quote:

My point is that it is okay to have tools that cost hundreds or a few thousand to help us make better guesses. Tools that cost in the Tens of thousands should produce results more definitively and consistantly.

I doubt that there is any definable relationship between tool cost and results.

Walt

hello Mr Aditya
Can i have your personnel email id?

thanks & regards

Jignesh

E-Pete: I just read the McKinnon paper. Page 2 states: "With the exception of some rotors, the resulting graph of inductance will typically display waveforms sinusoidal in shape." The way I read it, residual magnetism is always present to a more or less extent, thus 'low' & 'high' influence rotors. The change in residual magnetism is the indication of the broken bar.

The two rotor bar patterns that I attached earlier are not identical. The good rotor's graph is approximately sinusoidal, with a few flat spots at the trough, which could be data scatter. For the bad rotor, the graph is more erratic, especially Phase A. Towards the end (Positions 20 to 24), the impedance is barely 192, reaches 199 for all the other peaks. Thus, you can distinguish between a good & bad rotor.

Dan: No hard feelings, in fact sorry for being touchy. I think your experience was in line with what all on this board feel, that LV testing is not the way to go to detect weak insulation. I have detected weak insulation issues a few times by All-Test, but would not make such predictions based on those readings. I use it mostly for rotor bar fault confirmation & for turn-to-turn shorts in DC armatures & fields and synchronous rotor poles. I have been taking data on induction motors also, but mostly for my reference. If you need a basic electrical tester; take a look at www.metrel.si. Excellent low cost stuff to start with.

Jignesh: You can mail me at adityakorde@yahoo.com.

Regards,

Aditya

quote:

Originally posted by Aditya:
Too much of nasty stuff being written about LV testing! I think the main points about LV testing are being ignored, viz. quick assessment of the rotor bars & the high frequency impedance measurement can pick up shorts missed at power frequencies. The phase angle & phase balance stuff is not of much relevance.

There was an overwhelming amount of information posted in last few days. I believe it deserves a little more discussion.
Let me start with Aditya’s post: The low voltage testing is not supposed to be a: “quick assessment” from the point of view of the low voltage proponents. The low voltage testing is supposed to be the crux of the matter. We hear it over and over again.
The most attention deserves your statement: “…missed at power frequencies”. I wish somebody directed me to the explanation of this phenomena: Somehow the turn to turn fault does not show at, let’s say, 60 Hz and 50 millivolts turn to turn. (Please, refer to previous posts, where I calculated turn to turn voltage with low voltage tester). Now I am supposed to accept that if I increase the frequency to 400 or 600 or 1200 Hz, the same 50 millivolts turn to turn will indicate a problem!??? I am at a loss! I do not have any knowledge of any mechanism that will provide for that. If you have any information where I can get this knowledge (besides the manuals of the low voltage testers) I will truly appreciate it great deal.
jank

I think what Aditya is talking about is hard shorts in windings (which may persist even during operation for dc windings such as sync motor rotors).

At low frequency including power frequency, the circuit may be primarily resistive, and the impedance is proportional to the number of turns.

At higher frequencies, the circuit appears more and more inductive, and the change in impedance resembles the change in inductance which is proportional to number of turns squared.

So if we have 100 turns with 1 of them shorted, we might see 1% difference at low frequency and 2% difference at high frequency. The improvement in sensitivity is even more dramatic in one of the examples that Aditya posted (08 April 2008 08:23 AM ), and I think the reason must be some series resistance that further hides the problem when doing the lower frequecny and dc testing.

Jank,

I don't think the problem was missed at power frequency, there is a variation in the impedance values for the defective coils at 50 Hz also. However, this variation is much higher at 400 Hz.

Why does that happen? I'll leave that to you or the other brains at PdMA & All-Test. E-Pete may have the answer.

Regards,

Aditya

Aditya,
I have been going through your reports. By the way, very impressive reports. Can you just tell me what is the frequency response all about? For example you have measured impedance 8.48 Ohms on pole #1 @ 400 Hz and the frequency response was –41%. I know that it has something to do with doubling the frequency??? Also the phase angle was 57 degrees. What is the angle? It does not seem to be angle between voltage and current, is it?
Also in table 16 is a comparison of the inductance between DTIPL and JEAUMONT. I noticed that the readings on the poles 2 and 8 are lower despite the poles being new. However there is no difference in JEAUMONT readings. I assume both readings were done at 250 Hz.
Regards,
jank

Jank,

Thanks for the comment about the report.

The Jeumont test report was from their manufacturing works in France. The coils were subsequently shipped to India & we were testing those coils prior to installation. I suspect coil nos. 2 & 8 got damaged in handling or shipping. The surge test & MCA were done in front of both IPCL & Jeumont engineers. I don't think the Jeumont engineer was very happy about the result, but the tests were repeatable & he had to accept it.

Frequency response is All-Test's term for change in current with doubling of the test frequency. I guess the phase angle would be that between voltage & current. All-Test recommends looking at these values to detect winding shorts. I find them surreal & am happier with the impedance values.

Regards,

Aditya

When a rookie in this field in the late 70’s my great late mentors already had discussions about the effectiveness of various insulation integrity test methods. It seems there are so many factors involved, some of them easily overlooked outsmarting even professionals and reputated vendors.
Maybe the time is right for something new under the sun, so here is my addition to the pool of shared knowledge:

Dan started off his interesting posting pointing towards PWM inverters, so let me return to the beginning: when frequency converters are involved why not (ab)use the frequency converter as an insulation integrity tester?
In VFD induction motors the stator winding system is continuously subjected to very rapid dc bus voltage switching between the phases, several thousands of switchings in a second, rise time of some thousand volt in a microsec. This is some kind of a huge dc step surge tester, right?
Where I am looking at is this: With a clip-on high frequency rogowsky current transformer the common mode current in the three phase cable between frequency converter and motor is recorded with a high speed data acquisition system on my laptop. I am especially interested in the common mode current high frequency damped oscillations as these oscillations represent leakage current passing through the stator winding insulation towards ground. The basic idea behind it: these oscillation patterns should be a reflection of insulation integrity.
It is just an idea and I am trying to collect data as much as I can whenever I am involved in in-the-field VFD diagnosis to investigate the feasibility of such a test method option.
Is this High Frequency Switching Test (HFST) the test engineers dream? Just easily clip-on your dirt-cheap rogowsky current transformer gadget around the three phase leads. It is an on-line, in-situ test method. It is a test performed at actual operating conditions and under actual voltage and voltage rise time condition (residual magnetism cancels out?!). No need to interrupt service of the drive, zero downtime. Portable. 1 hp .. 500 kW.

The ultimate compromise between ac/dc hi-pot and low-voltage testing systems?
Dan Steinwenders holy grail?

A basic limitation of such a HFST method might be that the voltage rise time will not be able to penetrate deep enough into the insulation. If so, then who cares, it will not harm the insulation either, an intrinsic non-destructive test method! Another one: > 500 kW, > 3 kV line CT’s will probably limit bandwidth, however there are not many > 3 kV VFD’s, the bulk is low voltage < 690 Vac.
Whenever I have gathered enough consistent test results I will come back here. Meanwhile others may pick up this idea and collect test results too and verify the feasibility of the proposed HFST? And what about DOL ac motors, and dc motors?
Hope this posting triggers improvements. Like to hear your opinions!
IMO: rotor integrity is the domain of PPF vibration diagnosis and MCSAnalysis.
BTW: yes, anonymous identity should preferably be avoided, greetings from behind the NL dikes,
www.ariemol.nl

quote:

Originally posted by arie mol:

I am especially interested in the common mode current high frequency damped oscillations as these oscillations represent leakage current passing through the stator winding insulation towards ground. The basic idea behind it: these oscillation patterns should be a reflection of insulation integrity.
uRL=http://www.ariemol.nl]www.ariemol.nl[/URL]

It should be an invitation to discussion. Those oscillations will indeed cause a leakage to ground (if the source is referenced to ground). But the overwhelming majority of the current measured will not be present because of the insulation integrity, but because of the capacitance to ground. The insulation integrity is the function of the WEAKEST spot. Till you find a method how to distinguish the current through the weakest spot from 99.9999% of the other currents, you will be in the dark. The traditional methods of the insulation testing are of definite help. The low voltage testing is not.
-----------------------------------
QUOTE:
“BTW: yes, anonymous identity should preferably be avoided, greetings from behind the NL dikes,”
END QUOTE
--------------------------

Hmm, I wander whom you are talking about. The “BTW” is a response to what???
Did anybody ask if anonymity should be avoided? For some unknown reason you want to know identity of some people, but you do not have the guts to tell it straight. It is my view, that the identity of a guy I am talking to on the net is totally irrelevant. I just want to know what he is talking about. He has the freedom to tell anything he wants, behind the dikes or not. I have already asked one guy what he needs my identity for. He did not answer. Maybe I will get an answer from you.
jank

Mr Aditya
i have asked so many times to ATP User on forum that why ATP 31 and ATP IV are not giving same results while they are using same Testing method? ( reason i found on forum is ATP IV using default frequecy) but atleast at one frequency the reading of Both Gadget should be same.
Can you provide the best suitable guideline for using these both instrument together for data intepretation.
will ask later on this with example, right now in hurry.
thanks & regards

Jan, thanks for your interest and prompt response.
What is a definition of the weakest point? Isn’t it ultimately expressed in L-C-R: inductance, capacitance and resistance? Therefore I am focussing on oscillation patterns in terms of frequency and damping, not on the current itself. In My Opinion changes in pattern or abnormal patterns should indicate a potential loss of integrity.
By The Way, I was not at all referring to a specific person when I commented on the identity discussion earlier in this thread. You are right: on the internet basically identity is irrelevant and any persons choice for anonymity should be respected. However this forum is not only a knowledge base, it is a community of humans too and I just have some human interest. The dikes refer to my cradle way below the sea level, I do not intend to promote hidding places. (BTW: Amsterdam Airport platforms are 40 feet below sea level, welcome to the Netherlands in your 747 submarine !)
Arie Mol

Jank,
If the current measured by the coil will be due to capacitance to ground, why can't that be used to assess insulation condition? Aren't capacitance tip-up & capacitance mapping standard procedures for insulation assessment anyway?

Arie,
There has been work published on using the RTDs in the generators to detect PD without having to install any couplers & using that to assess insulation. Cutler-Hammer claims to have achieved success with this. That would be a good on-line, non-intrusive method. Others state that RTDs aren't shielded & thus prone to signal noise, also not sensitive for the high frequencies. I don't know if that issue is settled yet.

Thermographer,
I know that the AT-IV defaults to 200 Hz, you can select with the AT-31. The AT-IV will occasionally change the test frequency for one of the phases, giving an impedance almost double that of the other phases. As per All-Test Pro, this is an early indication of winding faults. I have not yet got any explanation for this from them & hence prefer the AT-31. I would use the AT-IV for resistance only (maybe inductance, but cross-check impedance with the 31).

Regards,

Aditya

Arie

I wonder as well what the impact of stray induced currents in the frame of the motor might have on trying to discern the true magnitude of common-mode current. Often with the use of VFDs there are currents that are created in the frame and shaft of the motor due to poorly managed magnetic coupling and these currents can be found in even new installations. Sometimes these currents come back and haunt us in the form of destructive bearing currents.

Nevertheless, Arie, I would be interested in looking for leakage current via common-mode current but like Jank said it might be hard to see it but I think more so due to lack of very accurate measuring instruments. Another problem that I have seen in the past has been due to EMI being picked up in any CTs used within electrical panels containing VFDs. Its like an AM broadcasting station in your face.


Aditya

During the demo of AT-IV and AT-31, I did notice that despite that both testers were verified by the salesman to be operating at the same frequency, the units did have noticeably different readings for impedence between each other. The salesman said this was typical of the units and it was more important to just compare readings of one unit against previous readings carried out with the same unit. I found the explanation weak and now suspect that he was incorrect in assuming that the AT-IV was at the same frequency. Aditya, you were probably correct in observing the AT-IV changes frequency sometimes.

DanS

quote:

Originally posted by Aditya:
Frequency response is All-Test's term for change in current with doubling of the test frequency. I guess the phase angle would be that between voltage & current. All-Test recommends looking at these values to detect winding shorts. I find them surreal & am happier with the impedance values.

The numbers generated by All Test Pro AT-31 are very intriguing. This time I looked at table 4, R-phase. If the Phase angle is the phase shift between voltage and the current, then the resistance of the circuit can be calculated from the formula: R= Z*cos (phi). (Z is the impedance).
I have got following numbers:
400 Hz ……….4.20 Ohms
200 Hz………...3.89 Ohms
100 Hz…………3.86 Ohms
60 Hz………….3.37 Ohms
50 Hz…………...3.11 Ohms
30 Hz…………...2.47Ohms
25 Hz……………2.18 Ohms

It is quite surprising that the resistance is changing so dramatically (unrealistically) with frequency. On the top of it is miles away from the measured resistance line to line of something like 0.160 Ohms (table 3) (ratio up to 1: 26 ???).
I just wander if the engineers from All Test could explain this.
Also interesting is the frequency response. The change in frequency from 200 to 400 Hz caused the impedance to increase from 20.4 to 40.2 Ohms, but the frequency response increased only from 49 to 51. If I take the tiny resistance of the winding (0.160 Ohm) it should have gone from 20.4 to 40.8 Ohms (double almost exactly!)Where are those numbers coming from? I have read the All Test manual from the web site, but it gave me absolutely no clue what is the science behind it. Luckily there is a number of satisfied users (Hi Bob!) who can give as a detailed lesson!
jank

Jank,

Now why should resistance change with frequency? Far as I remember, resistance is a DC characteristic & the change in impedance with frequency is due to the change in inductance (2xPIxFxL). Maybe I'm missing something?

Your inference about the frequency response is incorrect. Frequency response is a ratio; thus its value @ 200 Hz is the ratio of the current drawn at 200 Hz to that drawn at 100 Hz. Similarly, its value @ 400 Hz is the ratio of the current drawn at 400 Hz to that drawn at 200 Hz. So, basically what we are seeing is that the current drawn by the circuit approximately halves each time the test frequency gets doubled. That would be as expected.

Regards,

Aditya

Hmmm,

What I see is that DC resistance is essentially negligible in calculating the total impedance in the test conducted by Jank. The equation can be simplified in this case to Impedence = 2*pi*freq*inductance. Therefore if you halve the test frequency then the calculated impedence should come out to half as well.

This does not happen based on Jank's data the inductance ( not impedence )of the winding is changing with each change of test frequency. The winding at 400Hz is 1.7mH and at 25 Hz grows to 13.8mH.

This makes no sense unless I suppose we begin looking at significant digits in each part of the equation. Yikes call in the Math experts....
In this case

quote:

Originally posted by Aditya:
Now why should resistance change with frequency? Far as I remember, resistance is a DC characteristic & the change in impedance with frequency is due to the change in inductance (2xPIxFxL). Maybe I'm missing something?

I am certainly not trying to invent something totally new. I am merely taking the data from the report. I have absolutely no doubt, that the data was recorded with utmost accuracy. The whole report speaks for itself both by content and by the form.
At the same time, if there is a number giving me the impedance, and if there is a number giving me the phase angle, anybody can calculate the resistance (as I did). The secret behind it is not any secret from the times of Pythagoras (500 B.C.). I have just calculated the resistances from the data in the table 4. The resulting numbers do not make any sense. Since I added just a little math (that is beyond any discussion) the result is, that the input numbers, numbers generated by your low voltage tester, do not make any sense. It is my opinion that it is not you, who should defend the numbers. You have just followed the instructions. There are thousands upon thousands of satisfied users of the All Test instruments, and they will without any doubt straighten this mess out, never mind the engineers from All Test. For a while I thought that I am the only one confused. From the post above this one, DanS has some doubts too, thank you. I would somehow expect some opposition from more than one guy.
jank

P.S. The resistance grows with frequency due to the skin effect.

Jank,

AC resistance (not impedance) & DC resistance values do not necessarily have to be equal (skin effect, eddy current & hysteresis, as you pointed out). So, comparing the two as you have done may be a wrong approach. Then again, I can't argue with Pythagoras. So I looked at some of my data (see attachment, Page 13, Tables 4 & 5):

The DC resistance as measured by a rather precision micro-ohmmeter & that given by the All-Test IV are nearly equal. Does All-Test give AC or DC resistance? - I don't know.

The XL value as calculated from (Z x sine phi) does not match that calculated as (2 x pi x f x L).

As seen from the last report's comparison with Jeumont, All-Test & Jeumont's impedance values are almost identical.

So, now I'm really confused. I have data which shows that All-Test resistance & impedance values match those from other instruments, yet the impedance, inductance & phase angle numbers don't make sense. Is capacitance playing some role here?

This is not about defending numbers or instruments, its about understanding whats going on. I have used All-Test instruments to find faults successfully so I do trust them, but many things have never been explained.

I had sent a mail to All-Test to join the discussion, as well as taken up the matter with the PdMA Regional Manager. Both companies responded that they want to be neutral, so its left to us users to figure out the theory.

I know PdMA & All-Test users in India and have heard good & bad stuff from them. Surprising that this thread is basically down to 3 or 4 persons. Maybe we should take it up at eng-tips.com.

Regards,

Aditya

GTG_5_Report,_Feb_2008.pdf (925 Kb, 9 downloads)

Aditya its not just a 3 to 4 persons facing problem? Too Earlier i have raised this issue but after not getting reply from AllTest i consider wat i was thinking is true as if they are so confident on their technology they should prove technically or atleast try to satisfy the customers.
i have checked a new motor. ATP software generated report results are as below

R 0.036 0.036 0.036
Z 30 33 27
L 6 6 5
PHI 74 75 75
I/F 43 43 43

and the ATP recommendations are
1. Possible contaminated or overheated winding
2. evaulate impedence and inducatace pattern match. ( i have done and found same readings )
3. recommend check at motor if tested from mcc ( i have checked motor at motor terminal)

Now wat could be the conclusion as this is a new factory tested motor which was received just in warehouse. Is it should be contaminated? (bull **** and so if ATP cant predict or false predict , should we buy it for just like Micro Ohmmeter?)

One more thing as you all discussed about phase angle but you need to confirm 1 that is this a Phase angle between Voltage & current or it just a technical word ATp using for different technology.

thanks & regards