The Basic Question Megger

GA

Using megger, one is checking for quality of insulation RESISTANCE. Even in inductive or capacitive components, one looks for resistive component which can withstand the applied voltage. Hence DC is used and not AC. What fails is insulation RESISTANCE which does not have inductive or capacitive values. So this is the basic reason. You can use AC for checking inductance or capacitance. Once you bring in AC, question of frequency also arise adding to complexities !!

Measurement of Insulation Resistance (IR) is done to get not only the value of resistance of the insulation, but also the condition or health of the component under test. A parameter "Polarization Index" (PI), which is the ratio of IR value after 10 minutes to the value after 1 minute, is very important and is a guiding factor for performing the High Voltage withstand Test. If the PI is less than 2, it is considered that insulation contains moisture and a dry out is necessary before performing the HV test. As rightly stated in other replies, because of exchange of energy during each half cycle of AC application, the value of PI would not be achieved (or may take much longer time). Charge holding capacity goes down if moisture is contained in the insulation. Under such condition, charging process completes much quicker and IR becomes almost constant and PI value is less than 2. All this information can be achieved only through the application of DC High Voltage and not with AC High voltage.

I have two identical electric motors. 2.2 kW, 230/400V, 3 phase, which motor would you consider having the best isolation?

motor A isolation resistance test done at 1000 Vdc

• at start 300 mega ohm,
• after 1 minute 400 mega ohm,
• after 10 minutes 420 mega ohm
• PI = 1.05

motor B isolation resistance test done at 1000 Vdc

• at start 750 kilo ohm,
• after 1 minute 2 mega ohm,
• after 10 minutes 5 mega ohm
• PI = 2.5

If the two motors are identical, then the data given for motor A and B seems to be assumption (theoretical) and not actual one. Therefore cannot be commented about which one is better. If 1 minute insulation is higher, this would mean that initial charging current is low and therefore time required to charge to full voltage would be higher compared to the motor which has low IR, mean higher charging current and would require less time to charge to full capacity.

These are measurements from an actual motor.

Motor A is after rewinding, motor B is before rewinding.

Both motor are normal IEC motors TEFC, B3, IP55

Measurement is done at room temperature +/- 25°C

Measure tool is fluke 1520 MegOhmMeter

If these are actual readings then is could be due to change in insulation material of or rewind machine and / or quality of insulation consolidation. A high PI is always considered better indicating good consolidation of insulation. Low PI definitely is indication of moisture absorption and should be dried out. During initial dry out process IR will fall but after some time it will increase and heating should continue till Time vs Ir curve flattens out.

I missed in your first comment that you are referring to a low voltage machine. These motor have very low capacitance and therefore hold low charge and generally PI criteria may not be applicable to theses low voltage machines. But this is very important to high voltage machines (of the order of kilo volts)

You have posed a very interesting porblem. I think I am 90% sure that it may be either the motors are new or there is moisture inside (how - I do not know). As you apply voltage from megger - kindly check power loss (or megger current- if possible). The heating effect results in moisture evaporating and insulation improving !!!! Actually insulation degrades and does not improve. May be here you have this moisture phenomena. I hope somebody else can also respond - as to why you are seeing imrovement in insulation resistance !!

Other people have already answered this correctly, but, it may not be clear.

We often add capacitors between (say earth and 0V) points which are required to be galvanically insulated from each other.

What is the DC resistance of a capacitor?

What is the AC resistance of a capacitor?

Resistance of any capacitor is given by Xc = 1/wc where Xc is capacitive impedance (or you may call it resistance) w is product of 2time pai(3.14159....)times frequency. C is capacitance in farads. In case of DC frequency is zero, so Xc. is infinite. In case of ac, it would depend on the frequency of the voltage. Higher the frequency, lower would be the value of Xc. But no capacitor is perfect and each one has a resistance which is considered connected in parallel to capacitor. This value is same for ac and dc voltage

We do use AC voltage for testing motors and insulation.

AC Highpot information regarding insulation testing.

The comment about the low voltage motor and DC Megger PI testing is correct.

DC PI testing is usually performed on solid insulation with long runs but sometimes DC can damage the cable depending on the "type" of cable. So AC highpot testing is performed instead at very low frequencies VLF.

For an understanding on DC high potential testing. The term "MEGGER" is a common used term for this because the vendor Megger makes many of these instruments.

Read "the lowdown on High-Voltage DC testing" it explains the concept of capacitance (low capcitance DUT and high capacitance DUT). PI testing is not usually performed on oil filled devices due to masking effects and provides limited information although there are procedures for performing PI tests in "the lowdown on High-Voltage DC testing". For information about masking effects due to swirling oil See "a guide to diagnostic insulation testing above 1kV" and read the last paragraph of the Polarization Index test section.

Please read Megger publications all free at this location. You can also read some of the equipment manuals.

The lowdown on High-Voltage DC Testing

A stitch in time

A guide to Diagnostic Insulation testing above 1kV

Regarding AC Hipot testing.

AC High Potential testing is performed on HV switches like Trayer Switches and Circuit Breaker Vacuum Fuses and for many other applications.

Power factor/capacitive dissipation testing is performed on oil filled transformers, and oil filled circuit breakers, HV bushings. See Megger publications on these.

AEMC also has some good publications free and available.

There are new products that perform live online testing using Electronic Signature Analysis

Here is one vendor of many and I do not endorse this vendor. I just found it in reference to the ESA on an internet search.

FYI. It is always good practice to use vendor documentation with vendor equipment.

Always remember to RTM. read the manual.

AVO Megger has training classes in the proper use and theory and safety regarding

electrical testing.

Where I agree with your comments which relate to High pot test, the question raised by OP is regarding use of DC or AC for the purpose of measuring insulation resistance. I restricted my replies to that point only. Thanks