摘要:

The different kinds of electrically driven conveyors which are used underground for carrying the coal from the face to the haulage road are first described.The different classes, viz.shaker conveyors, belt conveyors, and scraper conveyors, and, in association with these, gate-end loaders and similar apparatus, are considered in turn.The special advantages of using conveyors and loaders in groups are discussed and the difficulties to be overcome in using such combinations are described; special stress is laid on the switching arrangements necessary to enable full advantage to be taken of such grouping.The special points to be attended to in the design of electrical motors to be used for driving this class of apparatus are indicated.The economic advantages of electrical conveying are then discussed; and examples of typical results obtained, showing the savings gained by the new methods, are given

DOI:10.1049/jiee-1.1932.0082

出版商:IEE    

年代:1932

数据来源: IET

摘要:

The development and advantages of commercial cooking by electricity are set out in detail, and particulars are given of the types of apparatus used in this country and abroad.Representative installations in establishments of different characters are described, and the effect of the commercial cooking load on central stations in various countries is discussed.

DOI:10.1049/jiee-1.1932.0087

出版商:IEE    

年代:1932

数据来源: IET

摘要:

The paper is concerned with the behaviour of earthed open- aerial receiving systems. The analysis of such systems is based on the classical transmission-line equations, as modified by Moullin to apply to distributed excitation.Part 1 gives the theoretical formulæ so obtained for the case of a plain aerial with uniformly distributed constants, and the detail of the variation with frequency of the resistance and reactance components of the effective impedance of such an aerial.It is shown that successive resonances (i.e. conditions of zero reactance) are given by an equation of the formλ=(4h/n)(1+Ah2)wherenis any integer andAis a small factor depending onnand on the attenuation constant of the aerial considered as a transmission line. The corresponding values of the resistance are shown to beRh/2(approximately) whennis odd, and a very high value whennis even,Rbeing the total effective resistance per unit length of the aerial.The actual variation of the impedance of a plain uniform aerial as a function of frequency was determined experimentally, and a detailed analysis of the results in relation to the theory is given.The observed variations of resistance and reactance are shown to be in substantial agreement with those deduced from the analysis, but it appears that the actual length of the aerial (h) must be replaced by an effective lengthh+ ε, ε being about 5 per cent ofh. This is in agreement with a more rigid analysis by Abraham, and with experimental results obtained by Wilmotte. The observed aerial resistance was found to be mainly due to parasitic eddy-current and dielectric losses. It was reduced to less than one-third of its value by using a single-wire “earth screen.”The validity of the fundamental assumptions, to a useful degree of approximation, having been established by measurement, Part 2 of the paper is devoted to obtaining additional information by purely analytical methods. The analysis is mainly based on the case of a plain aerial divided into three parts in each of which a uniformly distributed e.m.f. is assumed to be induced, the intensities being different in the three parts.By an analytical manipulation of this three-part case a line integral formula is found for the effective e.m.f. induced in a uniform aerial by a non-uniform field (Section 7). The variation of the effective e.m.f. induced by a uniform field is considered as a function of frequency and aerial height, and it is shown that no advantage is gained by makingh>λ/4unless certain parts of the aerial are compressed (Section 8).The relative effectiveness of different parts of a receiving aerial is considered in Section (9), and it is found that in an L aerial short compared with the wavelength the addition of a down-lead from the open end nearly to the ground may actually increase the total effective e.m.f. in spite of the reversed e.m.f. induced in the added length.An earthed aerial of this shape with the vertical members of length λ/4 and the horizontal member of length λ/2 is shown to have a figure-of-eight polar diagram with a law of the form cos(½12πcosΘ (Section 10).The effective e.m.f. formulæ of the three-element case are applied to the Franklin suppressed half-wave construction and afford means of allowing for the effect of aerial resistance in this system (Section 11).In Section (12) the effective e.m.f. of the present formulation is related to the original conception of “effective height.” It is suggested that the idea of “effective height” is one which has to a great extent outlived its usefulness.The analysis of a receiving aerial considered as a collector of energy from the incident wave is given in Section (13). The figure of merit from this point of view is |e6|2/Re, which is determined as a function ofhand λ. It reaches a maximum whenhis very approximately λ/4, and thereafter oscillates with rapidly diminishing amplitude. Considered as a collector of energy the suppressed half-wave construction will not apparently give an increase in useful power proportional to the increase in height, owing to the corresponding increase inRe.Current and potential distributions in receiving and transmitting aerials are discussed in Section (14). It is shown that they depend on the distribution of the exciting field, and that the current distribution in a receiving aerial may be totally different from that in a transmitting aerial of the same height. Current nodes in a receiving aerial may be any distance apart from 0 to λ, and are not necessarily separated by half a wavelength as in a transmitting aerial with a point source of excitation

DOI:10.1049/jiee-1.1932.0101

出版商:IEE    

年代:1932

数据来源: IET

摘要:

Designers of alternating-current machinery have long been able to make rapid estimates, by means of published curves, of the heating effect due to current displacement in slot-wound conductors. The wave-shape of the current in the windings of direct-current armatures is complex, since it is rich in harmonics. The estimation of the heating effect due to current displacement in direct-current armatures has hitherto necessitated laborious calculation, for each case on its own merits. The busy designer rarely has the time to carry out such a long process, and it is probable that in most cases guesswork is resorted to.In this paper an attempt is made to provide curves, by the use of which the heating of direct-current armatures may be estimated rapidly. It is believed that results are obtainable with fair accuracy, and with almost the same rapidity as is associated with the estimation of the loss in an alternating-current machine. It is not possible to construct one curve for use in all cases; a “standard” curve is given, which refers to a single-turn full-pitch winding, having 15 slots per pole and 45 commutator segments per pole. Two additional curves give coefficients which enable other full-pitch single-turn windings to be dealt with. The heating ratios so obtained do not differ from those obtained from individual calculations by more than 2 per cent in any practical case. Curves are also given for windings which differ from the standard winding in having (a) two turns per coil, (b) three turns per coil, (c) chording. The question of the critical height of conductor, and the question of the use of unequal heights for the two layers in a slot, are alsotouched upon.It is, perhaps, not generally realized that current displacement is present at all times in a direct-current armature winding; it is not confined to the period of commutation. A set of current-distribution curves has been calculated for one case, to show this.The author hopes to deal with the problem of the rotary convertor, and the problem of unusual winding arrangements designed to reduce heating losses, in a subsequent paper.

DOI:10.1049/jiee-1.1932.0106

出版商:IEE    

年代:1932

数据来源: IET