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This week’s Tech Note talks about what needs to be done after an Automatic Resistance Tester indicates a ‘low insulation alarm’.
Automatic I/R Testing and Monitoring systems are designed to detect early signs of insulation breakdown in Motors and Generators in order to prevent catastrophic failures. Once a low level alarm occurs there are several steps that can be taken to try and correct the problem before removing the motor or generator and sending it to a repair shop for servicing. Removing the motor or generator should be the last possible solution. The first steps are as follows:
Determine that the Automatic I/R Testing and Monitoring system is operating and calibrated correctly by pressing and holding the system “TEST/CAL” button. The meter needle should align with the “TEST” mark on the meter dial and after a short delay the alarm contacts will all change state, signaling that the unit is operating and calibrated correctly. If not, follow the operating instructions included with the unit for the proper calibration procedures. Note: If the Automatic I/R system does not have the “TEST/CAL” button feature then consult the factory for proper testing procedures.
Separate the stator windings from the power circuit to determine if the problem is in the motor or generator, or in the power cables and connections. Using a hand held Megohm meter, with the same test output voltage as the Automatic I/R unit, test the stator windings and the power cable circuit separately. If the low reading is in the power cable circuit then further isolation of the cables and testing is required to determine where the cause of the low reading is coming from. If the low reading is in the motor or generator stator windings, then proceed to step 3.
Check to see if the internal space heater circuit is installed and operating correctly. Make sure the space heater circuit is energized and doesn’t have a blown fuse or tripped circuit breaker. Make sure that the space heaters are sized correctly for that size motor or generator. Consult the factory for proper sizing of the space heaters. Note: The general formula for sizing space heaters is 1 to 1.5 watts per H.P. or KW. The heaters should be placed inside the stator housing, near the air inlet, at the 5 o’clock and 7 o’clock position. They should never be installed outside or on the feet of the motor or generator. The space heaters are designed to control the ambient temperature inside the motor or generator in order to prevent large temperature swings that can cause condensation to form on the coils. They are not intended to dry out the stator windings when they become wet, however they can help in the drying process as described below in step 4. If they are not operating then check the power source, run contact, and circuit protection to be sure they are working correctly. If the heaters are still not operating then test each heater for an open circuit with a volt ohmmeter. If the heaters test open then replace them with new space heaters.
The most common cause of low insulation readings in motors and generators is surface moisture and contamination on the windings. This moisture accumulates each time the motor or generator cools down too quickly after being shut off. Condensation forms on the surface of the coils and is held there by dirt and contamination. This surface moisture can normally be removed in place by applying some heat to the motor or generator, either internally or externally. There are several approved methods of drying electrical equipment in place, which are outlined below:
INTERNAL – USING ELECTRIC SPACE HEATERS Energize the internal space heaters located inside the motor or generator and cover the unit with a heavy tarp to contain the heat and restrict air flow. This process typically takes a minimum of 24 hrs to see a positive change in the megohm readings. It is very common during this process for the megohm readings to drop off significantly and then raise up again to a higher level, indicating that the moisture is being removed. In some cases it is necessary to add additional space heaters in the motor or generator to dry off the moisture. Typically these would be a flexible mat type heater that can be installed between the stator core and the housing wrapper if the inside of the motor or generator can be easily accessed. Once the insulation readings return to a safe level, the heaters should be turned off and the tarp left on until the motor or generator returns to the ambient temperature.
INTERNAL- USING DC CURRENT IN THE WINDINGS Another method is to use the stator windings as the source of heat to dry out the motor or generator. This method requires a well trained maintenance person to perform this procedure. There are commercial systems available for drying out electrical windings using this method that inject a controlled DC current into the stator windings to cause the heating. If one of these systems is not available, a DC welder can be used. A problem with using a welder is when it is connected across two of the three phases, the third phase is left with no heating. So it requires moving the welder connections during the dry out process to evenly heat the entire winding. The portable commercial drying units have the ability to place controlled current into all three phases to evenly heat the windings at the same time. A heavy tarp should also be used to aid in this process. Note: You must take extreme caution with this method not to let the current flow exceed 60% of the nameplate operating current of the motor or generator. Also, the stator winding temperature needs to be monitored and should not exceed 180 degrees at any point in the process. The goal is to build just enough heating in the windings to gradually dry them out without causing any steam pockets that could damage the windings.
EXTERNAL HEAT If the inside of the motor or generator cannot be easily accessed to heat the windings or if a DC current source is not available, then an external method of heating can be applied. It will require making a “tent” like structure around the motor or generator with a heavy tarp or other suitable material and inducing heat from an external source. Typically a portable type of fuel heater ( Salamander ) or an electric space heater source is used to create the heat needed to dry the equipment. This process takes a little longer than the internal heating process but it can be effective in removing surface moisture. Note: Caution needs to be taken not to get the heat source too close to the tent or the motor or generator to avoid a fire or damage to the equipment. The object is to raise the internal temperature in the tent high enough to dry out the moisture on the windings. Another method for generators is to dry out the stator windings by disconnecting the excitation circuit and running the prime mover. This creates air flow through the windings and induces some heat from the engine or turbine back into the generator housing which can help remove the surface moisture. Note: Cleaning the stator windings on site to correct insulation problems with air or solvents is not a suggested method for removing surface moisture or contamination. The cleaning process just forces the dirt, dust, and chemicals back up into the windings which restricts the airflow cooling slots and causes heating. This type of procedure needs to be done at a service facility where the motor or generator can be completely dis-assembled during the process.
If the proceeding steps do not correct the low Insulation condition, then at this time it is necessary to remove the motor or generator and send it to a repair facility for what is known as “ RECONDITIONING”. By preventing the potential failure before it caused any damage to the stator core and/or windings, the motor or generator will most likely only need to be reconditioned instead of rewound to correct any problems. The reconditioning process consists of the following: Dis-assemble the motor or generator, steam clean all the windings, bake the windings to remove all moisture present, Dielectric test the windings to ensure they meet the minimum insulation requirements, Hi-Pot or Surge test the windings for any defects, re-insulate the windings with new varnish or epoxy insulation material, ( Epoxy VPI process is preferred), bake the windings to cure the insulation material, re-test the windings again to ensure they are in proper condition, clean, sandblast, and repaint all housings and covers, replace bearings and any seals, replace any other defective or worn parts, re-assemble the motor or generator, run test, paint and ship.
Reconditioning vs Rewinding is about a 50 to 60% savings over a rewind cost and about half the time to do the repair work. This results in a significant cost and time savings and more importantly allows the customer to maintain the original operating life of the equipment. Anytime a motor or generator is rewound, the operating temperature rises due to core damage caused by the burn out process or slot failure and the operating life of the equipment is shortened. The motor efficiency is often reduced as well . If extreme care is not taken during the rewind “burn out” process or if there is too much lamination damage caused during the failure, the motor or generator operating life is reduced significantly and the motor or generator will need to be replaced very soon.
CONCLUSION: The use of Automatic Permanently Installed I/R Testing and Monitoring systems can be very effective in preventing catastrophic failures in electrical equipment. The systems provide an early warning signal to maintenance personnel, before the equipment is energized, in order to prevent a failure from occurring on start up. By detecting electrical insulation problems ahead of time, such as those caused by moisture, chemical or dirt breakdown, heat damage, vibration, etc., repairs can usually be made on site. If not a local repair shop can correct these problems at their facility by doing what is called a Recondition and thus eliminating the need for a total rewind. These systems also allow the maintenance personnel to do their scheduled I/R testing without accessing the control cabinets and being exposed to dangerous Arc Flash conditions and/or electrical accidents. This not only increases plant safety but also ensures maximum equipment reliability and increased operating life.