Category Archives: Tech Notes

Corrective action to be taken after an Automatic Insulation Resistance Tester and Monitor indicates a Low Insulation alarm in Motors & Generators

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:

Step 1
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.    

Step 2
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.  

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.

Step 4
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.

Step 5
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.

Tech Note : The importance of Hot Insulation Resistance readings vs Cold Insulation Resistance readings.

One of the major benefits to an automatic permanently installed Insulation Resistance testing system( MegAlert Motorguard and Genguard ) is that it allows technicians to capture the “Hot insulation” reading in a motor or generator winding, along with the “Cold insulation” level reading. Then by comparing the two readings you can determine the true electrical condition of the winding insulation and the amount of operating life that is left in the motor or generator. Trying to capture the Hot Insulation resistance reading any other way than with an Automatic Insulation Resistance Testing system is almost impossible. This is because the insulation resistance readings will change within seconds after the motor or generator shuts off and it takes too much time to Lock and Tag out the equipment and access the stator leads to capture this Hot reading with a manual I/R testing device.

  These Hot/Cold readings are very important since they show how well the insulation materials are rebounding from the change in their operating temperature level back down to the ambient level. This is critical in determining the electrical condition of the insulation materials. The lower Hot reading should always increase several times as the insulation cools and then end up at a higher safe level, as called out by the IEEE Standard 43. ( The  minimum safe level should be at least one megohm per thousand volts of operating voltage , plus one megohm )This information is similar to what a P.I. ( Polarization Index ) test is looking for by comparing a 10 minute reading to a 1 minute reading.

  The reasons why insulation materials change their electrical resistance during heating can be explained by the following :

  Temperature Effects on Electrical Insulation Materials which are classed as CONDUCTORS tend to INCREASE their resistivity with an increase in temperature. INSULATORS however are subject to DECREASE their resistivity with an increase in temperature. Materials used for practical insulators (glass, plastic, dielectric paper, etc) should only exhibit a marked drop in their resistivity at very high temperatures. They remain good insulators over all temperatures they are likely to encounter in use. If after the insulator material cools to the normal ambient temperature, the resistance does not increase it is an indicator that the insulator material is no longer working properly and should be repaired or replaced. The reasons for these changes in resistivity can be explained by considering the flow of current through the material. The flow of current is actually the movement of electrons from one atom to another under the influence of an electric field. Electrons are very small negatively charged particles and will be repelled by a negative electric charge and attracted by a positive electric charge. Therefore if an electric potential is applied across a conductor (positive at one end, negative at the other) electrons will “migrate” from atom to atom towards the positive terminal.

  Only some electrons are free to migrate however. Others within each atom are held so tightly to their particular atom that even an electric field will not dislodge them. The current flowing in the material is therefore due to the movement of “free electrons” and the number of free electrons within any material compared with those tightly bound to their atoms is what governs whether a material is a good conductor (many free electrons) or a good insulator (hardly any free electrons). The effect of heat on the atomic structure of a material is to make the atoms vibrate, and the higher the temperature the more violently the atoms vibrate. In a conductor, which already has a large number of free electrons flowing through it, the vibration of the atoms causes many collisions between the free electrons and the captive electrons. Each collision uses up some energy from the free electron and is the basic cause of resistance. The more the atoms jostle around in the material the more collisions are caused and hence the greater the resistance to current flow.

In an insulator however, there is a slightly different situation. There are so few free electrons that hardly any current can flow. Almost all the electrons are tightly bound within their particular atom. Heating an insulating material vibrates the atoms, and if we heated sufficiently the atoms vibrate violently enough to actually shake some of their captive electrons free, creating free electrons to become carriers of current. Therefore at high temperatures the resistance of an insulator can fall, and in some insulating materials, quite dramatically. In a material where the RESISTANCE INCREASES WITH TEMPERATURE it is said that the material has a POSITIVE TEMPERATURE COEFFICIENT. When RESISTANCE FALLS WITH AN INCREASE IN TEMPERATURE the material is said to have a NEGATIVE TEMPERATURE COEFFICIENT. In general, CONDUCTORS HAVE A POSITIVE TEMPERATURE COEFFICIENT and at high temperatures INSULATORS HAVE A NEGATIVE TEMPERATURE COEFFICIENT

  This explains why the dielectric strength of insulation materials used in electrical equipment should increase from the Hot insulation level to the Cold insulation level. If the readings only raise a small amount then it shows that the insulation materials are old and fatigued and are in need of repair. And if they don’t increase to a level at or above the minimum IEEE Standard safe level, then the insulation needs to be repaired as soon as possible. This can be done by a process called “Reconditioning” which is done by cleaning and re-encapsulating the windings with new varnish or epoxy to restore the electrical dielectric strength of the insulation. By preventing winding insulation failures using this type of testing and avoiding a rewind type of repair will help extend the life of the equipment indefinitely.