insulation resistance between Line to Line 2

[Krahim4]

Hi
In a 3Ph generator, 400 V 500KVA What is the expected value of PI for the insulation resistance between Line to Line

 

[Gr8blu]

Krahim4: What to expect depends - at least a bit - on who did the winding, when it was manufactured, and what kind of insulation system was used, the general environmental condition of the machine itself . . . and the method of testing.

Testing per IEEE 43 on a machine as it is ready to leave the manufacturer would give a polarization index (PI) value of roughly 2 for either a random or form coil winding from before 1970 because of the type of winding and insulating materials used. Performing the same test two years later on a machine which has been operating continuously in a wet or corrosive environment might give a result of roughly 1.5. Now do the test on a new unit manufactured fairly recently (within last 20 years or so): PI might be as high as 4 when leaving the factory, and degrade to perhaps 3.5 in the adverse environment.

The PI test might also provide (relatively) meaningless results, such as when the initial insulation resistance reading at 1 minute exceeds 10 G-ohm.

Last thing: remember to correct the measured resistance values to a known condition (for example, IEEE 43 recommends a 40 C temperature) before doing the ratio so that it is an "apples to apples" comparison. Likewise do this as part of the ongoing preventive maintenance program, so that the trend over time provides useful data.

Converting energy to motion for more than half a century

 

[che12345]

Dear Mr. Krahim4 (Electrical)(OP)4 Jan 22 12:29
"...In a 3Ph generator, 400 V 500KVA What is the expected value of PI for the insulation resistance between Line to Line. "
1. I am puzzled when you ask ... " between Line to Line". The (ohmic reading) across [Line to Line] would be the [winding resistance]. This is not what you ask, when you state " PI for the insulation resistance".
2. Do you mean " Line to frame " of each winding (of which you have to separate the windings), or the three windings together wrt frame? This is regarding the [testing procedure/method] which was not pointed out; otherwise learned Mr. Gr8blu has covered the subject expressively.
Che Kuan Yau (Singapore)

 

[Krahim4]

([highlight #FCE94F]1. I am puzzled when you ask ... " between Line to Line". The (ohmic reading) across [Line to Line] would be the [winding resistance]. This is not what you ask when you state " PI for the insulation resistance)

Thanks, I'm wanting to make sure is it standard to take IR between L1 to L2 or not, I'm also I will be owed to you if you explain more about this issue

 

[che12345]

Dear Mr. Krahim4 (Electrical)(OP)5 Jan 22 09:
"... Thanks, I'm wanting to make sure is it standard to take IR between L1 to L2 or not, I'm also I will be owed to you if you explain more about this issue."
1. To measure the (IR and then calculate the PI) is NOT by measuring the [resistance] between L1 and L2 .
2. Google "insulation resistance testing " will show you the method/procedure, connection, test voltage level, duration etc..., including PI computation [what is considered as good/..../bad] ; by all major insulation resistance testing meter. Most write outs are very informative.
Che Kuan Yau (Singapore)

 

[electricpete]

I agree with che12345

insulation resistance between Line to Line

I think you need to ask whoever wrote this phrase what they meant by it.

As che12345 says, "insulation resistance" for rotating equipment is typically measured phase to ground. If you take a measurement phase to phase, then what you're measuring is generally a winding resistance (resistance of the copper in that path).

With all that said, there are circumstances during testing of certain 6-lead machines where you would open up the neutral to do a phase-to-phase insulation test, but that is infrequent/specialized and generally only after you have first done a phase to ground test (with all 3 phases connected together via neutral or otherwise).

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(2B)+(2B)' ?

 

[Martin0951]

I have a motors maintenance manual withe following information regarding insulation testing of 3 phase motors:
Quote:

4.1.2 Measurement of insulation resistance
During and immediately after measuring, the terminals must not be touched as they
may carry residual dangerous voltages. Furthermore, if power cables are
connected, make sure that the power supplies are clearly disconnected and there
are no moving parts.

For rated voltage below 1000V, measured with a 500 VDC megger.
(b) For rated voltage above 1000V, measured with a 1000VDC megger.
(c) In accordance with IEEE 43, clause 9.3, the following formula should be applied:


R insulation ≃ Rated voltage ÷ ( 1000 + 1) x 10(MΩ)


(d) On a new winding, where the contaminant causing low insulation resistance is generally
moisture, drying the winding through the proper application of heat will normally increase
the insulation resistance to an acceptable level. The following are several accepted
methods for applying heat to the winding:
(1) If the motor is equipped with space heaters, they can be energized to heat the
winding.
(2) Direct current (as from a welder) can be passed through the winding. The total current
should not exceed approximately 50% of rated full load current. If the motor has only
three leads, two must be connected together to form one circuit through the winding.
In this case, one phase will carry the fully applied current and each of the others, one-half
each. If the motor has six leads (3 mains and 3 neutrals), the three phases should be
connected into one series circuit.