Polarization index testing benefits

Polarization index (PI) testing is an extension of the insulation resistance test and is designed to check specific issues in a motor such as moisture and insulation deterioration.


Image courtesy: Bob VavraTo ensure the optimum performance and condition of machinery, it is important to conduct certain tests that can also prevent costly repairs and unexpected downtime. For instance, polarization index (PI) testing is one of these techniques that can help machines to function properly. 

Overview of PI testing

PI testing is designed to check specific issues in a motor including moisture, motor fitness, suitability of the operation, and the gradual insulation deterioration of the machinery. In fact, it is an effective and efficient way to evaluate the performance of a motor. Machines with the higher index for the windings are ideal candidates for this type of test. A minimum PI value of 2.0 is necessary for induction motors.

The life and performance of a motor depend largely on the windings' insulation conditions. This is why it is necessary to determine the insulation condition through tests such as the polar index tests and insulation resistance (IR) testing.

To put it simply, polar index testing is more of an extension of the IR test. During an IR test, users will be able to identify and assess the conductors' insulator resistance to the rotor or the stator. If the resultant resistance comes with a low value, this means the motor is undergoing deterioration. Consider every reading from the PI since the values all pertain to the insulator of the motor and its condition.

Performing PI testing

To gain success from PI testing, it is important to follow these nine steps involved in this process:

  1. Remove internal and external connections before the testing begins.
  2. Discharge the windings to the grounded value.
  3. Apply a maximum dc voltage between the stator and conductor in the windings.
  4. Use a megohmmeter to measure the resistance.
  5. Obtain a reading at one and 10 minutes so the final value can be determined.
  6. Turn the power supply off after the readings have been received.
  7. Use the PI test formula to calculate the final resistance.
  8. The entire test duration must be performed within 10 minutes.
  9. Take note of the winding temperature at all times. Perform this portion since the test is of single- and three-phase induction motors. This is why the windings should be grounded based on the testing procedure performed.

As mentioned earlier, the polar index test aims to determine the condition of the motor's insulators. Since the winding insulation impacts the durability and integrity of a motor, it is important to keep this area in check to avoid untimely downtime and massive expenses for repairs. Hence, it is important to make sure that some factors that impact the insulation resistance are under control including the type of insulation, surface area, the age of the material, as well as contamination and moisture.

Insulation resistance also involves various components such as the capacitance, leakage, conduction, and absorption. The capacitance is usually the first to be determined at the onset of the test. As for the conduction, this can stay in zero because of the windings' dry condition. As for the leakage current, this area remains constant during the polar index test.

Benefits and uses

There are several reasons why it is important to conduct PI testing. One of these is to take note of the absorption current, which provides information on the insulation's condition and health. Users should also learn more about how the ground impacts motor insulation. This is why the PI testing ratio comes with a 10-minute to a 1-minute resistance.

In addition to these functions, users can also identify the time needed for the polarization of insulation molecules to resist current flow. When adding the current to the windings, molecules begin to align. The alignment rate provides details about the condition of the insulation.

Since the evaluation of the insulation resistance takes place between the conductors and ground during this test, users can obtain more accurate results from testing taped and sealed coils. Just be aware of air exposure skewing the results due to humidity, contamination, and the conductor surface.

To check the condition of a motor's insulators, it is best to conduct the PI test properly. By doing so, the motor's lifespan and durability can be prolonged while preventing unnecessary downtime, loss of profits, and expensive repairs.

David Manney is a marketing administrator at L&S Electric. This article originally appeared on L&S Electric Watts New Blog. L&S Electric is a CFE Media content partner.

Top Plant
The Top Plant program honors outstanding manufacturing facilities in North America.
Product of the Year
The Product of the Year program recognizes products newly released in the manufacturing industries.
System Integrator of the Year
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
October 2018
Tools vs. sensors, functional safety, compressor rental, an operational network of maintenance and safety
September 2018
2018 Engineering Leaders under 40, Women in Engineering, Six ways to reduce waste in manufacturing, and Four robot implementation challenges.
GAMS preview, 2018 Mid-Year Report, EAM and Safety
October 2018
2018 Product of the Year; Subsurface data methodologies; Digital twins; Well lifecycle data
August 2018
SCADA standardization, capital expenditures, data-driven drilling and execution
June 2018
Machine learning, produced water benefits, programming cavity pumps
Spring 2018
Burners for heat-treating furnaces, CHP, dryers, gas humidification, and more
October 2018
Complex upgrades for system integrators; Process control safety and compliance
September 2018
Effective process analytics; Four reasons why LTE networks are not IIoT ready

Annual Salary Survey

After two years of economic concerns, manufacturing leaders once again have homed in on the single biggest issue facing their operations:

It's the workers—or more specifically, the lack of workers.

The 2017 Plant Engineering Salary Survey looks at not just what plant managers make, but what they think. As they look across their plants today, plant managers say they don’t have the operational depth to take on the new technologies and new challenges of global manufacturing.

Read more: 2017 Salary Survey

The Maintenance and Reliability Coach's blog
Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
One Voice for Manufacturing
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Maintenance and Reliability Professionals Blog
The Society for Maintenance and Reliability Professionals an organization devoted...
Machine Safety
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
Research Analyst Blog
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Marshall on Maintenance
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
Lachance on CMMS
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
Material Handling
This digital report explains how everything from conveyors and robots to automatic picking systems and digital orders have evolved to keep pace with the speed of change in the supply chain.
Electrical Safety Update
This digital report explains how plant engineers need to take greater care when it comes to electrical safety incidents on the plant floor.
IIoT: Machines, Equipment, & Asset Management
Articles in this digital report highlight technologies that enable Industrial Internet of Things, IIoT-related products and strategies.
Randy Steele
Maintenance Manager; California Oils Corp.
Matthew J. Woo, PE, RCDD, LEED AP BD+C
Associate, Electrical Engineering; Wood Harbinger
Randy Oliver
Control Systems Engineer; Robert Bosch Corp.
Data Centers: Impacts of Climate and Cooling Technology
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
Safety First: Arc Flash 101
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
Critical Power: Hospital Electrical Systems
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
Design of Safe and Reliable Hydraulic Systems for Subsea Applications
This eGuide explains how the operation of hydraulic systems for subsea applications requires the user to consider additional aspects because of the unique conditions that apply to the setting
click me