Up Time Magazine- June/July 2008

test amongst all three phases. The difference should not be more than +/- 3% without the rotor in the stator. The spin test consists of placing 10% of the nameplate voltage across all three line leads. A current reading is taken and compared. Then a ball bearing or test rotor is inserted into the stator core. If the windings are correct, the bearing should rotate within the stator core, or the test rotor will operate in the same direction as it is brought around the inside of the stator core. All test results are recorded for future reference. solid slot fill. The varnish also collects on the end turns. In considerably less time than two dips and bakes, the stator windings will have the equivalent of three dips and bakes (1 to 2.5 hours as opposed to 16 to 20 hours). Vacuum Pressure Impregnation (VPI) – due to expense, this process is not recommended for low voltage motors but is a must for medium voltage, form wound cores. It consists of a voidless slot fill (as with the trickle method), but requires taping the windings to keep the The current and voltage are measured and recorded; if the motor had been tested during the disassembly phase of the repair, the final results are compared with the first. Also, the temperature of the stator is checked and should remain cool to the touch when operated at no load. The measured current readings are compared and, if found to be in excess of 5% of each other, the phases are rotated. For example: Phase A is rotated to the Phase B location, B to C and C to A. If the unbalance remains in the same incoming phase, then the unbalance is due to the power supply. If it follows the motor leads, it is a problem with the motor and the rewind repair is suspect. The motor would then be disassembled and inspected. Motor current should not exceed the nameplate rating during a no-load test. The ‘rule-of-thumb’ for two-, four-, and six-pole motors is that the no-load current will be in the area of 25% to 50% of nameplate. Once all the running tests are complete and acceptable, the motor is electrically suitable for operation. Depending on the required specification, additional tests are normally requested such as vibration, temperature, etc. Varnish Insulation The final step in the rewind process is to varnish the stator. The purpose of the insulating varnish is to increase the mechanical and electrical strength of the stator windings. As with the slot insulation, it is best practice to use Class F or H varnish on the stators. There are several basic methods for insulating rewound stators: Dip and Bake – the stator is pre-heated, then dipped into a tank full of insulating varnish. This is normally done a minimum of two times to Figure 3 - Trickle Varnish Impregnation System ensure a full coat of varnish. Care must be taken as voids, varnish in between conductors. The stator is which may collect moisture or other conwarmed in an oven, then placed in a VPI tank taminants, may be left within the stator coils. where a vacuum is drawn within the tank. Additionally, all of the surfaces, including Varnish is flushed in from a holding tank and machined areas, are covered with varnish, pressure is applied to the tank. Air bubbles which must be removed. While the slots are within the insulation system that expanded receiving a reasonable amount of varnish to in the vacuum constrict and draw the varnish allow for heat conduction, a blanket of varnish through insulating tapes. The stator is then collects on the outer surfaces of the motor, placed in an oven to cure. reducing its ability to cool itself. Trickle Varnishing – the stator is placed on a turntable and connected to three-phase power. This both serves as a heating source for the windings and as an additional powered test (the coils should heat evenly). The stator is heated horizontally and monitored with an infrared sensor. Once the windings have reached a pre-determined temperature, the turntable is tilted to 35-45 degrees and varnish is trickled onto the windings through several tubes. The varnish is drawn through slots by gravity and capillary action, creating a Conclusions As shown, there is more to an electric motor repair than a good paint job. The type and quality of work required for returning a ‘good as new’ electric motor following a rewind repair is extensive. It is now apparent that a motor repair customer must work closely with a motor repair center to ensure that the equipment that is sent out for rewind repair is handled in a manner that does not reduce efficiency or reliability. End users should have pre-qualified an electric motor repair shop to ensure that their equipment will be repaired to their expectations. This pre-qualification should include a review of capabilities, equipment, a recognized quality control program (ISO 9000 or equivalent), Final Tests Once the stator has been varnished and cleaned, noting that abrasives on the stator laminations may cause shorting between laminations, the motor is assembled. An insulation to ground test is performed once the motor has been assembled and should be at least 1,000 MegOhms. The electric motor is then tested at no load and all rated voltages for 30 minutes, or until the bearings reach a stabilized temperature after 30 minutes. 44 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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