There are several factors that have an effect on insulation resistance, and these include the age of the material, moisture & contamination, surface area and the type of insulation. Moreover, key components make up insulation resistance such as absorption, conduction, leakage, and capacitance. When you conduct a Polarization Index test, the capacitance impacts the first few seconds of this test. It is also typical for the conduction to remain “zero” due to dry condition of the windings while the leakage current remains constant.
Uses of the Polarization Index Test
The Polarization Index Testing is conducted for a wide range of purposes. For instance, it is used to determine absorption current, which gives you an idea of the health and condition of the insulation. Furthermore, the test also identifies the relation of the motor insulation to the ground. At this point, the Polarization Index Testing ratio involves 10-minute resistance to a 1-minute resistance.
In Polarization Index Testing, it determines the time necessary for insulation molecules to polarize, so these can resist the flow of current. Once current is incorporated to windings, alignment of molecules occur. The rate of alignment provides you with an idea of the insulation’s condition.
Considering how Polarization Index Testing aims to measure the insulation resistance that exists between the ground and conductors, coils that are sealed and taped can be tested with much accuracy. However, it is important to note that the results may be skewed due to some factors including the surface contamination and humidity, as well as the exposure of the conductor surface to air.
Additional Facts about Polarization Index Testing
As a means of ensuring the performance and overall condition of motors, certain tests may be performed such as Polarization Index Testing. This type of test checks various aspects of a motor including motor fitness or health, amount of moisture present, suitability to the operation, and insulation deterioration. Generally, this type of test is reliable and provides accurate results, which makes it important in determining the performance of motors. In each reading, specific values are measured. For instance, Polarization Index Testing is referred to as the ratio of 10-minute to 1-minute insulation resistance value of a machine’s windings. It is usually recommended that machines with the high index for their windings are tested. In the case of induction motors, 2.0 is the value of the minimum PI.
How Polarization Index Test is Done
The insulation condition present in the windings has a huge impact on the life of the motor. The commonly used tests to determine the condition of the insulation include Polarization Index Test and Insulation Resistance (IR). IR test is designed to identify the conductor’s insulator resistance to the rotor or stator. Ideally, the value of the resultant resistance obtained after the test must be higher since a lower value depicts gradual motor deterioration.
When it comes to Polarization Index Testing, all the readings obtained are noted, and these are all taken into consideration. However, very low or very high values for each reading both mean one thing – an existing problem in the insulator of a machine.
In order to perform Polarization Index testing, essential steps must be performed to achieve accurate and reliable results. These steps include the following:
- Removing external and internal connections
- Discharging all windings to the grounded value
- Applying a DC voltage between the windings’ conductor and stator, in which the value of the voltage is very high
- Using a Megohmmeter to measure resistance
- Obtaining readings after one minute and 10 minutes, so the final value can be calculated accurately
- Turning the power supply off after obtaining readings
- Using the Polarization Index Testing formula in calculating final resistance value
- Making sure that the testing period is performed within 10 minutes
It is also worth noting that the winding temperature must be considered as the Polarization Index Testing is performed since the test is used on single- and three-phase induction motors. As a result, grounding of windings must be ensured based on the procedure involved in the testing phase.
Overall, the Polarization Index Testing determines the condition and performance of a motor’s insulators. Since winding insulation has an impact on the durability and lifespan of the motor, it is only right to have this checked using prescribed motor testing techniques that provide reliable results for a more precise evaluation and analysis.
In addition to Polarization Index Testing, L&S Electric performs a wide variety of motor tests to examine the overall health of your electric motors.