Megger and Bridge Test of Motor

As all the results are symmetrical - i.e. the same for each winding - and above 20MΩ for the insulation resistance it's pretty safe to assume there's nothing wrong with them.

that was not my question pal. i want to know a reference resistance against which i will compare my test results. and there should be some standard or formula that will dictate the value of the reference insulation resistance for a given rating motor. say if that reference resistance was 20 mega ohm and my test yielded 60 mega ohm then it means my motor is ok. but the problem is how will i find this 20 mega ohm reference resistance ?

I very much doubt that a "reference resistance" exists. The value measured will be highly dependent on the measuring conditions (particularly the humidity). If such a reference existed, the only people who could provide it would be the manufacturer (or the lab that did the type-test on the motor, assuming one was done).

TonyS is correct re. the insulation test on the separate phases - unless you have three isolated windings, they will always give the same reading.

_{And please don't call me 'pal'.}

Your figures for FLC don't add up

Contact the manufacturer for CR readings.

BTW when you did the first IR test you tested all the windings to earth in one go, they're all connected together at the star point!

Pure fiction!

There are tests that you can do if the stator star point is brought out to an external JB.

That is disconnecting the star and do individual tests for IR (Ph - Ph & Ph - E) and CR for each individual phase. 9 tests in all.

What voltage are you using for you IR tests?

REading you post I assume that you are conducting an experiment, normally to test a motor to make sure its OK to use, requires a fluke meter and an insulation tester.

The fluke, to tell you that each pair of windings, that is, if the motor is internally connected in "Star" as you test "two" windings at a time, has good continuity, and the insulation tester to prove the motor winding insulation is good.

Now if you require what is regarded as manufacturers data on the winding resistance then you have to either ask for it, or when the motor is NEW, test it before you use it.

Your readings tell me the motor is good, all you have to do is follow the nameplate details for the voltage, HP and Hz.

As a theoretical test/experiment in a lab, then what you are doing is fine if part of a course, but in the real world, a fluke and insulation tester are the preferred testers of choice!

And there is one other test you can do to make sure that your motor is good... use your nose!

a friend of mine ( who is manager in WEG ) told me the following rule of thumb for IR testing :

if IR > ( ( rated motor voltage / 1000) +1 )Mega ohm ) , then my motor is ok.

sya if rated motor voltage is 400 volts. then 400/1000 + 1 = 1.4 Mega ohm

now if in the megger test for a motor , IR > 1.4 Meg ohm then it means my stator insulation is ok. .

but i doubt above formula a bit coz there is no account for the type & thickness of insulation.becasue if we use thichker insulation for the same rated motor then it IR will be higher. so

my question was basically , do any one have such formula for IR test ???

In this context, there is no such thing as an insulation resistance measurement which is too high. Your 60MΩ is fine. Stop fretting over nothing.

(Or were you "just testing" in your OP (original post)?).

I think you will find that is for generators, once upon a time I worked for Alstom Generation, and that the "rule of thumb", actually is their SOP for testing/ commissioning new generators.

As stated by JohnDG, if your readings are high when you test the insulation, then your motor can be considered OK, as for a formula, you could try this idea...

number of turns per phase coil/length of conductor in the phase coil x number of coils per phase = length of conductor per phase divided by resistance per 1000M. Then look at the manufactures spec for the insulation of the conductor per 1000M. Also to remember to take into consideration the temperature and moisture content

Who or what is WEG?

WEG electric motors is brazilian based motor manufacturer company