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Using the AC Drive Motor as a Transducer for Detecting Electrical and Electromechanical Faults

By Mark Lane


Condition monitoring of AC motors is a subject area that has received extensive research. Whether this monitoring is carried out on a scheduled basis by engineer intervention, or continuously using an on-line unit, the results of this testing enable preventative maintenance work to be a carried out earlier, before any major failure occurs. Monitoring using vibration analysis is the most common and depending on the plant, can be done once or twice a year. This is usually limited to the condition of motor bearings and is not commonly used to detect failures in the motor electromagnetic systems. Monitoring units that use motor current measurements are also available, but these are less widely-used and usually on major plant motors (>250kW for example) that have a large capital outlay to replace. \ud \ud \ud The industry drivers – as always – are maximum plant and machinery uptime, with the minimal amount of scheduled maintenance. If maintenance is carried out too regularly, costs rise significantly not only due to the maintenance activity itself, but disruption to production schedules. Maintenance schedules that are too infrequent can result in an unacceptable rise in total failures of plant that are unexpected and may cause a significant amount of production disruption and downtime, especially if this occurs during out-of-hours working time. However, industry now faces another big challenge and one that has had a good share of exposure over the last few years. It is of course, the drive to reduce carbon emissions and with it the amount of energy that a plant itself consumes. What has brought this more to the fore recently is the significant rise in energy costs. Whilst product margins have to remain the same, many companies energy costs have seen a two to three-fold increase in energy budgets in the last few years alone. For industry processes that have a significant amount of fan and pump applications, the manufacturers of low-cost AC inverters have saturated the lower-performance market of inverter drives such that any drive can control these type of fan and pump applications, where accurate speed control is not a major driver.\ud \ud \ud Unfortunately, this can be a step backwards for end-users of plant that use equipment to monitor motor condition via motor current signals. Additionally, vibration analysis that relies upon ‘base-lining’ motor data when the AC motor is running at base speed may not give accurate readings when the motor is under inverter control and running at a different speed. \ud \ud \ud For manufacturers of AC inverter drives in this low-end market, it can be difficult to sell a product from one manufacturer over another without the unit having a “USP”, or Unique Selling Proposition. Most decisions taken on inverter equipment purchase at this level are usually in favour of the equipment that costs the least to purchase. Credibility of manufacturers based on product history and perceived reliability cuts little ground with an ever cost-conscious industry. \ud \ud \ud This is where the research into diagnosis of faults on inverter driven motor systems can provide just this USP for manufacturers. If the incorporation of on-line diagnosis for simple inverter applications can be brought to a typical inverter unit at a reasonable cost, the manufacturer who can offer this gains a unique foothold in the marketplace – a drive that can monitor and signal that the motor it is driving is showing signs of early failure. \ud \ud \ud It will be sensible to limit this research to simple inverter applications as high-end inverter drives that operate equipment such as high-speed printing presses, rotary shears will be more difficult to model and simulate than a fan or pump application.\ud \ud \ud It is hoped that a typical inverter drive can relay enough detailed information about the load which it is driving to allow this to be used for abnormal motor load conditions as this will provide a platform on which to extend the research beyond this MSc and into the realms of incorporating such technology into a drive manufacturers equipment. If this can be done without major modification to an inverter, then it may be easier to implement in equipment offered by different manufacturers. \ud \ud \ud It is quite possible that this technology could be licensed under a name that guarantees the performance of the condition monitoring algorithms and reliability from one drive manufacturer to another

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