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Advanced Engineering 2020

NEC, Birmingham(B40 1NT)

04/11/2020 - 05/11/2020

The UK's largest annual advanced manufacturing trade show, Advanced Engineering is your opportunity to (more)

Drives & Controls Exhibition

NEC, Birmingham(B40 1NT)

25/01/2021 - 27/01/2021

The show brings together key suppliers of state-of-the-art equipment representing the multi-tasking culture (more)

Beating motor failure

Beating motor failure In any study of the factors that cause motors to fail, the logical place to start is the power supply. With a properly conditioned and regulated power supply, the output to the motor should be a perfect sine wave on each phase at the motor's ranted voltage and frequency. However, this is rarely achieved, and the result can be a host of problems including: harmonics, which cause overheating and reduced efficiency; overvoltage and undervoltage, the former reducing efficiency and power factor while the latter increases current and causes overheating; and voltage imbalances, which cause overheating and reduce efficiency. Added to these problems can be others such as voltage spikes - from capacitor switching and VFD cable trays - which cause motor insulation failures, frequencies under 50Hz from VSDs - which require additional cooling for the motor, and motor bearing damage from shaft currents, also the result of using VSDs. This problem can be overcome using an insulated bearing sleeve, electro-conductive grease, or a shaft grounding system. Overheating is one of the major causes of lubrication breakdown. However, even without this condition, the pitfalls of lubrication are many. Users tend to under-lubricate and over-lubricate in equal measure, and do not take enough care to ensure that the lubricant does not introduce contaminants into the motor bearings. In addition, the practice of mixing greases with different bases can cause grease constituents to separate and run out, with disastrous effects on the motor. The contamination that can be introduced into motors by lubricating motor bearings can be excluded with care. However, contamination, generally, cannot be completely excluded, either by total enclosure or by using explosion-proof motors. This is a concern as in many operating environments the resulting problems of abrasion, corrosion and overheating can destroy motors very quickly. Another major source of contamination is humidity. This becomes a problem when a motor is de-energised long enough to drop near the dew point temperature. At this point, moisture ingresses the motor, weakening the dielectric strength of electrical varnish and other insulating materials. It also contributes to the corrosion of bearings and other mechanical components. The key to avoiding these problems is to keep the motor (or motors) warm. Where possible, motors stored in humid environments should be pre-warmed before start-up to drive out insulation moisture. Last but not least is the problem of unusual or unbalanced loads. A variety of mechanical conditions exist that can either overstress bearings, leading to early failure, or distort the motor frame, causing asymmetric air gap which in turn can cause vibration and bearing failure or winding overheating. The conditions that cause these problems are to be avoided. They include: couplings that are misaligned; drive belts that are overtightened, or shelves that are misaligned; dynamic imbalances of the load or internal balance of the motor rotor; motor feet not in the same plane, and/or poorly shimmed; and mis-applied bearings.
 
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