Value/Condition of % Inductance Imbalance HELP

We have big controversy,
 The Stator Fault Zone, with PdMA
Inductive imbalance on the stator windings
What are the levels that we have to consider as Test Limits?

Value/Condition of % Inductance Imbalance
High or Low
Caution  XX%
 Alarm  >~ XX%

Air Gap is OK
Poles Drope Voltage are OK
No Shorted rotor fields OK
No Broken amortisseur winding bars OK
Ground insulation  OK

our motors are synchronous motor > 1250 HP
RIC test for 200 RPM  !!!! 

ohm Ph 1 to 2 0.2606
ohm Ph 2 to 3 0.2596
ohm Ph3to 1 0.2606
mH Ph 1 to 2 8.295
mHPh2to3 7.165
mH Ph 3to 1 7.605
Average Inductance 7.685
% Res. Imbalance 0.26
% Ind. Imbalance 7.89

Original Post

A RIC Test for a 1250 HP Synchronous Motor  is somewhat complicated!!

An apology for my English
What I am looking for or my question is

 What level of inductive Imbalance should be considered as Caution level

What are the levels that we have to consider as Test Limits? % Inductance Imbalance



electricpete posted:

"RIC test for 200 RPM  !!!! "

I'm not clear what is meant. Did you do a RIC test? What are results?

Hi Electricpete

The RIC test has not been performed.

For a Synchronous Motor that features for this RIC test we would have to perform.?

with your experience that inductive Imbalance should be considered as Caution level


Saludos cordiales / Best regards

Buenas Noches

edortiz posted:

Average Inductance 7.685
% Res. Imbalance 0.26
% Ind. Imbalance 7.89

Buenas noches! No lo recuerdo bien pero creo que viene en el estandar IEEE 1415 (creo, no tengo mis notas a la mano). Un desbalance Inductivo menor al 8% es aceptable, en motores de baja tensión... te debo la información completa, amigo!

Un saludo!

Hi Edortiz,

There is no international standard of inductance imbalance. The best way to know that is good or bad inductance imbalance is trending. So, we have to have the baseline of inductance imbalance. For normal squirrel cage rotor, good inductance imbalance is lower than 6-8%. But,  it can be 12-15% in some rotor shifted cage.

For your testing result, resisntace imbalance is very low. It indicates good stator winding. For 7% inductance imbalance, I think ou have to double check by RIC or MCSA to know exactly about airgap and rotor condition.


I've been down this rabbit hole countless times. PdMA themselves says they've seen it as high as 20% on an otherwise perfectly normal motor. I've seen values up to 15% even brand new in the packaging in the warehouse. Doing all that other testing finds nothing every time. If I see anything up to 15% unless it's out of trend, I ignore it. At 20% and higher, that's a different problem. Impedance (online testing) usually fails too and eventually we'll find something else wrong but unless there is some other test (particularly online impedance testing) that indicates a problem, just ignore it.

Theoretically this test should be equivalent to the Baker surge testing. I haven't been able to document enough cases yet to conclusively prove that one test is superior to the other. It's the one offline test that I can't really trust all that much. That and capacitance which is basically just a confirmatory test for insulation resistance.

We actually don't use PDMA, but I'm interested in it. Commeting based on my limited understanding.

[quote]Theoretically this test should be equivalent to the Baker surge testing. I haven't been able to document enough cases yet to conclusively prove that one test is superior to the other. [/quote]

I guess you're saying they're equivalent from the standpoint they are both intended to detect turn to turn shorts.  In that respect I'd point out an obvious difference. Surge testing creates a higher turn to turn stress than motor operation (especially on the first few turns which are most vulnerable to damage from system transients).   In this respect is is similar to a high-potential test:  it is more likely to find a weakness but also more likely to damage a motor during the test.  That's a fine line to walk, but still makes the surge test valuable imo when used with proper understanding (don't do it unless you are prepared for a failure with a spare or time to rewind the motor).


A disadvantage of the inductive imbalance imo - no standards. And as you say many false alarms.  Although I think many of the false alarms can be ruled out as a stator (shorted turn) problem based on RIC test.  But if we have high rotor influence, we don't know whether it is actual rotor problem or just normal residual unbalance… still a lot of unknowns.  Doesn't seem really useful to me.  Most motor repair shop people I talk to say that for purposes of evaluating the rotor they would very much prefer a different test:  the "single-phase" rotor test at roughly 25% voltage with manual rotation of the rotor and checking for stator current variation.  But of course that is a much more difficult test to set up.

The output from a Baker surge test is the oscillating frequency which can be directly related back to the coil inductance. Lots of people use the graphical output and try to interpret it in various ways but the fact is that we're just seeing oscillations from an underdamped LC tank circuit that is smacked with a good size voltage stored in the capacitor. The charge voltage is within a fraction of the limits of healthy insulation which hopefully the Baker tester screened out using a megger test ahead of time and locking the technician out if it doesn't pass the megger test. As per Baker themselves it really only excites the first few turns. We've seen a few motors with a defect mid-winding that Baker doesn't detect so from shop wisdom we've confirmed this.

The PdMA tester takes a different approach. They are a little secretive on exactly what they are doing but they are using a couple hundred volts or so in the test compared to hi pot levels of voltage and varying the frequency of the test voltage, and achieving steady state resonant measurements. This much I can kind of see because on motors where I knew I had a lot of inductance to begin with, it took a lot longer to get readings compared to much smaller motors. This is a technique that is also used for determining inductance in transformers in IEEE standards so what they are doing is certainly not radical in any way. In fact if it wasn't for such a long industry experience with Baker, I'd even go so far as to say that the PdMA test is far less radical than the Baker one. Theoretically their test should also pick up on defects anywhere in the coil and not just the first few turns since it isn't a decaying wave they are using but rather a fully developed sinusoid at least based on the sketchy description of what it does.

But so far I just haven't had a lot of time to test them side-by-side for comparison purposes. The Baker tester is more of a shop/lab instrument while the PdMA is more of a field testing instrument. I work in the field so we use PdMA. The shop guys work in the shop and they've always used Baker for years so there is no value in looking at anything different to them. Wish there was some A vs. B data out there but right now there's not.

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