Up Time Magazine- June/July 2008

motor testing upload Anatomy of an Efﬁcient Motor Repair Ensuring Efﬁciency and Reliability in Rewound Motors by Howard W. Penrose, PhD I n part one of this article in the last issue, we discussed the importance of a motor repair customer working closely with the motor repair center to ensure that the repair is completed so that there is no loss of efficiency or reliability. We looked at the results of motor rewind studies that concluded there could be efficiency losses ranging from 0.5% to 3.4%, unless proper steps were taken to mitigate these losses. We also discussed: several reasons for AC induction motor losses, the importance of checking all of the mechanical fits on a motor and techniques for restoring the bearings to fit properly, and initial winding tests to be performed. In this article, we will cover coil removal practices, stator winding, post winding tests, varnish insulation and final testing. Mechanical Stripping (Dreisilker/Thumm Method) – Using a heat source, such as gas jets, a distance away from the core, the back iron and insulation is warmed until the windings become soft and pliable (approximately 10°C above the insulation class of the varnish insulation). The coils and insulation are removed using a slow, steady hydraulic pull. Temperatures remain low, stripping times extremely fast (about 2.5 hours for a 350HP motor), and there are no significant airborne by-products or disposal problems. Attempts at duplicating this process using pneumatic pulling methods have resulted in core laminations being pulled apart. Therefore, pneumatic machines of this type should be avoided. Coil Removal Practices At this point, and for the purpose of this article, it is assumed that the motor has failed at least one of the tests outlined above. The stator will have to be ‘stripped,’ meaning that the copper windings will have to be removed, before re-insulating and rewinding the motor. The best practice is to perform a core test before and after the stator is stripped. The wattage per pound of steel loss should be recorded and should not be found to increase or decrease. In all of the motor stripping practices, one end of the coil winding is removed. The length of the coil endturns must be measured first and any connection and/ or other information collected and recorded. Then one of the following methods is used for removing the remaining wire: Direct Flame – A flame from a torch or other source is directed onto the core and winding. In some cases, the stator is physically placed in a bonfire! The temperature is uncontrolled and severe damage to the core will occur. The varnish is reduced to ash and the windings removed. Burnout – The stator is placed into a burnout oven, which is set for a recommended temperature of 650°F (345°C). It is kept at this temperature until all of the varnish and insulating materials are turned to ash (eight or more hours). If the temperature exceeds this value, damage to the stator core and frame distortion may result, reducing motor efficiency, mechanical reliability and increasing soft-foot. Gasses and other by-products are exhausted through a ‘smoke stack’ into the atmosphere. Figure 1 - Dreisilker/Thumm Stripping System Mechanical Stripping (Water Blasting) – A high-pressure stream of water is used to blast the coils out of the stator slots. This is a fast method of coil removal. Personal injury due to high water pressure and mechanical damage can be avoided by experienced personnel and safety devices. 42 june/july 2008

Table of Contents Feed for the Digital Edition of Up Time Magazine- June/July 2008

Up Time Magazine- June/July 2008 Contents Upfront Upclose: The True Cause of Most Machine Failures Information Technology: Finding Future Perfect Lubrication: Building an Oil Analysis Winner Maintenance Management: Hidden Ways to Increase Safety Infrared: Air Barriers Lower Energy Costs, Increase Profits Motor Testing: Efficient Motor Repair, Part Deux Precision Maintenance: Good Maintenance, Smart Maintenance Reliability: Common Errors in Root Cause Analysis Ultrasound: Avoid the Baghouse Blues Vibration: Don't Let Your Sensors Get Eaten Alive Upgrade: Lining It All Up

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