I want to check the insulation of the 3-phase 400VAC asynchronous squirrel cage motor.

Motor's stator windings are connected in delta.

I've read in some textbooks it is necessary to open motor's junction box and disconnect terminals and remove jumper bars (which are used to establish delta or star connection) and only then perform insulation test between:
a) each winding and earth and
b) each of the windings.

I think this might not be necessary and that it is enough to test the insulation from the motor's contactor output between any of the phases and earth.

This will let us know if there is an insulation problem between windings and earth.

However, testing this way doesn't allow us to measure the insulation resistance between each of the windings.

Questions:

1. How likely is it that there is a low insulation reading between windings and that this reading will not manifest itself when performing insulation test on the motor's contactor output between any of the phases and earth?

2. I am using Megger MIT330 and testing 3-phase asynchronous squirrel cage motor which operates at 400VAC.
Which voltage should I apply when testing and for how long should the voltage be applied?

3. If I apply 1000V for 20s on motor's contactor output between any of the phases and earth, I notice an increase in resistance from 50MΩ up to 250MΩ during the first 20 seconds and then it remains steady around 250MΩ. Why does this happen?

4. What is the acceptable insulation resistance when testing resistance on motor's contactor output between any of the phases and earth for the 400VAC motor?

5. Why is this considered acceptable insulation resistance?

 

gubavac111
Stop doing what you say you're doing!
Following is an excerpt of USA Std, IEEE Std. 43-2000:
"New limitations on the use and evaluation of PI were issued. This test is no longer as easy and straightforward as it once was because most new insulation systems start with test results in the Giga-ohm and Terra-ohm (billions and trillions of ohms) range. As a result, leakage current between the windings below the micro-ohm range require the instrument and test methods used, including how to position the conductors, to be extremely accurate." It also emphasizes that readings must be taken at the machine itself, not from the starter.

I suggest you find an equivalent Standard in your country!
Regards, Phil Corso

 

gubavac111
Stop doing what you say you're doing!
Following is an excerpt of USA Std, IEEE Std. 43-2000:
"New limitations on the use and evaluation of PI were issued. This test is no longer as easy and straightforward as it once was because most new insulation systems start with test results in the Giga-ohm and Terra-ohm (billions and trillions of ohms) range. As a result, leakage current between the windings below the micro-ohm range require the instrument and test methods used, including how to position the conductors, to be extremely accurate." It also emphasizes that readings must be taken at the machine itself, not from the starter.

I suggest you find an equivalent Standard in your country!
Regards, Phil Corso

Understood, thank you, Phil.

 

The gradual reading increase has to do with a combination of the insulation response to the voltage (AC or DC) and the line capacitance for DC

 

To perform an insulation test (Megger test) on a 3-phase 400V asynchronous squirrel cage motor, follow these steps:

1. Disconnect the motor from the power source.
2. Megger test equipment to measure insulation resistance.
3. Connect the megger's positive lead to the motor's winding and the negative lead to the motor's frame.
4. Apply the megger's voltage and record the insulation resistance reading.
5. Compare the reading with manufacturer's specifications to ensure proper insulation integrity.