Dr Antony Anderson : Electrical Engineering Consultant : Electrical Consultant : Electrical Fault and Electrical Machine Failure Investigations

Winding failures on High Speed Magnetic Bearings [UK] The control systems for magnetic bearings have to act very fast. As a consequence, the bearing position control windings are repeatedly subject to rapid rates of rise of voltage, which, in turn tend, to cause partial discharges between turns in different winding layers, particularly if the windings are poorly wound and are not properly impregnated. The cure in this case was detailed attention to uniformly rewinding the coils, the use of sufficient inter-layer insulation and thorough vacuum impregnation, so as to minimise the risk of localised high voltage stresses.

In-service stator winding failures on deep well pump induction motors [Libya]: Water was circulated round the pump motor by an elementary centrifugal pump - with water being drawn in along the axis of the rotor and pumped out towards one stator end winding by centrifugal action through four holes placed around the rotor shaft. The spray action, rather similar to that of a garden water sprinkler, subjected the end windings to pulsating forces which in turn caused the end winding turns to vibrate. Because the windings were insufficiently restrained, both in the end winding and in the slots, the amplitude of winding vibration in the slots was high. Fretting against the stator core wore through the winding insulation, eventually causing winding shorts to ground. The failures occurred not in the end winding, but in the stator slots, some distance from the core end. Recommendations included stiffening up the stator end winding supports and protecting the winding from direct water spray impact by means of baffles.

Rotor winding failures on generators [EU and South America] the generators in question, built by different manufacturers, were driven by reciprocating gas engines and were therefore subject to high levels of vibration. The rotor winding failures were progressive, with more and more turns short-circuiting over time. On examination windings showed multiple sites where micro-arcing had occurred. The damage was consistent with the initial inter-turn breakdown not resulting in a hard short circuit as might be expected normally but in an intermittent contact that could be made and broken by vibration. As a result of the making and breaking of current high transient inter-turn voltages would have been set up that could intiate further breakdowns. The remedy was to provide a higher degree of insulation on the rotor winding turns.

Unreliable hot drinks dispensing machine. [UK. France] The hot drinks dispensing machines worked satisfactorily in the UK, but when exported to France had a habit of tripping, as if on overload. The question was why. It turned out that in the UK customers tended to want mainly coffee and tea dispensed, but in France hot chocolate was more popular. The charge of chocolate was heavier than the charge of tea or coffee, with the result that when the machine was fully loaded with cups for dispensing hot chocolate, the carousel ran very slowly and the timer , detecting what it thought was a jammed cup, tripped out. It turned out that some time previously the original motor - a substantial single phase ac induction motor suitably geared down - had been replaced by a much smaller geared dc motor originally not designed for the purpose, but for use as a car seat adjuster. Because the gear ratio was wrong for the new application, the motor had to be run significantly below its design voltage of 12V in order to get the carousel to run at the right speed. As a result, there was insufficient power to drive the fully loaded machine. The remedy was simple - replace the DC motor with the original AC motor. Why the original AC motor had been replaced by the DC motor in the first place was a bit of a mystery - The AC design was much simpler and had no drive electronics. It had more than adequate capacity to drive the heaviest load and never went wrong. The moral: if it ain't bust dont fix it.

Alleged electric shock [UK]from an electric lift call button. A hospital porter was wheeling a trolley down a corridor towards a lift so that he could take the trolley up to a higher floor. According to his story, he touched the lift button and received such a substantial shock that it threw him back through an open door into the laundry cupboard. There were no witnesses. Once in the laundry cupboard, after a while he recovered sufficiently to stagger out of the cupboard and call for help. He was rushed to A & E where he was medically examined, diagnosed as possibly suffering from post-traumatic shock and was kept in overnight for observation. He was later seen by medical experts who all agreed with the initial diagnosis. However, in the medical records there were no photographs and no observations of the physical signs of local burning that might be expected in the case of serious electric shock. The Lift Maintenance Company was called in to examine the lift and found nothing wrong with it. In particular, the metal plate surrounding the lift button was found to be well earthed, as it should have been. The lift was brought back into service without any modifications, none being required. In the records, there were several complaints of lift passengers getting minor electric shocks from time to time. The alleged victim of electric shock claimed a large sum of money from the hospital for post traumatic shock. A visit to the hospital was arranged for the electrical expert. It soon became apparent that by walking across the floor any individual became electrostatically charged and on touching either the lift button or the earthed surround resulted in them discharging themselves. The spark discharge was audible, just visible to the naked eye, and was detectable with a small radio receiver. It was estimated that the maximum electrostatic build up of charge - with the weather dry and the porter fully charging both himself and the trolley - was about one eightieth of that required to cause any kind of significant muscle contraction. In the opinion of the expert there was no way in which the electric charge discharged could possibly have caused sufficient current to flow to cause the claimed degree of muscular contractions that might have resulted in the claimant involuntarily ending up in the laundry cupboard. The case was settled on the basis of the expert's report without going to court.

Electrical Investigation of a Chicken Hatchery fire . The fire started in a room where the plastic trays used for the hatchlings were washed. The incident occurred early one morning just prior to the changeover from the night shift to the day shift. The initial report written by the insurance company's expert noted the presence of arc damage in the overhead wiring above a stack of fire-damaged trays but concluded that the arcing was the result of the fire not the cause of it. The expert's hypothesis was that someone had used a cigarette lighter to set fire to the plastic boxes and that these had set fire to the cable insulation and that the arcing in the overhead wiring was the result. The only evidence cited was that the fire brigade had been able to set a stack of boxes on fire by holding a cigarette lighter to the side of one box for 40 to 50 seconds. An electrical expert was not called in at the time to give a second opinion on the significance, or otherwise of the arcing. Having fixed upon arson as the likely cause of the fire, the insurance expert then proceeded to identify the presumed arsonist, who was in due course arrested and charged. An independent electrical expert's report was commissioned by the Defence shortly before the trial which showed that the likeliest cause of the fire was a wiring fault, itself the result of poor maintenance, which caused arcing in the overhead ducting and a subsequent cable fire, which then spread to metal-clad polystyrene wall and ceiling panels and thence to the plastic trays. The presence of high levels of moisture, as a result of the use of high pressure sprays in the washroom, may have been an additional factor in initiating an electrical breakdown. As a result of the Defence electrical expert's report, the case was dismissed four working days before the trial.

Intermittent electronic malfunctions in automobiles [USA, UK, Sweden, France] - a number of investigations of sudden uncommanded acceleration of vehicles in the USA, UK, Sweden and France. One case of suspected instability in a vehicle fitted with electronic stability control [UK].

Vehicle electrical fires - One battery fire [UK], one fire caused by a faulty cruise deactivation switch [USA], one fire where vibration fretted electrical insulation and caused a short circuit[UK].

Dr Antony Anderson C.Eng FIEE/FIET

Electrical Fault & Failure Investigations

Examples of electrical fault and other investigations carried out recently by Electrical Consultant Antony Anderson :

© Antony Anderson 2000