Electric Motor Discussion Group
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Electric Motor Maintenance & Reliability
On the 19th November 2008 the NSW Industrial maintenance roundtable held it’s last Roundtable meeting for the 08 year. There were three formal presentations in the morning and after lunch attendees either went on a site tour of the OneSteel Rolling Mill plant hosted by James Taylor or took part in some discussions on Electric Motor Maintenance and Reliability. The notes below are a introduction to some of those motor discussions.
Introductions
Brian Miller and Ernie Miller attended from OneSteel – Ernie is currently involved with setting up specifications for repairing and buying AC motors for OneSteel looking at efficiency and reliability. One key question is what size motor should be thrown out as opposed to get repaired considering Life Cycle Cost and efficiency. They are currently using a number of suppliers including Dowdy & Mills, United and Potential and use a wide range of motors including WIG, Teco and CMG.
Perry Johns & Stephen Crebbin attended from Shell at Clyde Refinery. Perry indicated that Shell tend to achieve good reliability out of its motors. Shell is very close to the Dowdy & Mills motor workshop and they tend to use them for motor work exclusively and they are happy with the service and quality of workmanship they receive. Shell work to API specifications and Dowdy & Mills advise on cost of upgrades vs replacement required by API requirements (flame proofing) when motors come out of service. With high voltage motors shell go for three quotes from suppliers such as Toshiba, ABB or WEG but they don’t necessarily take the lowest price motor. On a recent quote prices varied from $70k to $110k to $180k at 40 to 52 weeks delivery. Shell look at motor efficiency and payback time as one of the decision items but in this instance motor size was the key restrain. Yong Chai and John Loughton attended from Sydney Water. They had most interest around their very substantial population of submersible pumps, which generally has good reliability. These pumps are mostly supplied to Flyt when they get removed are sent back to Flyt for overhaul.
The meeting was attended by Bradd Spinks, Mark Easton and Goce Caldrmoski from the Bluescope Steel Electrical Shop. The Bluescope electrical shop carries out most of the motor rewind work for the steelworks as well as doing a significant amount of on-site support services for motors. Along with the OneSteel guys they were very interested in the decision of when a failed motor should replaced with a new unit and when the old item should be rewound. The point was made that many new motors purchased on a lowest cost basis are of very poor quality and although rewinding the old item would cost a lot more the reliability and service performance may make the additional cost worthwhile. An example was given for some pump motors on the salt water cooling water supply system where older style motors had been replaced with newer motors and there had been significant reliability problems and strip downs had found a number of deficiencies including an amount of metal shaving in the motor. Zivko from the Bluescope’s Condition Monitoring group also attended. He was interested in condition monitoring methods for motors.
Mike Davis attended from MachineMonitor. Mike indicated that a technical term that has developed from the problem of metal shaving being left in motors in Magnetic Termites. This is where the swarf in the motor get picked up by the magnetic field and impacts on the insulation, bores in causing significant damage. This is cased by manufacturing short cuts and lack of attention to detail.
Motor Quality from Suppliers and Purchasing for Lowest life Cycle Costs
Mike from Motormonitor mentioned that even motors suppliers like Toshiba who have historically had an excellent reputation are now selling cheaper motors sourced from lower quality suppliers, to be able to compete in the market. Perry from Shell gave the example of their recent quote for three motors where when they looked at issues like power factor and efficiency buying the lowest cost item was not the best business option even though supply strongly supported buying the lowest cost item. Brad from Bluescope gave the example where they had quoted the plant $40k to rewind a failed motor but the lowest price quote for a new motor was only $44k. He suggested that non-electrical managers that often make the final decision often don’t understand the potential quality and reliability issues with lower cost motors and tend to go with the new replacement option.
Old Vs New Motors
Bluescope indicated that one of their very large Strip Mill DC motors had been replaced with a new AC motor. The old motors have lasted 50 years with up to three rewinds of the armatures. The major problem now with these old DC motors is with brushes and the commutators where regular brush changes get very expensive. The electrical shop guys have to stone the comms occasionally and sometimes on-site machine the comms. Mike from Machinemonitor suggested that there has been a reduction of the quality of the motors supplied in the last 20 years with manufactures looking for shortcuts to reduce manufacturing time. Windings are now often just glued together and for higher power machines it is likely that the windings will start moving, which will significantly shorten the motors useful life. Mike suggested that as this new Toshiba motor will have a three year warrantee, it is very important to do a very detailed internal inspection before the three years are up to identify any deterioration or symptoms that would indicate future deterioration.
Condition Monitoring of Motors
Zivko from Bluescope ask what electrical condition monitoring was carried out on motors. Most attendee organisations were carrying some vibration analysis of accessible motors. Perry from Shell indicated that they also do 12 monthly PM’s on their more important motors. Zivko indicated that they have been investigating the testing instrumentation from PdMA, Alltest Pro and SKF. Zivko indicated that one organisation that he has spoken to currently use the Alltest Pro test all motors that come into their store as new or overhauls as well as all existing store motors. If an existing stored motor fails the test they scrap it. As this occurs quiet regularly this was a concern to Zivko as he worried that they may be scrapping useful motors. They had one installed motor in their plant that was tested and it failed the test but two years later it still seems to be running OK. Ernie from OneSteel indicated that they do similar tests for their DC motors but they take much more notice of the trend of the condition parameters over time than the absolute value of the tests. Mike from Machinemonitor suggested that a major component of what makes up an PdMA or Alltest Pro meter is just an impedance and resistance meter with some complex modelling and they have a reputation of giving a lot of false positives. Bradd from Bluescope indicated that Alltest Pro found a “serious problem” with a motor in their Sheet and Coil plant and the motor was removed and sent to their shop. On inspection it became obvious that the problem was just a terminal connection issue that could have been easily fixed on-site. An example was given of the successful use of this technology as a project at Bluescope in Western Port where they used the Alltest Pro meter to find a large number of poor terminal connections. The suggestion was that this type of detect could be picked up using a micro-ohm meter to detect resistive imbalance between phases. Mike from Motormonitor suggested the Alltest Pro meter will only detect interphase resistance issues when a problem becomes significant. He indicated that a mico-ohm meter that uses 10 to 40 amps will be more much more sensitive to resistive issues such as poor connections then the low current systems used by equipment such as the AllTest Pro.
Mike from Motormonitor indicated that for High Voltage AC motors (>4KV) the is an excellent CM method is Partial Discharge (PD) analysis that can give up to 10 years of warning for failures. He suggested that on-line PD testing plus detailed 12 monthly inspections give a good monitoring strategy. Mike stressed that any PM/ CM analysis should group motors on Voltage as well as criticality. Below 4KV the failure mechanisms for motors change so high voltage test such as PD or DLA are not useful. Mike stressed that what is important to understand is the specific failure mechanisms relevant for each motor and often these can be confirmed with direct physical inspection and so this should be a central part of any maintenance strategy. For example if winding shorted turns is likely to be a failure mode, then interphase inductive imbalance would be a monitoring parameter. Mike suggested that he has been people monitoring for issues in motors that are not a possible problem for the particular motor.
