Cables skin effects

Exciting developments based on electromagnetic induction raced along from that time, giving us the sophisticated products our everyday lives depend on. During most of the period productive uses for eddy current technology were few and few people believed in it as a usefiil tool eddy currents caused power loss in electrical circuits and, due to the skin effect, currents flowed only in the outer surfaces of conductors when the user had paid for all the copper in the cable. The speedometer and the familiar household power meter are examples of everyday uses that we may tend to forget about. The brakes on some models of exercise bicycle are based on the same principle. 

The influence of skin effects in a multi-core cable is almost the same as that of a multiphase busbar system, discussed in Sections 28.7 and 28.8. However, unlike a busbar system, the resistance and inductive reactance for various sizes of cables can be easily measured and are provided by leading manufacturers as standard practice in their technical data sheets. To this extent, making an assessment of skin effects in cables is easy compared to a busbar system. Since all the phases in a cable, of a 3-core or 3 72-core are in a regularly twisted formation throughout the length of the cable, they represent the case of an ideal phase transposition (Section 28.8.4(3)) and almost nullify the effect of proximity. 

However, there may not be an appreciable improvement in the proximity effect between each section, unless the transpositions are increased infinitely, as in the case of a stranded three-phase cable which has continuously twisted conductors and represents an ideal transposition. In addition, there is no change in the skin effect. This arrangement therefore has the purpose primarily of achieving an inductively balanced system and hence a balanced sharing of load and equal phase voltages at the far end. 

Current flowing in a cable produces PR losses. When the load current contains harmonic content, additional losses are introduced. To compound the problem, the effective resistance of the cable increases with frequency because of the phenomenon known as skin effect. Skin effect is due to unequal flux linkage across the cross section of the conductor which causes AC currents to flow only on the outer periphery of the conductor. This has the effect of increasing the resistance of the conductor for AC currents. The higher the frequency of the current, the greater the tendency of the current to crowd at the outer periphery of the conductor and the greater the effective resistance for that frequency. 

The capacity of a cable to carry nonlinear loads may be determined as follows. The skin effect factor is calculated first. The skin effect factor depends on the skin depth, which is an indicator of the penetration of the current in a conductor. Skin depth (5) is inversely proportional to the square root of the frequency ... 

The ratio of the resistance of the cable at a given frequency to its resistance at 60 Hz is defined as the skin effect ratio, E. According to this definition,... 

Low technology does not imply wrong or bad technology. In fact, ease of maintenance, ease of repair in the event of failures in the field (one can use a piece of heavy cord to get by if the control cable should break), and the intuitive understanding of pulling on one end of the cable to effect motion at the other are probably major reasons for the success of Bowden cables in prosthetics. This is in addition to the ability of users to sense prosthesis state by the pull or feel of the control cable and harness on their skin. 

A 16.6 Skin and proximity effects in a multicore cable 16/547 A 16.7 Short-time rating of cables 16/547 A 16.8 Termination of cables 16/548... 

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