Measuring Dielectric Losses 1

[mriechert]

Hello,

I have been reading up on Dielectric losses and came across the following definiton:

"Under AC conditions dielectric losses arise mainly from the movement of free charge carriers (electrons or ions), space charge polarisation and dipole orientation. Ionic, space charge and dipole losses are temperature and frequency dependent....."

Does this mean that an insulation resistance test will not be able to determine ionic, space charge and dipole losses???

thanks for your help

 

[MMAshwin]

Hi,

Maybe I'm using the wrong type of insulation meter but I didn't know they were capable of measuring space charge loss etc. The meters I used would tell what the insulation resistance was. Insulation failures can occur on DC lines as well as AC but require different types of test.

The one you refer to is("Insulation resistance test&quot would use a DC supply free from AC/spikes and step up the DC voltage until a resistance reading of the insulation is given. It would then be up to the operator to decide is the resistance was high enough for its given purpose.

However if you wish to measure dielectric strength then a high-pot or dielectric-strength test would be carried out. This puts AC down the sytem under test and gives a pass/fail answer given a certain Ac voltage.

Now, from the rest of the statement it seems to refer to an Antenna, is this what you wish to test ?. If so what sort of information do you wish to obtain from the tests. Let me know & I'll see if I can help.

Regards

 

[mriechert]

thankyou for your response.

What I am trying to do is get an understanding of exactly what is happening when an insulation resistance test is carried out, and the same goes with the Hi-pot test.

One thing I am confused about is the difference between insulation resistance and dielectric strength. It seems to me that they are measuring the same "characteristic" (ie the ability for an insulator to withstand conduction) but utilising different units (ie "ohms" for insulaion resistance and "ampere" for dielectric strength)

Therefore I was looking into the definition of Dielectric Loss, shown above. When the term dipole is used in the definition it is referring to a molecule having a charge.

So hence my question is, What information about the insulation does a hipot test give that an insulation resistance test does not? And vice versa?

I note the easy answer is, that the former gives insulation resistance and the latter gives dielectric strength. And therefore I have arrived at the beginning again......Help!

 

[MMAshwin]

Hi,

A High-pot test is a pass or fail test and tells whether the insulation is suitable for carrying the required voltage.
During a high pot test the potential is applied between conductors and/or ground and providing the insulation is up to it, will indicate a pass or fail. Failure will occur when there is a short (no insulation) or the dilectric breaks down at that voltage as electrons are ripped off the atoms and conduction occurs.

An insulation resistance test however gives the resistance of the insulation. Basically the same principle as a multimeter, however there are much higher voltages involved due to the very big resistances.

So what is the difference between resistance and dielectric values of an insulator.

Well resistance is frequency independent and is just the opposition to current flow (r = v/i in its simplest form).

Dielectric losses come about as the insulation absorbs the some of the electric field being generated by the flowing current. The losses are proportional to the frequency and the material it is made from.

All that aside, a hi-pot test is a pass/fail reading, a resistance check gives a value of its resistance to direct current flow.

Any help ?, yes no let me know.

Regards

 

[mriechert]

Thankyou laffalot for taking your time for such a detailed response.

However I believe that the insulation resistance test is actually conducted at approx 500V dc while a AC Hi-pot test is at much higher voltages ie 1.5kV ac.

So taking all the above into consideration how do you decide if an insulation test is all that is required or a hi-pot test is necessary?

 

[MMAshwin]

Hello Again,

Just to clear one thing up. In my previous post I said

"however there are much higher voltages involved due to the very big resistances"

I was of course comparing a megger test with a regular resistance test on a multimeter. Both use DC but a multimemter usually uses just 9 volts. Megger testing is in the 100's of volts.

Anyway to your question.

A resistance insulation test gives you a value in mega (or greater) ohms of the insulation resistance.
The Hi-pot test is a go/nogo test at a given voltage i.e. if it fails then the insulation has borken down and conduction has occured.
Now some say the latter is preffered because it is not subject to interpretaion. The test is a yes or no to the question "Will it handle the voltage"
A resistance test gives a value of the resistance and it is left to the operator as to whether it is suitable.
As an example lets say the insulation between conductors was 100 Megaohms and it should be 200 Megaohms is the cable capable of carrying a specific voltage ? Its up to you to decide. A hi-pot test will either say yes or no to the above question.

However the disadvantage with a hi-pot test is if it does fail because this means the insulation has broken down and may have made the problem worse. But in reality the tester was just bring forward the impending failure of the equipment.

I did read once that some association or other recomended Insulation Resistance tests on older motors and Hi pot on new ones, for the very reason you pointed out i.e the applied voltages during each test.

See the following links. They may be of some use.

http://www.rhombus-ind.com/app-note/hipot.pdf

http://www.avointl.com/products/insulation/xtra/notes/mtech1/tech.html

Any help ?, yes no let me know.

Regards

 

[230842]

I´ll try to explain the differences between insulation resistance, dielectric strength an dielectric losses.
Insulation resistance: Insulation and resistance are antithetic concepts: if insulates doesn't conducts, and in that case, resistance has no sense. Unfortunately, insulators are not perfects, and behave as very bad and non-linear conductors; so its "resistance", temperature and moisture dependants, evaluates the "perfection" of an insulator. Insulation resistance is meassured using direct current under cortrolled voltage, temperature and damp conditions
Dielectric Strengh: When a dielectric material is affected by an electric field (voltage image) its molecules become stressed in such a way that positive and negative charges tends to move away under electostatic forces (dipole orientation). When such a force overcome mechanical strength, molecule is destoyed and dielectric as well. The limit of admissible electric field is the Dielectric Strengh (in Volts per mil when specified .in material itself) or in kvolts when in insulating parts). It is specified and tested both for d.c. and a.c.strains
Dielectric losses: Subsequent to small conduction, insulators have power losses due to Joules law: Resistance times current to the square. Additionally, when insulator is under a.c. stress, other phenomena take place, mainly the so called displacement currents or changes on dipole orientations which waste some work per orientation change, resulting thus in an additional frecuency dependant power loss.
Hope to help you Julian