Discussion at Session 1

Authors' Introductions

Mr S. A. Andrews: Paper 1

In many applications in aero-gas turbines a considerable amount of trouble had been experienced with slip-rings. One of the shortcomings was the limited useful life of the contacting brush and ring. This was particularly so when the surface speed at the point of contact was large; rapid wear of the brushes then occurred, which produced graphite dust and hence lowered the electrical insulation. Also, if the contact point between the brush and the ring was at a high temperature there would be a high level of thermal noise. High surface speeds also made brush bounce and transient resistance changes more likely. Although the effects of these resistance variations could be nullified by the method described in §§ 68 and 69, the spurious thermal e.m.f.'s still remained. These would be reduced if there was a reduction in the temperature at the point of rubbing contact. Some experiments carried out by Mr Andrews' company using silver graphite brushes impregnated with PTFE had produced quite encouraging results. Using this material it had been possible to lower hash ten to one, with only 5ftV of noise.