Form Wound Electric Motor Repair Specification

Transcription

1 Specification for the Repair of Form Wound Electric Motors 1000 Volts AC to 6000 Volts AC Up to 12,500 Horsepower 1. General 1.1. The intent of this specification is to prescribe the minimum standards to be followed by repair centers in servicing form wound electric motors from 1000 Volts AC to 6000 Volts AC (hereafter referred to as motor(s) ). Not all portions of this specification will apply to all repairs Some repairs may require procedures not part of this document (i.e.: a special test or special rotor bar fitting to accommodate an oversized slot, etc.). Such work shall be performed as specified between the owner and the repair center and confirmed in writing The owner reserves the right of having one of its representatives review or witness any of the repair procedures or tests. The repair center must be informed no less than 24 hours before such visits are performed Nothing in this specification, or in Purchase Orders, shall relieve the repair center of the obligation to follow good shop practice using methods and equipment generally accepted within the motor repair industry and referenced to known and accepted standards The repair center shall furnish all labor, supervision of labor, engineering, tools, rigging, and all other equipment necessary to overhaul motors or motor components All repair work shall reference the appropriate IEEE, NEMA, ANSI, and other accepted repair standards, including: NEMA MG : Motors and Generators IEEE Std : Recommended Practice for Testing Insulation Resistance of Rotating Machinery IEEE Std : Guide: Test Procedures for Synchronous Machines IEEE Std : Guide for Insulation Maintenance for Rotating Electrical Machinery (5hp to less than 10,000hp) IEEE Std : Guide for Testing Turn-to-Turn Insulation on Form-Wound Stator Coils for Alternating-Current Rotating Electric Machines IEEE Std : IEEE Recommended Practice for the Repair and Rewinding of Motors for the Petroleum and Chemical Industry 1.7. Repair centers must have a known or recognized quality control program in place All test equipment used must be calibrated and calibration certificates available upon request All mechanical measuring and test equipment must be calibrated at least semi-annually and checked before each use All electronic equipment must be calibrated at least annually. 10/02/05 1 of 10

2 2. Scope of Work 2.1. Assessing and documenting the As Received condition: The motor shall receive a unique identification, which shall be provided by the repair center to the owner Record complete nameplate data and accessories received with the motor on a work order Whatever the anticipated repairs, the first steps shall be to ascertain and document the complete condition of the motor or motor parts as received in the repair center. Parts not involved in the supposed failure shall nevertheless be checked to determine if other failures have occurred or are about to Commencing with the motor or motor parts in their assembled condition, and proceeding through all stages of whatever assembly is needed to complete the evaluation, the as received condition of all components shall be recorded. Such description shall include, but not be limited to: General cleanliness Cracked or broken welds or castings Missing hardware Wear or rub marks Evidence of overheating Looseness of winding ties, slot wedges, hardware, lead lugs, or bearings in their housings Corrosion or moisture Lack of, or poor condition of, bearing lubricant Damaged or missing lubricant piping, fittings, gauges, or other accessory devices Movement of the rotor on the shaft, or the stator in the frame Photographs of any deficiencies found are strongly recommended as part of the repair report. In the absence of clear photographs, any drawings, diagrams, or descriptions of conditions found shall allow no uncertainty as to the location of such conditions Obvious Failures: When the nature of a motor failure is known beforehand, and a known repair is needed, the repair center shall proceed with that work in accordance with the applicable sections of this specification, including consultation with the owner as described therein. If the process of that work, or in the initial inspection of the machine, some other defects needing correction are identified, these shall be reported with supported recommendations before proceeding with such additional repairs General Overhaul: When a motor is in the repair center for general overhaul or for a breakdown having no obvious cause, so that no specific repair is contemplated beforehand, all conditions found in the as received examination shall be reported to the owner with supported recommendations for evaluation before proceeding with any repair work. 10/02/05 2 of 10

3 Initial Testing: All stator and rotor windings shall be tested as part of the initial evaluation of the motor. If that testing indicates the winding(s) to be in satisfactory condition, and if no other defect indicates that the motor should not be operated, it shall be energized at as nearly rated voltage as possible and readings taken of the three phase currents A winding not in satisfactory condition shall be reconditioned upon approval of the owner. If the repair shop judgement is that such reconditioning will be unsuccessful, and a rewind is recommended instead, work shall not proceed without the owner s written approval. 3. Initial Testing 3.1. Visually inspect the motor and record physical damage on the data sheet Perform a High Voltage Megger test at the Table 1 value for one minute and correct for temperature per Figure 1 of IEEE Std The value of the test should be no lower than 1 MegOhm plus 1 MegOhm per kv rating of the motor for safety purposes. However, a MegOhm reading below 200 Mς should be considered suspect Check continuity between phases If the Megger reading is satisfactory, there is continuity, and the shaft turns freely, energized the motor and allow to run for one half hour before performing the following tests: Current balance between phases. If the unbalance exceeds 5%, rotate the phases and recheck. If the unbalance follows the test cables, then the unbalance is due to the supply, if the unbalance stays with the motor leads, then the motor is suspect Perform a vibration analysis Perform a visual inspection of the motor in operation and note any defects Record all readings on the data sheet. Note any defects during operation. Table 1 (IEEE Std ) Rated Motor Voltage Megohmmeter Test Voltage, DC 240 through through through 13, or Disassembly 4.1. Before any disassembly is started, parts must be marked (brackets, frame, covers, brush holders) and coupling distances measured and recorded Brackets and bearings are to be identified as pairs Frame-mounted devices must be identified and recorded Wiring is to be recorded, sketched, and marked before disconnecting. 10/02/05 3 of 10

4 4.5. Check fan blades for damage and cracks visually or using a penetrating dye system As parts are removed, record all noted damage or special markings Check shaft extension runout compliance with original motor specifications. If other information is not available, reference the applicable section of NEMA MG Visually check for evidence of rubbing at outside diameters (fan, shrouds, end rings, armature iron, and others) If possible, check for tightness of the rotor core on the shaft. Visually inspect for signs of axial and radial movement Inspect the condition of bar joints, end rings, windings, slip rings, key ways, threaded fits, synchronous pole pieces, etc Check the following items with a micrometer and record information on the data sheet. If any of the following items are found out of tolerance, the repair center shall advise the owner and obtain written approval for needed repairs: Bearing clearances at three equally spaced locations Bearing journals vertically and horizontally at three equally spaced locations Bearing housings at three equally spaced locations Bearing fit in bearing housing Critical machine surfaces Megger test insulated bearings and housings, as applicable Check the integrity of any RTD s. Record RTD resistance on the data sheet Check the integrity of the space heater. Record findings on the data sheet Check power leads for thermal and mechanical damage. Check for corona tracking and other electrical damage in the motor junction box Perform a Polarization Index per IEEE at the Table 1 potential. Potential must be applied for 10 minutes with a recorded measurement every minute, or better. The results are to be graphed on standard graph paper and the 10-minute reading is to be divided by the 1-minute reading. The ratio must be 2.0:1 or better (above 8.0:1 should be checked for dry and brittle insulation systems) and there are to be no dips in the graph. 5. Cleaning 5.1. Steam clean and/or high pressure water clean stator, rotor, housing, endbells, etc. to remove build-ups of contaminants. Stator windings to have all contaminants removed between coils and air passages. Ensure all air passages in the stator and rotor are clear of any debris or blockage Bake the stator and rotor to remove moisture from the insulation: Sufficient heat must be provided to produce a temperature on the end windings of not higher than 90 o C (196 o F) until the winding is dry. 10/02/05 4 of 10

5 The rate of temperature rise should be limited to 5 o C (41 o F) per hour to avoid damage to the insulation by excessive gas or vapor formation The progress of drying may be observed through one of the following methods: Polarization Index One-minute megger readings corrected to 40 o C (per Figure 1 of IEEE Std ) With the application of heat, the insulation resistance will initially drop and then will rise again over a period of time, finally approaching a constant value A final Polarization Index must be performed and compared to the Polarization Index performed in section 4. If there is a significant increase, the stator will require re-varnishing of the stator or rotor windings. 6. Post Cleaning Electrical Tests 6.1. Windings in apparently good condition should receive a DC overpotential (hipot) test for one minute at a voltage calculated in Formula 1: Formula 1 (IEEE Std ) T = 0.65 (2Em V)(1.7) Where Em = rated motor nameplate voltage 6.2. Perform a Surge Comparison Test at a value figured in Formula 2: Formula 2 (IEEE Std ) T = 0.75 (2 Em) 6.3. An Impedance Balance test should be performed on the stator. Impedance imbalances of over 3% indicates shorted turns Induction rotors must be checked for broken rotor bars with a growler or visual inspection as a minimum For synchronous motors, a DC and AC voltage drop must be performed on the rotor fields: The DC Voltage drop method can be used to detect short-circuited turns only when connections between the coils are accessible. The test is made, with the rotor at a standstill, by passing a constant DC through the entire field winding. The drop in voltage of each coil or pair of coils is measured by means of a voltmeter. If these readings vary more than 2% from the average, it is an indication that there may be short-circuited turns in the coil, or that part of the winding is wound with the wrong number of turns or size of conductor The AC Voltage drop method is a more sensitive test for short-circuited turns and is made by passing constant amplitude AC through the entire field winding. If there is access to connections between the coils, with the rotor at a standstill, the voltage across each coil or pair of coils should be measured. The voltage across a coil having a short-circuited turn will be substantially less than that across a sound coil. The voltage across a sound coil adjacent to the coil with a short-circuited turn will be somewhat less than that across other sound coils because of the reduced flux in the short-circuited coil. Comparison of the measured voltages will readily locate any coils that are defective. 10/02/05 5 of 10

6 6.6. Visually inspect the stator and rotor windings and record findings on the data sheet: Deterioration or degradation of insulation from thermal aging. Examination of the coils may reveal general puffiness, swelling into ventilation ducts, or a lack of firmness of the insulation, suggesting a loss of bond with consequent separation of the insulation layers from themselves or from the winding conductors or turns Abnormal mechanical, electrical, or thermal forces may cause coil distortion. Such distortion may cause failure of turn or ground insulation Shrinkage and looseness of winding washers or supports permit coil movement during periods of acceleration and deceleration with the probability of abrading turn insulation and breaking or loosening of connections between coils Insulation supporting the brush rigging should be checked for evidence of flashover or carbonized leakage paths The stator core and rotor laminations must be inspected for evidence of damage due to rubbing, loose laminations, or interlaminar shorts following coil failure. If there is damage to the core, a loop test should be performed by looping several turns of wire through the core, applying current, and looking for hot-spots with the back of the hand or, preferably, an infrared camera If any of the above tests fail, the motor will require rewind. It is recommended that if the stator or rotor requires rewind repair, that both are rewound, or if the rotor fields require rewinding, that all fields be rewound. 7. Reconditioning 7.1. Dip and bake windings in a Class F, or better, varnish insulation and bake dry per varnish manufacturer s recommendations The windings may instead receive a Vacuum Pressure Impregnation cycle and baked dry, depending on the condition and age of the insulation system All air passages and machined surfaces must be cleaned of all varnish Upon completion, perform a Megger test to verify the stator condition. 8. Winding Removal 8.1. The windings are to be removed using a low temperature, hydraulic-mechanical system. Chemical stripping and stator burnout are not acceptable substitutes due to environmental problems and potential stator efficiency and reliability degradation After the stator has been stripped it must be cleaned. This is to be performed by hand or sand/bead/cobb blasting the stator A loop test is to be performed on the stator to look for hot spots. Once identified, hotspots must be corrected. If they cannot, the owner must be contacted for disposition. 9. Rewind Repair 9.1. General: Wire sizes, turns per coil, coil pitch or span, and winding connection shall be as originally specified and used by the motor manufacturer unless otherwise specified. Any 10/02/05 6 of 10

7 proposed redesign of the winding to alter its electrical or magnetic properties shall be carried out only if approved by the owner. Any windings specified as sealed shall be warranted in writing as being capable of passing the water immersion or spray test of IEEE Std Any temperature sensors installed in or on the winding shall either be removed intact for reuse, where possible, or shall be completely identified for proper replacement before their removal from the original winding The insulation system used shall be rated for Class F operating temperature or higher The system used for taping and VPI of all form wound coils shall be the ViPak system Coils must be formed from continuous lengths of properly sized and insulated magnetic wire. Splices are not acceptable under any circumstances Form wound coils must be insulated with a turn insulation of a minimum of one ½-lapped layer of mica based tape There must be enough tape to cause the coil to fit snugly into the slot without danger to the insulation when the coil is being inserted into the slot Prior to installing each insulated coil into the stator, the coil shall be tested for shorts at a value calculated in Formula 3. Formula 3 (IEEE Std ) 9.8. Coil Support: V1 = ( 2 / 3) V L Where V L is the motor nameplate voltage Spacer blocks inserted between adjacent coils shall be single or multiple thickness, as required, of Dacron felt, folded over at least once, to be compressed during insertion. Depth of spacers shall be at least equal to coil height. The final winding impregnation process shall saturate these spacers with resin or varnish prior to oven cure. Spacers cut from flat, pre-cured sheet material shall not be used Coil support rings (Surge Rings) shall be replaced without change in either their cross sectional area or minimum thickness in the direction at right angles to the stator bore thickness in the direction at right angles to the stator bore thickness in the direction at right angles to the stator bore axis. Steel rings may be replaced with epoxy glass rope or tape, provided that no welded attachments are involved and that the replacement rings fit into any support arms or blocks provided as part of the original design. Regardless of the practice followed in the original winding, in a replacement winding each individual coil shall be tied to all support rings Intercoil connections shall be suitably supported and must be silver brazed Coil support rings, spacers, blocks, etc. shall be installed to the same extent as originally present. A reduction in the number of rows of arch binding spacers, or the replacement of two surge rings by one in a different location or of a different type shall not be permitted without the written approval of the owner All form windings shall receive two VPI treatments. The cycles shall be determined by the manufacturer s and varnish specifications All passages and machined surfaces shall have the varnish removed. 10/02/05 7 of 10

8 9.11. The windings are to be tested upon completion and before assembly: A MegOhm test is performed per table 1 and the temperature corrected (IEEE Std ) A DC HiPotential test is performed at a value calculated from Formula 4. Formula 4 T = (2Em V) (1.7) A Surge Comparison Test is performed at 5000 VAC An impedance balance test is performed and the values checked to ensure that all three phases are within 3% of the average Power is applied to the windings (10% of rated voltage) and a spin test is performed, when possible. 10. Leads and Electrical Connections All lead cables from the winding into the motor terminal box shall be thoroughly inspected for cracked or burned insulation, for tight attachment to terminal lugs, and for missing or broken cable strands insofar as possible. Any leads that are replaced shall have insulation of a thermal class equivalent to that of the winding itself, and be of at least the same total cross sectional conductor area as those originally installed. Joints between new cables and the winding itself, and be of at least the same total cross sectional conductor area as those originally installed. Joints between new cables and the winding itself shall be insulated using materials and methods compatible with the coil insulation system and with the motor environment In a VPI insulated machine, all lead cables, whether for the main power leads or for connection to winding accessories, such as temperature detectors, shall be treated as necessary prior to impregnation so that resin does not wick into the cable strands such as to solidify them during cure In no case shall any strands of cable be cut or bent back out of the lug barrel so as to more easily the cable into the lug. All strands must be fully attached to the lug Where any cables pass across or against sharp metal edges of motor structural parts in the assembled machine, cable shall be appropriately sleeved or taped for mechanical protection of cable insulation against abrasion. 11. Shaft Condition Unless specified otherwise by the motor manufacturer, permissible shaft extension runout shall be as defined by NEMA Std MG and If readings taken per that standard exceed allowable runout, and the cause is determined to be a bent shaft, the shaft measurements supporting that conclusion shall be forwarded to the owner for evaluation. Readings are to be recorded Unless otherwise specified by the manufacturer, bearing clearance on a side shall be.002 inch for the first inch of shaft diameter plus.001 inch for each additional inch of diameter Bearing journals may be repaired through one of the following methods: 10/02/05 8 of 10

9 Sleeve bearing journals may be repaired by cleaning up the shaft surface and adjusting the babbitt bearing to fit accordingly or by chroming and finishing the shaft surface Anti-friction bearing surfaces may be rebuilt by welding and machining the shaft, sleeving, or refabrication. Bearing housings may be resized by machining and sleeving Sleeve bearing repair: Any replacement of sleeve bearing babbitt liners shall be by the centrifugal or spin casting process. Adhesion of babbitt to bearing shell shall be at least 95% overall and 100% along the mating surfaces of a split bearing, as verified by ultrasonic testing After finish machining of the replacement babbitt, including all oil grooves, lands, or clearance reliefs, horizontal split sleeve bearings shall be fitted to the shaft journals by scraping or bluing Anti-Friction Bearings: All anti-friction bearings shall be replaced with original or equivalent bearings Bearings shall be installed using a bearing heater, oven, oil bath, or arbor press in such a way and temperature as to not allow damage to the bearing or bearing grease Bearings shall be handled in such a manner as to not allow dirt or foreign objects to enter the bearing Whether replaced or not, any insulation components used to interrupt possible shaft current flow through a bearing shall be checked in the assembled condition with a 500 Volt Megger with a 50 MegOhm minimum resistance being acceptable (no temperature correction required). 12. Balancing Dynamically balance rotor to a maximum of 2 oz. Inches Check the shaft in multiple locations with a dial indicator to verify that the shaft is true Before and after balancing and dial indicator data shall be recorded on the data sheet. 13. Reassembly The repair center shall provide any missing or damaged screws, bolts, connectors, gaskets, wire tags, or nameplates Reassemble and test the insulated outboard bearing, as applicable Pre-Lubricate bearings and check clearances at three equally spaced locations and record data Complete reassembly of motor and check for: Megger test Continuity between phases 14. Air Gap Check 10/02/05 9 of 10

10 14.1. In any medium voltage motors that have accessible air gaps after final assembly, either by removing bracket covers of through holes provided for the purpose, the air gap shall be checked with feeler gauges in not less than three locations on each end of the motor No feeler gauge reading shall deviate from the average of all readings at that end of the motor by more than 10% of the average. If the deviation is greater, the cause shall be determined and corrected. 15. End Play and Magnetic Center In horizontal sleeve bearing motors, the magnetic center shall fall within the mechanical limits of endwise movement permitted by the shaft and bearing locations. Depending on the motor design, achieving this result may require shifting the position of one or both of the bearings in the bearing housings, or moving the rotor on the shaft. Such adjustment shall not result in misalignment of the stator core length with the rotor core length beyond the limits allowed by the motor manufacturer at either extreme of the mechanical endplay Whenever magnetic center is checked, the motor shall be leveled such that the shaft is horizontal and not tipped Location of the magnetic center shall be visibly scribed on the shaft extension. 16. Final Testing After the motor is completely reassembled, the repair center shall perform the following tests and record this data: Do a four hour run test at full speed and full rated voltage (if the repair center is unable to run the motor at full rated voltage, then the repair center must take exception to this and state the highest voltage that the repair center can operate the motor on) monitoring the following: Voltage Current RPM Vibration Analysis: Vertical, horizontal, and axial in acceleration, velocity, and displacement. Compare to original Motor winding temperatures (one set every 15 minutes) Motor bearing temperatures (one set every 15 minutes) Perform a Polarization Index test and record data as performed in step 4.16 of this specification Paint motor If possible, block the shaft into position for shipping Provide a written report detailing before and after electrical and mechanical testing. 10/02/05 10 of 10