Usage:
Problems can develop in the generator windings under certain environmental conditions. Most of these problems can be avoided by recognizing the effects of the environment on winding insulation and what can be done to counteract these effects. This question and answer brochure acquaints you with the importance of protecting, checking, and maintaining the winding insulation of your SR4 generator.
Illustration 1. Cutaway of generator with main components exposed.
What, exactly, is the function of insulation inside the SR4 Generator?
To understand what insulation does inside a generator, you need to know a little bit about electricity. In its simplest form eletricity is nothing more than a stream of electrons. Electrons or electricity have a strong tendency to flow back to ground. A bolt of lightning demonstrates this idea. In a generator, the stream of electrons can be compared to the flow of oil in a hydraulic circuit. Higher pressure (voltage) in the circuit increases the chances of oil leakage. Stronger, thicker tubes, valves and connectors are used to contain the high pressure oil. Higher voltages in an electric circuit require better insulation. Each strand and bundle of wire is coated with non-conductive materials to prevent unwanted electron flow. This non-conductive material is the insulation. Many different types are available.
What types of insulation are used in a Caterpillar SR4 Generator?
The wire used to make up the windings in the stator, rotor, and exciter is insulated with several coats of polyester varnish before being installed around the rotor core or in the stator core slots. The slots are lined with a sheet-type of insulating material. Insulating phase sheets are placed between each phase of the staor windings as the wire is installed in the stator core. The phase sheet insulating material provides further protection against current leakage and possible damage due to the voltage differences between phases.
The primary insulating material is a special epoxy varnish. In addition to insulating, this epoxy functions as a bonding and sealing material. Epoxy varnish is applied by submerging the main stator and exciter stator windings in a dip tank. In the main rotor, the epoxy is brushed on the wire as it is wound around the rotor core. The main and exciter stators are given multiple dips (complete immersion) in the epoxy and then baked in an oven to cure the epoxy after each dip. A coating of asphaltic epoxy is then sprayed on these components for increased resistance to moisture and abrasion. A final coat of insulating paint is sprayed on all internal surfaces of the stator and the complete rotor.
Rubber and plastic types of insulation are also used to cover the stator leads. But the epoxy insulation is our primary concern. If it starts to deteriorate, current leakage can increase tremendously, and the windings could be damaged.
Why use epoxy to insulate the windings?
SR4 Generators are designed to be used under a wide variety of environmental conditions. This means the insulation must be able to withstand heat, cold, salt spray, high humidity, airborne abrasive particles, chemical attack, fungus growth, vibration, and scraping. Epoxy, which is very non-conductive and chemically inert, offers the best overall protection against these conditions. Epoxy also has one other advantage: it remains liquid until it is heat cured. This allows the epoxy to flow into any air spaces in the windings as it is applied by brushing, spraying or dipping. After baking, the epoxy becomes a tough, cohesive barrier against any humid and corrosive environment.
If the insulation deteriorates how is the generator affected?
Usually insulation breakdown occurs in specific locations rather than generally deteriorating throughout the windings. This means current leakage is concentrated in just those locations where the insulation has deteriorated. Localized current leakage through the insulation generates heat and causes further breakdown in the insulation. Eventually, the insulation capacity is reduced so much that short circuiting and winding damage can occur.
What causes the reduced insulation capacity?
Of all the factors that can affect insulation, none causes more problems than moisture. Moisture, in the form of water vapor or humidity, is present nearly everywhere. The cooling air drawn through the windings by the generator fans brings the humidity into contact with the insulation. If local conditions are very humid or if the generator is improperly stored, a small amount of the water vapor will be retained in minute voids or cracks in the insulating material. Water is generally a poor conductor of electricty, but will combine with other materials to provide a conductive pathway for electrons to leak through the insulation. The ability of moisture to conduct electricity past the insulation is increased if there happens to be a small amount of salt or similar conductive chemical disolved in the water vapor. Generators operated near seacoasts or unprotected onboard a ship are more likely to have current leakage as the insulation deteriorates. Also, generators operating under these conditions can experience a salt buildup on the windings.
How does salt buildup occur?
As the cooling air flows through the generator, some of the water vapor containing dissolved salt evaporates on the windings and leaves a salt residue behind. The salt buildup pulls moisture from the air and retains it when the generator is not operating and the windings are cold. This puts more moisture in contact with the windings.
Other types of airborne material can also become caked on the windings and cause moisture retention. These include grain dust, saw dust, limestone dust from rock crushers, dirt, and various chemicals. No matter what type of material is allowed to build up on the windings, the result is nearly always increased moisture in contact with the windings.
Does moisture cause problems other than reduced resistance through the insulation to ground?
Yes. If a conductive pathway exists between two bundles of wire because of moisture, current will leak from the higher voltage bundle to the lower voltage bundle. This produces heat and causes further breakdown and charring of the insulation. If the problem goes undetected the windings can be severely damaged. Illustration 2 shows a "hole" that was blown in the windings because of a breakdown of the phase insulation.
Illustration 2. Hole blown in stator winding.
What other factors can contribute to reduce insulation capacity besides moisture?
Potentially, any environment that contains abrasive or chemically active airborne material can have a bad effect on the insulation over a period of time. Epoxy is strong and chemically inert, but it can be gradually worn down by mechanical abrasion or nicking and chemical attack.
The operating temperature of the windings also has a great effect on the longevity of the insulation. The winding temperature is a direct function of load and ambient temperature. The following graph shows the sharp decline in the insulation capacity as the winding temperature rises.
Illustration 3. Time vs. temperature graph.
A frequent cause of reduced insulation life - though much less than moisture problems - is generator overload. This can occur when the generator is used to drive equipment having low power factor, such as welders, rock crushers, sawmills or conveyors. Low power factor is difficult to explain, but what you need to know is that it is very possible to overload a generator even if the engine connected to the generator is lightly loaded. In any application where AC motors are a significant portion of the load, the power factor may be low. This can cause the rotor and stator to be overloaded and not lug the engine. In this situation, more power flows through the rotor than normal. If this happens continually, the insulation around the rotor windings could overheat and begin to break down. A rotor turn-to-turn short circuit may not cause any visible damage. However, the ability of the generator to start an AC motor wil be reduced. Also, field current will be excessive and may cause the generator fuse or thermoprotector to open. If the generator continues to operate with excessive field current, more turn-to-turn short circuits will develop. Eventually, the heat damage to the exciter rotor and the main rotor will be evident.
How can I tell if there's a problem with the insulation in my SR4 Generator?
Use a megohmeter regularly. This device indicates the insulation capacity of various components in your generator (Illustration 4). Test components with the mehohmeter (megger check) at least once every six months, even if the generator is located in an enclosed area with low humidity.
Local conditions and applications will determine how often megger checks should be made. Record all readings to plot any decrease in insulation capacity. This will allow you to predict when the generator will require reconditioning. If the megohmeter shows a reading of less than one megohm of resistance to ground, a substantial loss of insulation capacity has occurred. Some type of corrective action must be taken.
Illustration 4. Service Technician performing a megger check.
If the megohmeter reading is less than one megohm, how can I correct this?
If the low megohmeter reading is simply due to moisture absorption in the insulation, space heaters or other drying methods will usually raise the resistance to ground to an acceptable level. The space heaters are standard equipment on marine generators and can be installed on any generator. Space heaters require an auxiliary power source because they only need to operate when the generator is not operating. Operate the space heaters for a least four hours, then do another megger check. In most cases the moisture driven off by the space heaters will increase the insulation capacity and give a higher megger check reading. Another way to eliminate absorbed moisture is to place the entire generator inside a drying oven for four hours at 65°C (150°F). However, make sure that a forced air oven is used for the heating. A radiant heat oven will cause localized overheating and could damage the components. If you have any questions regarding megger checks, space heaters or other drying methods, contact your local Caterpillar dealer.
If the megger reading increases to more than one megohm when dry but drops when moisture is present, the winding should be reconditioned. Reconditioning the winding usually involves a thorough cleaning and addition dips and bakes of epoxy varnish. The process can be done by most local electrical repair shops
What action is required if the megger check reading is still below one megohm of resistance after drying?
If the winding insulation is relatively dry and the megger check still shows an excessive amount of current leakage to ground, then a closer inspection of the windings must be made. Any charring, nicking, or abrading of the insulation can leave the bare wire exposed and provide the pathway for current leakage. If the wire itself has not been damaged the local area of insulation may be repaired by the application of epoxy. If no damage can be found in the windings, then the overall insulation capability has been reduced in the windings. It will be necessary to have the component rewound replaced.
What else can be done to keep the insulation capacity high?
Since moisture and temperature are natural enemies of generators and all electrical equipment, every effort must be made to keep your SR4 Generator as dry as possible, even during storage periods or when the generator is used on a standby basis. The rate of moisture absorption can increase during storage if the insulation has been damaged by high winding temperatures. If possible, operate the space heater inside the generator whenever the unit is not operating. Ideally, the generator should be stored inside an enclosed area. Seal off air vents and any other openings on the generator to prevent birds or small animals from getting inside. If the generator cannot be stored inside a building, make sure it is completely covered with a waterproof tarpaulin tied securely for best possible water runoff. You may want to apply some type of rust preventive if you anticipate a long storage period.
Before the generator is stored, do a megger check and record the readings to get a data base. Always do a megger check before starting up any generator that has been in storage. Also, make sure that all the voltage connections are correct. To avoid overheating the generator, a properly sized circuit breaker and ammeter should be installed somewhere on the lines going to the load. The ammeter is a good way to monitor the actual load on the generator.