摘要:

Partial-discharge measurements are being used increasingly to detect localised or damaged areas in modern high-voltage stator windings. However, ambiguities may arise because of the complexLCnetwork between coils. Discharge pulses propagate in a variety of modes, each having different attenuation factors. Tests with diamond-wound motor stators and a 500 MW generator stator show that pulse detection in a limited frequency range, for example 20–300 kHz, is most suitable. Here, attenuation errors are restricted to within a factor of two to three, whereas a wideband detection with the bandwidth in excess of 1 MHz may produce up to a 50-fold attenuation of signals from sites remote from the terminals of the stator windings. Electromagnetic probes and ultrasonic scanning techniques are useful for locating the sites of high discharge. Their accuracy and limits are reported.

DOI:10.1049/ip-b.1985.0034

出版商:IEE    

年代:1985

数据来源: IET

摘要:

The paper describes a chopper circuit using three gate-turn-off thyristors in which the need for regrouping contactors, to change from motoring to braking or from braking to motoring has been eliminated. As the braking and motoring components are independent they can be rated for their specific duty cycles. Not only do gate-turn-off thyristors permit a higher chopping frequency than achieved with choppers using conventional thyristors, but they also enable the circuit to be reconfigured during the chopping cycle. In this way, line-voltage changes and other disturbances can be dealt with more precisely, within the chopping cycle, rather than more slowly over a period of several cycles as is the case when conventional thyristors are used.

DOI:10.1049/ip-b.1985.0035

出版商:IEE    

年代:1985

数据来源: IET

摘要:

To achieve loss reduction in large power switches based on bipolar transistor technology, the injection of a compensating voltage into the Darlington loop is investigated theoretically and experimentally for several configurations. The theoretical model predicts that a total loss reduction of 50% in the steady-state could be possible in comparison to conventional Darlington and directly-driven single bipolars. Experimental measurements on a power switch configuration for realising all three types of switches compared theoretically bear out these expectations. The complications of the technology to achieve this compensation, as well as the influence on turn-on and turn-off, are subsequently evaluated. It appears that simple solutions for realising the compensation by high-frequency Royer oscillators are feasible, and the compensation does not affect turn-on time or falltime. Turn-on and turn-off technology as developed for single bipolars and Darlingtons achieves the same results in the compensated Darlington power switch. The storage time corresponds to what is found for single bipolars. It is concluded that the compensated Darlington high-current power switch is a viable possibility for application in convertors where low losses are combined with requirements for fast turn-on and turn-off.

DOI:10.1049/ip-b.1985.0036

出版商:IEE    

年代:1985

数据来源: IET

摘要:

Traditional PWM AC motor drives perform well over a wide speed range and have many positive features associated with their simplicity. However, the waveforms used are significantly less than ideal at high modulation depths and low switching rates, particularly with regard to harmonically induced motor losses. An improved strategy for digitally producing PWM based on conventional triangulation methods is presented. A novel technique for reducing the harmonic losses resulting from sinusoidal PWM waveforms is analysed in depth and realised on a 40 kVA inverter. Tests with this inverter on 7.5 kW and 15 kW motors confirm that the inverter's performance is significantly enhanced for certain operating conditions, without any significant sacrifice.

DOI:10.1049/ip-b.1985.0037

出版商:IEE    

年代:1985

数据来源: IET

摘要:

Clay-graphite and carbon-bonded silicon-carbide crucibles are used in nonferrous induction melting furnaces. Silicon-carbide crucibles especially have encountered premature failure when used at high power densities and operating frequencies. This is thought to be related to their nonuniform properties. To gain a more thorough understanding of this problem, an equivalent circuit analysis has been applied to the composite load of crucible and metal charge. An explanation of the analysis is put forward and results given including electrical characteristics, the distribution of power input between charge and crucible, and resultant meniscus height for typical industrial installations. The paper also explains the use of probes to measure the current density and magnetic field strength on the surface of these crucibles. The results are used to estimate electrical resistivity and power densities at different axial points of the crucible.

DOI:10.1049/ip-b.1985.0038

出版商:IEE    

年代:1985

数据来源: IET

摘要:

The torque available from small synchronous machines having high-coercivity rotor magnets is calculated from purely electromagnetic considerations. A general number of stator slots is considered together with circular and trapezoidal slot profiles. A method of optimising the stator lamination is presented for both slot forms. Torque performance and lamination details are given as a function of the airgap flux density produced by the rotor magnets. Experimental data is given in support of the findings.

DOI:10.1049/ip-b.1985.0040

出版商:IEE    

年代:1985

数据来源: IET

摘要:

The paper analyses the requirements for transient phase balance when operating 3-phase induction motors from a single-phase supply. It is shown that exact phase balance can never be achieved with a single-capacitor phase convertor. The required parameters of a two-element phase convertor are determined as functions of motor slip. AC phase control and AC current chopping are used to maintain the convertor parameters at the required values. It is shown that balanced phase conversion can be achieved from standstill to full speed in a direct-on-line starting, cage induction motor drive. The system is capable of accelerating a loaded motor and of maintaining supply balance to the motor as the load changes.

DOI:10.1049/ip-b.1985.0041

出版商:IEE    

年代:1985

数据来源: IET