May 06, 2005
6 Motor Testing and CBM
Today’s topic will concern insulation to ground testing, its test values, what it accomplishes and its strengths and weaknesses.
VP, Electrical Reliability Group
T-Solutions, Inc.
Insulation Resistance Testing
The purpose of insulation resistance testing, sometimes referred to as Megger® testing, is to evaluate the ground-wall insulation between the frame of the machine and the conductors closest to the frame. This is performed by placing a relatively low DC voltage between the conductors and ground. The meter then measures the current across the dielectric (insulation) and presents the value in terms of MegOhms to ground. This type of analysis is an evaluation of the insulation’s ability to polarize and to ensure that there are no direct paths to ground.
The values used for DC testing are:
Machine Rating Test Value (Vdc)
< 1000 500
1000 – 2500 500 - 1000
2500 – 5000 1000 - 2500
5000 - 12000 2500 - 5000
12000 + 5000 – 10000
The test limits are:
Winding Description Min Insulation Resistance
1kV plus 1 MegOhm Most windings made before 1970
100 MegOhms Form wound stators after 1970
5 MegOhms Random wound stators under 1,000 Volts
These values are referred to after 1 minute of testing and corrected for temperature (see temperature correction tables referenced to IEEE Std 43-2000).
The purpose of applying the voltage for 1 minute is to ensure that the data is more easily referenced. If you were to observe the insulation resistance, you would see the insulation to ground value increase over time. This occurs as the dielectric polarizes and builds its charge.
What Does Insulation Resistance Tell Us?
If you have been performing insulation testing, you may have noticed that the MegOhm value may fall under the recommended value in some instances, yet the machine still operates. In other cases, you may have a very high MegOhm reading, but the machine does not operate. This is because the insulation resistance test only detects the condition of a small portion of the electrical insulation system.
Strengths:
- Detection of a potentially unsafe condition between the conductors and frame of the machine;
- Direct path between conductors and ground; and,
- Significant contamination that has impacted the insulation between conductors and frame.
Weaknesses:
- Does not check for inter-turn insulation breakdown (shorts);
- Does not detect overheated insulation;
- Only detects condition where a breakdown path exists directly between conductors and the frame;
- Does not detect continuity of windings; and,
- Due to temperature, dewpoint and other humidity conditions, it is difficult to trend insulation resistance tests with any degree of confidence.
One of the keys to understanding the limits of insulation resistance is to consider an example of what a test result indicates, the correction and what it really means:
An electric motor (480 V, 3 Phase, 50HP) is found with an insulation resistance of about 200 kOhm to ground (ambient temperature above dewpoint and temperature corrected). The insulation path has a higher resistance than the motor winding, so it is determined that it will be operated to see if heating the winding has any impact. The motor is run for 2 hours and the insulation resistance reading increases to 100 MegOhms. If the motor had been tested just after operation, the original condition would not have been realized. In this case, the results indicate that there is some developing issue that has occurred with the insulation system. This condition may be corrected by cleaning and re-insulating by dipping and baking.
The next topic will be Dielectric Absorption and Polarization Index.
