Common Problems & Unusual Conditions Both Addressed

A negative sequence event or generator motorization can leave a unit significantly damaged. With the clock ticking on lost revenues, our goal is to get trained technicians and engineers to your unit with right testing equipment as quickly as possible. That can mean getting the right people and equipment to your unit's location or making expedited arrangements to get the unit into our service facility. With our full battery of visual inspections and electrical and mechanical tests, supported by test data and photo documentation, we can recommend the the best repair options for getting your unit back into operation. READ MORE about NEC's generator repair capabilities.

Jump to sections on this page for:

• Turbogenerator rotors
• Salient pole rotors
• Stators
• High-voltage motors

Rotor Visual Inspection & Testing – Generators & High-Voltage Motors with Cylindrical Rotors

When a rotor arrives at the NEC service facility, a lathe run-out (photo left) will be performed to gather baseline data about the unit before to inspection. Prior to many tests and inspections, both retaining rings may be removed using Mannings induction heating equipment. The temperature of the rings is monitored throughout this process with contact pyrometers.

Detailed visual inspections look for:

• signs of physical damage such as dings, dents or rubs to any part of rotor
• slot wedge migration and contact with retaining ring
• unusually large gaps between rotor end turns and retaining rings
• signs of overheating or arc damage in the retaining rings (photo just below), nose ring or along rotor body
• blocked ventilation ducts
• localized overheating at pole cross slotting
• loose slot wedges or end turn blocking
• migrated turn insulation
• distorted top turns
• shifted or displaced turns
• loose blocking
• torn or detached Nomex® tabs
• popped out rivets
• torn or displaced slot liner extensions or slot armor
• stress corrosion cracking in the retaining rings, fans or forging, including rotor dovetail or tooth-top cracking
• unusual surface conditon of rotor journals such as rubs, cracks or streaking
• coupling distress shown by wear of coupling bolts in holes, rabbit wear and fretting, or other signs such as dings, dents or rubbing

Testing for rotors includes:

• Lathe run-out tests to gather baseline data
• DC f ield winding resistance measurements and comparisons with previous test results to assess the mechanical and electrical integrity of copper conductors and brazed joints
• Polarization Index measurements and megger tests and comparisons to verify insulation resistance and condition of the windings
• Pole balance and stationary impedance across windings and between axial leads and pole cross overs to identify turn to turn shorts
• Hardness testing of the retaining rings and fan components
• Dye penetrant and magnetic particle tests on forgings, retaining rings and fan components

READ MORE about NEC's Turbogenerator rotor repair capabilities. RETURN to top of this page.

Rotor Visual Inspections & Testing – Generators & High Voltage Motors with Salient Pole Rotors These types of generators and motors have four poles or more. The rotor poles have a laminated steel pole piece surrounded by a copper winding. Each pole is mounted with bolts and blocking directly on the rotor shaft or, in the case rotor rims that are supported by a spider, mounted onto the rim with bolts or tapered keys. Photo at top left shows dye penetrant tests to locate cracks in the rotor spider forging. Center left shows the results of lead failure on a hydro pole. Photo at bottom left shows a six-pole (salient pole) motor rotor with rim in place. Top right photo shows an incoming four-pole (salient pole) generator rotor. Pole laminations just adjacent to the mounting blocks were damaged during the unit failure. SEE MORE photos and case studies

Visual inspections for these units look for:

• loose pole pieces or rim segments
• loose or cracked or failed connectors
• damage to pole piece laminations
• signs of overheating, arcing or other localized damage indicating presence of turn to turn shorts
• fatigue cracks in the rotor shaft, spider or rim
• distortion or warping of the field pole back iron or rotor rim face

Testing for salient pole rotors includes:

• insulation resistance / polarization index
• AC pole drop test (comparison test)
• Dye penetrant and magnetic particle tests on forgings and castings

READ MORE about NEC's salient pole rotor repair capabilities. RETURN to top of this page.

Stator Visual Inspection & Testing – Generators & High-Voltage Motors

Photos at top left shows use of wedge tapper. Top right shows damage to teeth of stator core laminations. This type of damage usually can be fixed without removing laminations. Center right shows badly damaged stator wedges. Bottom right shows core loop test.

Detailed visual inspections look for:

• loose or damaged wedges or side fillers
• paint discoloration or cracking
• dusting, greasing and other signs of movement of coils/bars or bus, surge and connection rings or their supports, including the axial support system
• deterioration or coil/bar corona supression system
• loose or missing axial support hardware
• broken or missing ties or blocking
• blocked vent duct passages
• damaged laminations
• oil on end turns or other signs of oil leaks

Testing for stators includes:

• Winding resistance tests to identify poorly brazed joints, or shorted or open strands
• Polarization Index measurements and megger tests and comparisons to verify insulation resistance and condition of the stator winding ground insulation
• Tests for RTD continuity and resistance
• Wedge tightness survey including wedge tapping for tightness, compression and movement, deflection measurements of top ripple springs, and slippage and movement of side fillers
• Electromagnetic Core Imperfection Detection (EL CID) test to identify shorted core iron laminations. In some cases a Core Loop test is used in addition to verify results of an EL CID.
• Core bolt torque check on on through bolts and building bolts

READ MORE about NEC's fossil stator repair capabilities.

READ MORE about NEC's hydro stator repair capabilities.

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Exciter Visual Inspection & Testing Because their size makes shipping reasonably easy and cost-effetive, exciters are usually inspected and tested at our Columbus, Ohio service facility. Detailed v isual inspections of exciters look for: bearings requiring re-babbiting or machining out of tolerance bearing clearances to the journal and shaft seals out of tolerance conditions for the Permanent Magnet Generator (PMG) air gap and fan to shroud clearnance sites of excessive dirt or contamination at heat sinks, field poles, fuses, connectors, armature windings, diode wheels, collector rings, PMG, RTDs etc. deterioration of glass banding shown by fraying or axial cracks damaged insulation materials including those related to the diode wheel, coils and heat sinks

• internal wiring that is frayed, cracked or brittle
• damaged or worn screening
• loose, damaged or distorted hardware and connectors, including phase leads, pole to pole connectors, standoff insulators and washers that have become worn or dished
• foreign objects that can lead to damage such as loose shims, unattached hardware or missing tools
• sites of inadequate electrical connections caused by dirt, contamination, vibration or worn hardware
• sites of moisture contamination, such as cooler leaks

Testing for exciters includes:

• Measurement of diode forward and reverse bias resistance and voltage drop
• Measurement of resistances of fuses for diodes. Results may indicate necking down of fuse element.
• Verification and comparison of capacitor charging with Fluke meter on highest resistance scale
• Verification of the operability of bearing thermocouples
• Electrical tests to check for open capacitor fuses
• Electrical tests on the rotating armature:
  • insulation resistance / polarization index
  • DC winding resistance of individual taps (comparison test)
• Electrical tests on the stator field pole coils to verify winding integrity
  • insulation resistance / polarization index
  • AC pole drop test (comparison test)
• Dielectric and resistence tests on the PMG

READ MORE about NEC's exciter repair capabilities. RETURN to top of this page.

High-Voltage Motors

With few exceptions inspect and test regimens for high-voltage motors are similar to those for generators. Please use the links below to jump to the related sections on this page for more information:

Stators

Motors with two-pole cylindrical rotors

Motors with salient pole rotors

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