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.  

2 Common Factors That Affect Traditional Insulation Resistance Test Results

Temperature
Temperature and electrical resistance are closely related. As temperature increases, electrical resistance will decrease; this also is true for the inverse. The parameters and minimum/maximum acceptable IR test values are based on fixed reference temperature, the IR test numbers have to be corrected (to the reference temperature) to be made any sense of.
Humidity
The amount of moisture in an environment will also affect the IR test measurements. The lower the moisture content in the air, the higher the IR test reading. The inverse is also true, so when possible, IR tests should not be carried out in very humid atmosphere. Because there are no standard formulas to adjust for humidity, you should record the relative humidity of each IR test to be used as a comparison when engaging in traditional IR testing.