A REVIEW OF CLEANING METHODS FOR MOTOR WINDINGS

I. M. Culbert (MIEEE)

Rotating Machines Engineer IRIS Power LP

ABSTRACT: Stator winding contamination by oil, dust, and a host of other materials that may be present in a cement plant can lead to its failure from accelerated thermal aging and electrical tracking between phases, or to ground. Also the reduction on winding cooling that results from contamination will increase motor I2R losses to reduce motor efficiency. This paper discuses the effectiveness of winding cleaning methods for removing debris from stator windings and cores which include steam cleaning, dry ice blasting and scrubbing with solvents. But, of course, preventing contamination in the first place remains the most effective cure.

INTRODUCTION One of the most common maintenance tasks for motors with open enclosures involves cleaning the windings to remove contamination and debris. Most motor operators perform this task on a routine basis based on historical information, or if the stator winding temperature detectors indicate a significant upward trend for the same operating load. If done properly, a significant outage is necessary to perform the cleaning, which is often most effectively done by removing the motor and sending it to a local service shop. Thus, the cost of cleaning can be considerable. To minimize the cost, cleaning should only be performed when it is needed. And, when done, the cleaning should be performed with the most effective methods and materials available. This paper discusses five issues, which are;

Sources of contamination Effects of contamination on stator winding insulation life The most effective methods for removal of contamination Test methods to verify that the stator winding is clean and dry after contamination removal Prevention of contamination

SOURCES OF CONTAMINATION Contamination and debris may come from inside or outside the motor. One of the most serious sources of contamination is oil, or grease released from the motor bearings. (a) Oil, or Grease This is one of the most common types of motor contamination. In most cases oil contamination comes from a leaking bearing oil seal that allows oil or oil mist (vapour) to enter the motor enclosure. This generally occurs when differences in air pressures within the ventilation path and the bearing oil reservoir permit oil to be drawn out of the reservoir around the bearing seal area. One common source of oil misting is over the standpipe in vertical motor oil bath lubricated bearing assemblies. This occurs of the rotor shaft is not concentric with the standpipe and acts as a pump to suck oil mist into the motor, or the venting of this area is not well designed such that it is not maintained at atmospheric pressure. This oil is eventually distributed throughout the stator and rotor by centrifugal and ventilation action, creating an oily film everywhere. The most common cause of stator winding grease contamination is re-greasing bearings that do not have automatic grease relief devices, or have not had grease relief plugs removed. The consequences of this

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are that the grease relieves into the motor enclosure and on to the windings. In the most extreme cases the motor enclosure is filled with grease (Fig.1) (b) Dust and Debris Common sources of this are;

Pollen, sand, and dirt from the plant environment (open enclosures) Insects (open enclosures) Debris left inside motor during manufacture, or after maintenance (all enclosures) Carbon dust – wound rotor and synchronous motors with internal brushgear and sliprings Particulates produced by the cement making process (all enclosures)

(c) Moisture

Humidity in the atmosphere can enter a motor enclosure and condense onto the windings surfaces if the motor is not running and it is not fitted with space heaters. This is most likely to happen in motors with open enclosures that are outdoors, or that operate in a high humidity indoor environment. In the most extreme cases if water is sprayed from a hose, or leaking pipe in the vicinity of an open-enclosure motor it can enter it to contaminate the stator winding. Also totally-enclosed, air-to-water cooled motors can be exposed to moistures if a cooler leak develops.

EFFECTS OF CONTAMINATION Contamination can affect winding condition in a number of ways which including thermal degradation due to restrictions on cooling air flow winding failure from electrical tracking. These effects are discussed as follows: (a) Oil or Grease The presence of oil by itself may not cause significant degradation in modern global vacuum pressure impregnated (VPI) stator windings, unless the motor leads are insulated with EPDM, or similar materials. This lead insulation tends to swell and degrade if contaminated with oil. On the other hand if the winding is manufactured from individual hot pressed resin-rich, or individually VPI’d coils the slot wedges may not be well “glued” to their stator core groves and the presence of oil may cause them to migrate out of the slots which leads to the stator winding becoming loose. This can cause winding failures from ground insulation abrasion and in the case of high voltage motors rated 6kV and above from slot partial discharge activity. If a motor bearing is frequently over-greased the high pressure that develops on the bearing housing can force grease between it and the shaft. This can result into the motor enclosure becoming partly or completely filled with grease (Fig. 1) and this will cause winding overheating due to restrictions in cooling air flow. (b) Dust and Debris If these materials enter the motor cooling circuits they can cause significantly higher winding operating temperatures, which increase the rate of thermal aging to reduce insulation life. In motors with open-type enclosures these materials get into the motor endwindings and cooling air ducts to clog them up. This type of contamination is accelerated if it occurs in combination with that from oil, grease, or moisture (Fig. 2).

Figure 1 - Motor Winding

Contaminated from Over Greasing

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Motors with TEFC and air-to-air tube coolers can also overheat of their external cooling air circuits become contaminated to reduce, or block cooling air flow. If abrasive dust enters the motor enclosure it can start to erode the insulation on the stator endwindings and connections to the extent that it can cause an interphasal, or ground fault. The entry of conducting debris, such as carbon dust, to a motor enclosure can also lead to tracking between phases and to ground which can also lead to a winding failure. Metallic objects left within a motor during manufacture or maintenance may immediately fail a stator, or rotor winding if they are near the coils, since magnetic or moving air effects can cause them to come in contact with it and cut through its insulation to cause a short. Also, smaller bits of metallic, ferrous debris also may result in winding shorts. Debris such as steel filings or small curls of metal from drilling or tapping holes in the machine tend to move to areas of high current, due to the magnetic fields. They then tend to bore through the insulation, causing a short. This can sometimes be a very slow process. (c) Moisture If a stator winding is not adequately sealed against moisture then if this substance contaminates a winding it can cause it to fail between phases, or to ground.

DETECTING THE PRESENCE OF CONTAMINANTS (a) On-Line Tests As described below temperature and partial discharge monitoring provide affective means of detecting stator winding contamination. 1. Temperature Monitoring An effective non-invasive technique of detecting contamination that affects winding cooling, especially where debris is partly or completely blocking the cooling system, is to trend the stator winding temperature (where such monitoring is fitted) over time. Under the same load and ambient cooling water/air conditions, the winding temperatures should not vary more than about 1°C. If there is a gradual increase in temperature from the same sensor over the years, then one cause may be that the cooling is becoming less effective, possibly due to debris and contamination. (Since there are generally multiple temperature sensors, a rise is generally observed in more than one). In the past, trending of temperature under the same load/ambient conditions was tedious. However, with the modern computer systems now used in many plants, this data is archived. A simple selective database search can often easily uncover any gradual increase in temperature. If a motor does not have winding temperature monitoring then trending exhaust air temperature, should also detect significant internal cooling air restrictions by an increase in this temperature with time. Such temperature measurements can be obtained by infrared thermography, or other temperature measuring device.

Figure 2- Winding Contaminated with Oil and

Dust

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2. Partial Discharge Testing Another powerful method to detect stator winding contamination and debris is on-line partial discharge (PD) detection. If the insulation overheats, the mica layers within the groundwall delaminate, creating air pockets within the insulation. Air pockets within coils connected to, or close to, the phase terminals (i.e., operating at high voltage) will then electrically break down, creating small sparks. This is called a partial discharge (PD). Thus, PD is a symptom of thermal deterioration, and, hence, the contamination which accelerates thermal deterioration. Similarly, if the contamination causes electrical tracking, by definition, the tracking process creates PD. Finally, where oil contamination accelerates the loose winding failure process, the abrasion will gradually wear away the black semi-conductive coating in the slot and on the surface of the groundwall insulation. Once an air gap appears between the coil surface and the stator core, PD (also known as slot discharge) occurs. (b) Off-Line Tests and Inspections Confirmation of contamination can be obtained from IR and PI tests and visual inspections. 1. Insulation Resistance (IR) and Polarization Index (PI) Tests Detection of contamination or debris without disassembly is more cost effective and less invasive. To do so, in a brief shutdown, then IR and PI tests can be performed. Such tests should be performed from the motor terminals. As described in IEEE Standard 43-2000, a low insulation resistance (or low PI) suggests that there are partly conductive films over the stator endwindings or rotor windings.

2. Visual Inspections When a machine has been completely or partly disassembled, it is usually easy to detect severe contamination that may lead to any of the above problems. That is, the cooling ducts will be partly or completely plugged, the spacing between the endwindings may be closed off, and/or there is an oily film over the endwindings. If one investigates the stator endwindings more closely, then one may also see the formation of electrical tracks (Fig. 3), if they are present.

STATOR WINDING CLEANING METHODS A variety of cleaning methods are available for rotor and stator windings, with varying degrees of effectiveness. (a) Steam Cleaning and Pressure Washing This is the most effective method for cleaning badly contaminated motor stator windings. Such cleaning is best done in a service shop that has a dry-out over to remove remaining moisture from the winding insulation and a dip, or VPI tank to apply a sealing layer of resin after dryout is complete. When performing such cleaning a maximum steam pressure of 30 psig, or power washer nozzle pressure of 200 psig should be used and care must be taken to ensure that contaminants are not forced into core ventilation ducts to remain there. If a detergent is used in a power washing process the winding should be thoroughly rinsed to remove any soap residue that might affect the winding insulation. These methods should not be used for older stator windings having asphaltic mica insulation.

Figure 3 - Stator Winding Tracking

from Contamination

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(b) Particulate Blasting The most thorough and rapid cleaning on-site method might be called particulate blasting. This is where materials such as ground-up corncobs, walnut shells, or dry ice (frozen CO2) are "blasted" at the winding surfaces at high velocity using special equipment. It is the electrical equivalent of sandblasting. The idea is to remove the bulk of any debris and surface contamination by the force of the propelled material. Skilled personnel are required for the blasting method since, if the nozzle that is blasting particulate is held over the insulation for too long, the coil coating and even the groundwall insulation can be rapidly abraded, creating a weakness in the coil. Usually, the stator or rotor winding must be encapsulated in a temporary tent to contain the particulates and debris, and personnel must use breathing apparatus. After particulate blasting is completed, a significant effort is needed to vacuum up the blasting material and remove contaminants, and for inspections to make sure any remaining material does not block ventilation gaps in the rotor or stator cores. An advantage of blasting with dry ice is that particulates do not have to be vacuumed up. However, dry ice does not seem to be as effective at removing severe oil contamination.

(c) Manual Cleaning A more labor-intensive process is to manually clean the stator endwindings and rotor windings. This involves equipping staff with rags and either a solvent or a water-based detergent, and cleaning with "elbow-grease." In the past, solvents such as trichlorethylene were very effective in removing grease and oil contamination. Such materials constitute a health hazard, and so it is no longer permissible to use them. Today, citrus-based solvents or just detergent and water are more common. However, with this approach, many areas of the winding cannot be effectively cleaned because they essentially are inaccessible. Care must be taken to ensure that the solvent or detergent/water mixture does not degrade the insulation. In addition, cleaning with liquids or solvents can “transport” conductive contamination, such as brush carbon dust into areas where it is inaccessible and can cause future problems. Usually most modern insulation systems such as epoxy mica and polyester mica having quality groundwall insulation are essentially impervious to common cleaning liquids. (d) Drying to Remove Moisture If the windings have been contaminated by moisture alone, or steam/pressure washer cleaned then they can be dried out in a service shop in an oven at a controlled temperature. Dryout oven temperatures should not exceed 200°F (93°C) during the first 6 hours to avoid the possibility of steam building up in the winding insulation to cause damage to it. The dried winding can then be sealed by dipping the complete stator in a tank of varnish and then curing the varnish in an oven. On the other hand if this cleaning has been performed at site then dryout can be achieved by either by blowing hot air over, or by circulating a current through the winding with a welding or specialist winding dryout machine (Fig 5). It may take days to dry a large winding. The length of time for applying heat or hot air can be determined from IR measurements for which a minimum value of 100 Megohms is considered acceptable for modern windings. According to IEEE 43-2000, a polarization index greater than

Figure 4 - Dry Ice Cleaning of Large Motor Stator

Figure 5 - Dryout by Circulating Current

Through Winding

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2 also is a good indication that the insulation has dried. After a site dryout the stator endwindings and connections can be sealed by spraying them with air-drying epoxy varnish.

PREVENTION OF CONTAMINATION The most effective means of preventing the ill effects of contamination is to ensure it does not occur in the first place. The prevention of contamination from the surrounding environment is best achieved by selecting the most appropriate enclosure for the motor, e.g., for a dusty wet environment use a TEFC, TEAAC, or TEWAC enclosure. For motors in dusty environments it is important to clean air inlet screens and clean, or replace filters, if fitted, regularly to ensure adequate cooling air-flow through the motor. When maintenance is being done at a plant, many companies find it prudent to restrict access to the machine, and ensure that all items (and especially metallic objects) that are not required for work within the machine are excluded from entering a well-defined perimeter. Service shop maintenance repair specifications should include Foreign Material Exclusion (FME) requirements. Finally, a thorough cleaning and visual inspection of the machine should be conducted before it is returned to service. The purpose is to ensure that both large and small objects are not left within the machine and become a hazard to the windings. As mentioned above, oil is a contaminant that can cause several problems, but steps can be taken to prevent oil from entering the machine. Good maintenance procedures and strictly following the manufacture's guidelines will minimize the likelihood of oil from this bearing dripping onto the rotor.

REFERENCES 1. Greg C. Stone, Edward A. Boulter, Ian Culbert, Hasnain Dhirani , “Electrical Insulation for Rotating

Machines – Design, Evaluation, Aging, Testing and Repair”, Wiley – IEEE Press, 2004

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