Up Time Magazine- June/July 2008

Mechanical Stripping (Hot Vapor Process Chemical Stripping) – A stator is submerged in a bath of non-chlorinated petroleum-based solvent at a temperature of 370°F (190°C) for a short period of time. The coils are then removed with high-pressure air. Once the windings have been removed, the stator may have to be cleaned. This may be done by steam cleaning and baking, bead or cob blasting, or low-pressure air. In some cases, additional copper that may have fused to the core at the time of motor failure will have to be removed. This is done with a small air grinder or jewelers’ files. The stator should then receive a loop test, or core loss test, which is performed to check for hot spots within the stator core caused by shorted laminations. If these are found, they may be removed by separating the affected laminations and insulating them, then pressing them back together. Other methods include a dip and bake before rewinding or vacuum pressure impregnating the stator core. In some cases, the core losses or hot spots may be excessive, requiring that the stator core be re-stacked or the motor replaced. Computerized Coil Winding Machines – the technician is free to perform other tasks while the machine winds the stator coils. Proper tension and turn count are maintained. The coils are then inserted by hand or machine. It is important to include phase insulation and ‘in-betweens’ in order to avoid phase-tophase or coil-to-coil shorts when the motor is returned to operation. Once the coils have been inserted, the coil ends are insulated and connected. The stator connection must be the same as the original and the coil ends crimped, silversoldered, or brazed. The lead wire must be of the correct size and type for the motor current and application. After this phase, the coil ends are tied down for mechanical strength. The ties should pass between each coil slot and be tied. Care should be taken not to pull up the phase insulation. Figure 2 - Computerized Winding machine tional area. A general rule of thumb is, for every three-wire sizes smaller, two wires will be the same. For instance, if one number 15 wire is required, two number 18 wires may have to suffice. If the cross-sectional area is made smaller, the I2R losses will increase, decreasing motor efficiency. If it is made much larger, there is the chance of over-filling the stator slots or increasing the motor’s half-cycle inrush current. It is best to create a sample coil to ensure that the coil ends are the correct length and the coils will fit in the stator slots. There are several coil winding methods: Hand-Winding – performed with a ‘tower-type’ winding machine and mechanical counter. The winding technician must try to maintain correct tension and layering of the coils, or the coils will be difficult to lay in the stator slots. In the worst-case scenario, there will be wires crossing, which will increase the turn-to-turn potential in the wire, creating an area that may short under certain operating conditions. Improper tensioning of the coils may cause more wire per phase, changing the impedance balance of the motor windings. Automatic Coil Winding Machines – maintain constant tension and proper count of the coils. They still require a technician to observe operation, but will still reduce labor time. Post Winding Tests An insulation to ground test should be performed on the rewound stator at 500 VDC. The windings should now show a resistance of better than 1,000 MegOhms (based upon experience). A Hi-Pot test should be performed at a value calculated in Equation 1. Passing results and methods are outlined in the initial winding tests. The surge comparison test should be the same as in the initial winding tests (in the Apr/May issue). Stator Winding Common rewind practice dictates that the paper insulation inserted into the stator slots be of Class F insulating materials or better. The most common type used in motor repair practice is Class H. This is to allow the motor insulation to survive any hot spots that may have been missed during the loop or core loss tests. This also has the effect of potentially increasing the insulation life of the motor beyond the original design and allowing some ‘forgiveness’ if the original cause of insulation failure has not been corrected when the motor is returned to service. It is best practice to rewind the motor with the same wire size and type of coil winding method (lap or concentric). In some cases, this is not possible. If the wire size must change, it must maintain the same cross-sec- VAC = 2Em + 1,000 V VDC = (2Em + 1,000 V) * 1.7 Equation 1 - Test Voltage Additional tests include an impedance test and spin test. The impedance test is a comparison www.uptimemagazine.com 43 http://www.uptimemagazine.com

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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