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Poor commutation traced to frame vibration
512
CURRENT TOPICS.
[J. l:.I.
fair to displace it. One of the most promising of these is lead azide, a salt of hydronitric acid. The chief characteristic of mercury ~ulminate which makes it valuable as a detonating agent is the ease with which it can be exploded by simple and ordinary means, and the characteristic mode of pressure development which brings about the explosive decomposition of the charge. A large number of chemicals are violent explosives and many exceed mercury fulminate in explosive power, but are less suitable as detonators, or are not suitable at all. Some are too sensitive to shock or friction, others too unsensitive ; mercury fulminate occupies a middle position. Many investigators have attempted to account for the initiating effect of detonating compounds, but so far no fully satisfactory theory of their action has been formulated. Poor
Commutation
Traced
to
Frame
Vibration.
ANON.
(Electrical World, vol. 69, No. 8, p. 372, F e b r u a r y 24, I 9 ~ 7 . ) - There are so m a n y well-known possible causes of unsatisfactory commutation that when a commutation trouble occurs with a machine of admittedly good design a trouble seeker is likely to direct his attention to eliminating the most common causes before looking for the more uncommon ones. In one particular instance the generator of a motor generator set, that consisted of a continuous current generator driven by an induction motor mounted on a rather long base, commutated rather badly. The sparking was not sufficiently bad to prevent operating the set, but so long as it continued the commutator could never take a polish. The operator had checked the speed, voltage, load, polarities, air-gap, field drops, brush sets, brush tensions, brush alignments, brush shift, and also the switchboard instruments, and everything appeared to be perfectly regular. The c o m m u t a t o r mica appeared to be flush with the bars, but, as it is sometimes difficult to tell with certainty whether or not mica is a little high, it was decided to slightly undercut it as an experiment. At this time a turbine construction man happened along (turbine men are usually very keen for any condition that is likely to produce vibrations), who noticed that oil was breathing out between the bearing cap and pedestal of the middle bearing. This struck him at once as a case of faulty alignment, with the possibilities and causes of which he was thoroughly familiar. T w o iron wedges were made and driven under the centre of the base on opposite sides of the machine. A relief was immediately noticeable, and the vibration that before had been evident only to the most skilful observer was missed at once. The sound emitted by the machine, especially at the brushes, became entirely different, and all sparking ceased. Investigation disclosed that the base of the machine as originally set was virtually resting on its four corners, so far as the distributing of the weight on the foundation was concerned, and the long span of the base had sagged.
CURRENT TOPICS.
[J. l:.I.
fair to displace it. One of the most promising of these is lead azide, a salt of hydronitric acid. The chief characteristic of mercury ~ulminate which makes it valuable as a detonating agent is the ease with which it can be exploded by simple and ordinary means, and the characteristic mode of pressure development which brings about the explosive decomposition of the charge. A large number of chemicals are violent explosives and many exceed mercury fulminate in explosive power, but are less suitable as detonators, or are not suitable at all. Some are too sensitive to shock or friction, others too unsensitive ; mercury fulminate occupies a middle position. Many investigators have attempted to account for the initiating effect of detonating compounds, but so far no fully satisfactory theory of their action has been formulated. Poor
Commutation
Traced
to
Frame
Vibration.
ANON.
(Electrical World, vol. 69, No. 8, p. 372, F e b r u a r y 24, I 9 ~ 7 . ) - There are so m a n y well-known possible causes of unsatisfactory commutation that when a commutation trouble occurs with a machine of admittedly good design a trouble seeker is likely to direct his attention to eliminating the most common causes before looking for the more uncommon ones. In one particular instance the generator of a motor generator set, that consisted of a continuous current generator driven by an induction motor mounted on a rather long base, commutated rather badly. The sparking was not sufficiently bad to prevent operating the set, but so long as it continued the commutator could never take a polish. The operator had checked the speed, voltage, load, polarities, air-gap, field drops, brush sets, brush tensions, brush alignments, brush shift, and also the switchboard instruments, and everything appeared to be perfectly regular. The c o m m u t a t o r mica appeared to be flush with the bars, but, as it is sometimes difficult to tell with certainty whether or not mica is a little high, it was decided to slightly undercut it as an experiment. At this time a turbine construction man happened along (turbine men are usually very keen for any condition that is likely to produce vibrations), who noticed that oil was breathing out between the bearing cap and pedestal of the middle bearing. This struck him at once as a case of faulty alignment, with the possibilities and causes of which he was thoroughly familiar. T w o iron wedges were made and driven under the centre of the base on opposite sides of the machine. A relief was immediately noticeable, and the vibration that before had been evident only to the most skilful observer was missed at once. The sound emitted by the machine, especially at the brushes, became entirely different, and all sparking ceased. Investigation disclosed that the base of the machine as originally set was virtually resting on its four corners, so far as the distributing of the weight on the foundation was concerned, and the long span of the base had sagged.