DOI:10.1049/pi-a.1962.0001

出版商:IEE    

年代:1962

数据来源: IET

摘要:

At the present time there exist no established, non-destructive methods for the measurement of depth of hardness in surface-hardened steels which are independent of the effects of chemical composition and quench procedure.Electrical non-destructive methods are dependent upon changes in the electrical and magnetic properties of steel which occur when it is hardened.The electrical method investigated by the authors was an a.c. one, based upon the measurement of the complex impedance of a search coil magnetically coupled to the test surface. Distinctly favourable results were obtained, and the paper is concerned with the theoretical and practical aspects of the procedure.

DOI:10.1049/pi-a.1962.0002

出版商:IEE    

年代:1962

数据来源: IET

摘要:

Ionic impurities in insulation not only affect its insulating properties but can result in undesirable electrochemical action and premature failure under electric stress. Impurities may ionize in traces of water the insulation, but it is also known that electrochemical effects can occur in the absence of moisture; in such cases the impurities ionize in the insulation itself. A sensitive test for ionizable organic impurities a material is the measurement of the electrical conductivity of an extract of the material in a suitable pure organic solvent. Details of recommended test are given. Solvent extraction tests are suitable for testing materials before they are assembled into equipment.Ionic impurities in a liquid-impregnated solid dielectric, such as in oil-impregnated-paper capacitors or cables, can be detected without harm to the assembly by studying the loss-angle/voltage characteristic. This technique may be used for studying changes in ion concentration during service.

DOI:10.1049/pi-a.1962.0003

出版商:IEE    

年代:1962

数据来源: IET

摘要:

A ‘pinhole’ in enamel on a wire is a spot of insufficient electric strength to pass certain tests. The B.S. method of counting pinholes requires the wire to be passed through a mercury bath. This is considered destructive because it contaminates the enamel surface. A non-destructive method is described in which the wire is run through an open tube held at a sufficiently high voltage for corona to occur on the passage of a pinhole. It may be applied on enamelling plant and winding machines.Corona onset conditions are discussed and a formula is derived for the minimum electric stress to be applied to the bare wire surface. Peek's empirical law for the onset of visible corona between coaxial electrodes is recommended for calculating the maximum permissible surface stress.A suitable high-voltage supply unit and pulse-detecting and -counting circuits are suggested. Attention is drawn to the data-handling problem arising from the accumulation of a large amount of test information which must be reduced to an accept/reject decision. The solution adopted in a multi-channel instrument is based on the use of magnetic-core sealers and paper-tape distribution recorders.

DOI:10.1049/pi-a.1962.0004

出版商:IEE    

年代:1962

数据来源: IET

摘要:

Specimens weighing 10g or less and carrying easily wound coils about 20 turns can be used for measuring magnetic properties. The small signals from these low-inductance specimens are easier to handle if they are alternating rather than direct current.Bridge networks adapted to measure low impedances are recommended for measuring permeability and loss, at either low or high flux densities and at frequencies up to 10Mc/s. Coercivity, remanence and saturation can be measured by subjecting specimens to pulses superimposed on various values of bias and integrating the resulting signals.Appendices discuss the choice of series or parallel circuits to represent loss in a magnetic material, and methods of measuring the time-integral of a signal.

DOI:10.1049/pi-a.1962.0005

出版商:IEE    

年代:1962

数据来源: IET

摘要:

Absorption of moisture causes paper to change its electrical resistance and permittivity. The second effect is approximately linear and little affected by changes in furnish. It is most conveniently measured by means of a fringing capacitor. Instruments for use statically and on a moving web are described. Theories of the water-cellulose bond are discussed.

DOI:10.1049/pi-a.1962.0006

出版商:IEE    

年代:1962

数据来源: IET

摘要:

Over a period of about 40 years no significant changes have been made in the methods recommended for the determination of the surface resistivity of dielectric materials. Recent work has shown that results obtained by these methods may not be accurate. The errors inherent in measurements of surface resistivity are caused by the shunt resistance through the bulk of the material and by contact resistance at the electrode-surface-film interface. The sources of these errors are discussed and methods of reducing them are recommended.

DOI:10.1049/pi-a.1962.0007

出版商:IEE    

年代:1962

数据来源: IET

摘要:

The insulation-resistance test is used as a non-destructive method of detecting the presence of water in insulating components in service. Water penetrates only the outer layers of oil-immersed Bakelized paper leading to the formation of a low-resistance surface skin. It is shown that, by taking account of the operating conditions, a critical surface conductivity can be estimated above which the insulation will fail in service.The correlation between conductivity under working conditions and insulation resistance is described. A method of estimating a rejection limit of insulation resistance for a component in service is suggested. The measurements needed are stated.

DOI:10.1049/pi-a.1962.0008

出版商:IEE    

年代:1962

数据来源: IET

DOI:10.1049/pi-a.1962.0010

出版商:IEE    

年代:1962

数据来源: IET

摘要:

Investigations of a number of generator stator-insulation failures have shown that breakdowns of several machines generating at 33 kV were initiated by discharge erosion of the insulation. The paper describes a method of measuring discharges in stator insulation. The technique consists in counting the number of discharge pulses exceeding a certain level. It is concluded that this proposed instrument has several advantages over other types of discharge detector.It is shown that, at the present time, this method, or indeed any other means of measuring discharges, cannot determine the depth of discharge erosion in generator stator insulation. In a complete stator winding, the internal discharges tend to be concealed by slot discharges, which may or may not be harmful, depending on the surface conductivity of the dielectric.

DOI:10.1049/pi-a.1962.0011

出版商:IEE    

年代:1962

数据来源: IET