking transient voltages, the choice of which particular scheme to use, purely on a voltage severity basis, is not as clear as might be initially considered.

DOI:10.1049/ip-b.1991.0013

出版商:IEE    

年代:1991

数据来源: IET

摘要:

This paper investigated the magnetic field distribution in a 1 hp three-phase induction motor. Utilising a triangular finite-element method, the magnetic field in the motor is analysed at various balanced magnetomotive forces and four different rotor positions. From the field solutions, the flux linkages and the slot leakages are found for a range of magnetising currents. An experiment using a straight laminated rotor core without aluminium bars was tested at the four locked positions by using a 20 kVA balanced three-phase transformer with a frequency of 60 Hz. The stator currents, line-to-line voltages and power factors were measured. The experimental data are compared with the calculated results and a reasonable agreement is found. Over a wide range of saturation level, the paper determines parameters such as the magnetising inductance and the core loss resistance in the machine equivalent circuit.

DOI:10.1049/ip-b.1991.0014

出版商:IEE    

年代:1991

数据来源: IET

摘要:

From the viewpoint of reactive power compensation, the decomposition of Czarnecki's reactive current component is discussed. When the number of harmonics present in the source voltage/current is higher than the number of reactive elements of the compensator, the reactive power can only be partially compensated. Thus the reactive current can be visualised as the sum of two components: the compensated reactive current and the residual reactive current. These two components are orthogonal, which can be used to estimate compensation effects.

DOI:10.1049/ip-b.1991.0015

出版商:IEE    

年代:1991

数据来源: IET

摘要:

A single-phase switching mode bilateral convertor using current-forced hysteresis switching control is designed and successfully implemented. The power circuit configuration is simple, since only one full-bridge convertor is required, and the lock-out circuits are not necessary. Based on the proposed current-forced switching scheme, the bidirectional power flow is very easy to control and the switching voltage jumps and losses can be reduced. Moreover, the line current can be sinusoidally regulated with unity power factor in the rectifier mode, and the voltage waveform distortion can be kept small in the inverter mode by instantaneous voltage control. To perform the control system design, the transfer function models of the proposed convertor system in two operation modes are built up, and then a generalised systematic design method is proposed to find the parameters of the controllers according to the prescribed specifications. Some experimental results are provided to demonstrate the effectiveness of the proposed convertor.

DOI:10.1049/ip-b.1991.0016

出版商:IEE    

年代:1991

数据来源: IET

摘要:

The relation between the stator current and the field current of a synchronous motor at a constant terminal voltage and with a constant shaft load is known to be a V curve. It is shown, by both experiment and calculations, that the stator current of a hysteresis motor also shows a V curve. The mechanism of the V curve of the hysteresis motor is discussed. The relation between the hysteresis motor and the synchronous motor is clarified.

DOI:10.1049/ip-b.1991.0017

出版商:IEE    

年代:1991

数据来源: IET

摘要:

A three-phase flyback AC-DC convertor, which has the step-up and step-down characteristics of output voltage, is described. The introduction of pulsewidth modulation makes it possible to obtain the sinusoidal supply currents with a near unity power factor. Three gate-turn-off thyristors provide the necessary control over the currents for unidirectional power flow. To improve upon the oscillatory transient operations, the tested system includes the digital PID controller using a microcomputer for output voltage regulations. The experimental results and state-space simulations confirm the usefulness of the presented convertor.

DOI:10.1049/ip-b.1991.0018

出版商:IEE    

年代:1991

数据来源: IET