I would like to hear opinions on the validity of the "rotor influence check" performed at low voltage using PDMA and similar testers in order to check rotor conditions.

Will it reliably detect open rotor bar problems?
Will it give false alarms?

Thanks.

Pete,

I understand the theory behind it, and it seems to make sense.

However, theory to practice, there are some pitfalls... I was advised that it is possible to conduct this test from the breaker... for all future RICs that we perform, we will go straight to the motor leads.

The attached file shows two RIC tests that were performed at the breaker and followup inspections at the motor leads. In these particular examples, there is also another breaker downstream from the breaker where the RIC was performed. What we found was that any resistance in that downstream breaker would affect the RIC test results.

We originally opted to try the test at the breaker due to the difficulty in working at the motor leads (nuke containment work).

In the future, we will only perform the RIC tests at the motor leads to eliminate all other 'influence' on the test results.

RIC_Test_Examples.doc (164 Kb, 107 downloads) RIC Test Examples

Pete,
In practice 'broken rotor bars' are not really bars that have broken. It is often the joint(s) between Cu rotor bar(s) and endring that has reduced conductivity. This conductivity may be load and temperature dependent. So I only have faith in a test at normal operating conditions. (pole-pass sidebands in 50/60 hz motor current and same for RBPF +/- 2*Fline).
Inevitable voids in Alu cast bars usually result in stronger fluctuations in RIC than with Cu. However this will remain constant over the life time of the machine.
BTW, Jamie's motor is a (approx.) 32 bar, 4-pole, Alu rotor machine?
Regards,
Arie Mol, WI

Jamie - Thanks for posting those. Those are interesting example of waveforms distorted by remote testing. Apparently the first one was an intermitted loose contact and the second one was steady high resistance on one phase. Did you conclude there was any problem with the cabling/conections that might require repair? Or maybe the test is so sensitive that even normal cable/connection resistance show up this dramatically?

Arie - good point that we can never rely on off-line test to prove a rotor good, due to thermal factors (although we might be able to "prove" it bad if it shows bad during off-line test and if we have enough confidence in our test). Sometimes you want to test a motor in a configuration that can't be easily loaded. For example when the need arises to evaluate the motor during a plant outage or in the repair shop. I think the only choices would be R.I.C. test or single-phase test. Any preference between those two?

And how did you draw the conclusion about Jamie's rotor being aluminum and having approx 32 bars?

I use All-Test IV as well as the EMPATH for testing clients motors. The former has a rotor test, but it has not been as successful as ESA.

I have attached data of two identical motors, 330 KW, 6.6 KV, 4-pole with 46 bars. ESA indicated broken bars on both, the rotor test indicated the same for only one. Inspection revealed hairline cracks in both rotors. Also, copper dust & small pieces were found.

Regards,

Aditya

Rotor_results.doc (4,167 Kb, 86 downloads)

Jamie,
You have some interesting patterns. I have done the RIC test only once, a few years ago. Looking at the PdMA website, the theoretical explanation of the RIC test can be found for example in Fault Zone Analysis “AIR GAP”. http://www.pdma.com/AirGapFaultZone.pdf
And it is a rather strange explanation. If you look at page 2, the 4th paragraph from the bottom beginning with: “ Evaluate the influence of the rotor’s residual magnetic field on the stator’s phase to phase inductance.”
As far as I know, the RIC is a static test, so the residual magnetism cannot have any affect on the stator, because there is no relative movement between the stator and the rotor. No change of flux means nothing is induced (Faraday).
Am I missing something?
Then there is an interesting the pattern with one inductance about 13% lower. I believe the PdMA talks about “2up, 1down”. It has been assessed as a typical stator short. Did it fail or did you have the motor rewound? And do you happen to have the readings after rewind?

There are some interesting questions to be answered. For example, what is the influence of the frequency (600 Hz). For sure, it will increase the eddy currents in the iron by the factor of 100. It will also squeeze the current flowing in the bars to about 3 millimeters below the upper edge of the bar. If the rotor slots are closed, the miniscule flux created by the high frequency and low voltage may simply flow over the surface of the rotor iron instead going around the bars. Are there any answers to those questions?
jank

OK...First off the disclaimer...I have a mechanical background and was fortunate enough to take over our site's motor testing program when it was transferred into our PdM group...needless to say, I have learned quite a bit about motors over the last couple of years administering the program...nowhere near to the level to which you guys have obtained...I have gained much of my knowledge from reading and attempting to understand your responses on this board.

Arie - These motors are Reliance 460v, 4 pole, 58 bar

Pete - The first test - we did not think that our connections were bad, but they may have been. We discussed the 'possibility' of radio interference - and concluded that we needed to test at the motor. The area of the breaker for the first test had many sources of possible radio interference, but it didn't seem likely to me that it was a realistic possibility. For the second test - we concluded that a high resistance connection could possibly be the culprit and moved on to test at the motor. The downstream breakers are not currently in our IR program, but will be added based on this experience.

jank - regarding PdMA's statement on 'residual magnetism', they present a case history in their data analysis class where a motor exhibited 'crazy' RIC data. Turns out that some repairs in the area resulted in welding leads being draped across the motor, 'removing' any residual magnetism in the motor. The motor was subsequently run and later re-RIC'd with satisfactory results. So... PdMA qualifies the RIC with the requirement that the motor must have been run previously (contain residual magnetism). I hope that I am making sense here. Additionally, when the RIC is performed, the phase to phase inductance is measured in 18 equal increments across one pole face of the motor. Is that the piece of the puzzle that you are missing? Regarding my attached test data and the two up, one down pattern - that data was obtained at the breaker, with another breaker downstream of the test location. The subsequent test was performed at the motor leads, with the motor determed. No repairs were made to either motor between tests.

I don't know about the test frequency, but am interested in learning more about this - difficult to perform - test.

When assessing the impact of the partial cracks in the rotor bars I would advise caution. I believe that those tiny cracks and also the voids are blamed for a lot, but in fact they are quite innocent.
In order to have some numbers, I have conducted a low budget experiment. I took a stripe of metal 20 inches long. 4 inches from each end I drilled a 3/16 hole for a screw. It means the holes (screws) were 12” apart. I fed the current on each end of the stripe and measured the voltage drop between the 2 screws 12 inches apart. In other words I had an equivalent of the rotor bar 1 foot long and ¾” wide (19 mm). The thickness is irrelevant. The resistance measurement was actually conducted with a sophisticated low resistance Ohmmeter.
The resistance of the intact “rotor bar” was 4225 micro-Ohms. Then I cut a slot in the middle with a hacksaw. Only 4.5 mm was left. So the “rotor bar” was 76% broken. The resistance increased to 4593 micro-Ohm.
The resistance increase was only about 8.7%.
I dare to say, that finding the partialy broken bar among the dozens of others is virtually impossible.
If you want to repeat this experiment, they have the metal strip in your shipping department.
jank

Gentlemen,
Your points are all quite valid and reinforce my decision to not by the MCE when first introduced. The ability of the MCE RIC test to find rotor 'anomalies' is based on requiring a 'ghost' gauss that was left at the time of power being cut. The US Navy specification for identifying broken bars utilizing a single phase current test is 'at least 70% nameplate current'. Therefore, if you have only the ability to test a motor 'off line', you will need alot of current and all bets are off if the rotor bars are not completed disconnected and can still carry the majority of the current. Before I go too much further here let me state that we purchased an MCE EMAX system. I had always explained to PDMA that I would be on-board the second they could look at the voltage and current of a loaded machine. That is ALWAYS the best condition for testing. Also, remember that there are multiple indicators of a bad rotor and you should base your call on multiple indications. These multiple indicators are all a result of the same thing, the inability of the rotor to evenly and smoothly carry torque due to the bar problems, therefore,
multiple pole pass sidebands in the vibration data.
pole pass sidebands less than 35 dB down in the current signature.
a drop in speed due to the inability of the rotor to carry full current (load)
periodic fluctuations in analog current meter on the MCC (again, these are the sidebands).
As far as testing motors at the MCC vs the motor leads, obviously the MCC is far easier and the user should always confirm at the motor leads any data that indicates a problem at the MCC.

Great comments all around. Thanks Aditya for those case studies. Again reinforces that the off-line tests may not be as good which seems to be emerging as a big consensus.n Among recent posts here and in the vibration forum we have a pretty impressive collection of rotor bar case studies including symptoms and findings which is very valuable to me since I have only been exposed to one rotor problem on a large motor at our site.

Jan - that's a lot to think about. (I'm still thinking). As far as the crack, my first reaction would be to be nervous about anything that was cracked, if nothing else from a pure mechanical perspective. We are talking about a crack at the joint from bar to endring- - would the crack be parallel or perpendicular to the current in the bar?

Ron - are you saying that in your view a major shortfall of the PDMA MCE EMAX is that it measures current and not voltage? That's somewhat ironic because in our plant environment I would not use the voltage inputs. It's so much easier to get people comfortable with putting a clamp-on onto equipment than hooking leads into a circuit (it would be difficult to cause a trip of a circuit with a clamp-on but somewhat more plausible if you happen to cause a short accross terminals.) The guys who do this testing for us are not electrically trained. And I have tended to think there is not a lot of extra useful predictive info that comes from voltage.... and I wrong about that?

EP,
No. I am fine with EMAX. My problem was with PDMA stating that a static RIC test could find bad rotors in large (over NEMA size) frame motors. I have had excellent results with EMAX, including reading from the CT's and PT's on high voltage motors.

Electricpete,
Yes I would be nervous about any cracks also. I was talking about impact of the cracks and voids from point of view of finding them by testing on assembled motor, testing from the stator. And I really believe it is a formidable task. You can see claims that it is possible, but I would like to see a reliable confirmation.
I have seen lots of cracks on a double caged rotor. They were perpendicular to the bar in the closest proximity to the ring. Where the silver solder joint ended, the crack started. The rotor lasted about 6 month and the motor died when the upper ring flew off.
Jamie,
What I am missing on the explanation of the RIC test is the sense.
a) They hook up 2 leads to the 2 stator leads and feed some current through.
b) By transformer action this current induces some current into the rotor.
c) The inducing happens only because they feed the alternating current (AC).
d) The residual magnetism cannot induce anything back from the rotor to the stator for a simple reason that the flux does not change in time; it is a “DC” flux (Faraday induction law).
e) If there is nothing induced back from the rotor to the stator the presence or absence of the residual magnetism is totally irrelevant.
I would suggest, they should first figure out how the test works. For sure it is not the residual magnetism. (If you let the Earth magnetic field through a coil of wire it does not induce anything either).
Somehow it seems to me, that performing a test that is not based on sound science is not an efficient use of time.
jank

Sorry Pete for being late with this response to your posting of 16th January. (I know your are right not to appreciate 'no replies').
a) No preference. However the best place to test is the machine at site, at load, at continuous operation.
b) The number of fluctuations on the RIC plot is approx. 32. Aliasing may mask 64, 96.
Regards,
Arie Mol, NL

I have resurrected this old thread, because I believe I have something interesting to post.
There was a discussion on some aspects of the RIC test in the thread: ”Sidebands 45 dB down…” I would like to continue the discussion by showing the difference between the impedance of the motor with open and closed rotor slots. The dependence of the impedance of a motor with closed slots on testing voltage has already been shown (see the thread “Sidebands 45 dB down”).
It may seem to be an unimportant detail, but the difference in impedance of a motor measured with low voltage testing equipment (such as PdMA, Alltest…) and measurement conducted according the standard IEEE 112 (with nameplate current) is so striking, that it deserves additional attention.
Attached are 2 curves of impedance for a) motor with open slots, and b) for a motor with closed slots. It may seem strange that I even talk about the open and closed slots. I do not think you have ever heard about that difference from the major “low voltage” testing proponents. However you have heard about the “LIR” motors, and this test may shed some light on the reason behind a motor being “LIR”.
I have measured impedance of two motors with voltages ranging from 150 Volts to about 1 Volt. Since the motors were of a different horsepower the absolute value of impedance in Ohms would be vastly different. The way to compare them, is using the “per unit” impedance. To calculate the “per unit” impedance you just divide the ratio of: Measured voltage / Measured current, by another ratio: Nameplate voltage / Nameplate current. If you seek the result in percent, you just multiply by 100.
That was done for 100 hp, 575 V motor with closed slots and for a 300 hp 550 V motor with open slots. The curves start at higher voltages (to the left) and then the voltage is lowered. In case of the motor with closed slots, the nameplate current was reached at 150 Volts; on the other hand for the motor with open slots, the nameplate current was reached at 110 Volts.
Both curves have a very important common feature: When you reach roughly the nameplate current (see IEEE 112) they both approach 20%. In other words, the locked rotor current for both motors is roughly 1/0.2=5x running current. The similarities stop right there!
The ratio is maintained for the open slot motor to the lowest possible voltages. The curve is totally flat.
But for the closed slot motor, the ratio grows from 20% to 190% at 10 Volts testing voltage!!! (60 Hz test). The enormous growth of the impedance appears right in the range of the low voltage testers.
If you want to do similar test for yourself, keep in mind that probably 95% of all motors are closed slots. There is a fair chance that a 3000 or 5000 hp motor has open slots and will be LIR. The “no LIR” motors are much more common. Not because of residual magnetism, but because of the closed slots.
The test I have just described can be done very quickly (1/2 hour) if you have the right equipment (variable voltage source). It would be very interesting to do a series of RIC tests with all different voltages (hours and hours of testing). Not only it would show the “Validity of the RIC Test”, it would show shed some light on validity of the low voltage testing.
jank

open-closed_comparison.pdf (40 Kb, 36 downloads) Comparison of impedance of 2 motors. One has open rotor slots, the other has closed rotor slots

I had a unique opportunity to test a 1000 hp, 1200 RPM motor rated at 4000 Volts. The unique part was a manufactured aluminum rotor (104 bars) with closed rotor slots. The rotor lamination was sitting on a 4-arm spider. As it is usual in this design, the rotor bars are inserted into the lamination, then it is heated up and shrunk onto the pre-machined spider. The outer diameter of the rotor is then machined exactly to size. As a result of the thermal shrinkage of the lamination, the thickness of the iron bridges over the bars is vastly different. It is very thin (or completely open) above the arms of the spider and much thicker over the bars between the spider arms. In other words, the rotor is not magnetically symmetrical to a random axis’. The reluctance is changing depending on the relative position of the rotor to the stator.
This was a good indicator, that the inductive unbalance will be quite large. Since I had only limited amount of time, I could not do a RIC test with variable voltage, but I did the next best test possible. I monitored a single-phase current with constant voltage while slowly spinning the shaft (CSI analyzer – monitor overall mode). I tried my best to rotate the shaft 180 degrees (mechanical) in about 45 seconds. After few tries it worked reasonably well.
The test was done for phase A to B at 5 different voltages (60 Hz): 2.46 V, 5.59 V, 7.76 V, 10.96 V and 22.3 Volts. I did not calculate the inductance; instead the current is plotted versus time. For the ease of comparison, the variation of the current from the average current between 0 and 180 degrees (or between 0 and 45 seconds) is plotted with the Y-axis in percent.
The results are shown in the ppt file attached. For voltages 2.46, 5.49 and 7.46 Volts, the current unbalance (which would translate into the identical inductive unbalance), is over 15%, approaching the 20% mark. The unbalance drops substantially at 10.96 Volts and at 22.3 Volts is practically nonexistent. Each graph has also the calculated average inductance: It varies from 170 mH to 30 mH at 22.3 Volts. Yet the inductance per phase at nominal voltage (4000 Volts) is ~7 mH (locked rotor current ~ 800 Amps). It shows again how is the inductance and with it, the inductive unbalance dependent on the testing voltage.
The last slide in the ppt file is comparison of the phase A-B to phase A-C at 5.59 Volts. (Somehow I have lost the third phase). The motor seems to be in big trouble by the low voltage test standards. It seems to have broken bars on the rotor, shorted stator turns and possibly scores of other motor ailments. However it is enough to increase the testing voltage, and the motor is miraculously cured.
The motor came to the shop for a routine overhaul. It has always worked flawlessly and all the classical tests confirmed a motor (and the rotor) in perfect condition. It is my believe, that it will perform well till a guy with a low voltage tester comes along.
jank

VARIABLE_VOLTAGE_RIC_TEST.ppt (132 Kb, 21 downloads) almost RIC test with variable voltage

In the previous posting I have tried to describe the magnetic asymmetry of the rotor in the 1000 Hp, 1200 RPM motor. Since one picture is worth 1000 words, I am attaching two pictures of the surface of the rotor in a ppt file
The first one is “ABOVE SPIDER ARMS".You can clearly see where exactly the upper edge of the bar is. The material over the bars is so thin that it even breaks open in few places.
The other is “BETWEEN SPIDER ARMS". This picture was taken 45 degrees from the first one (the spider has 4 arms). There is much more material over the upper edge of the bars and the surface looks quite smooth.
The manufacturer does have to be too concern if there is such a difference. In normal operation the lamination bridges over the upper edge of the bars saturate and the rotor behaves the same way as a motor with open rotor slots.
However this is not the case of the low voltage testing. The magnetic flux produced by the low voltage, high frequency tester cannot saturate those bridges and as a result the bridges act as magnetic shunts. The tiny flux travels over the surface instead going around the bars. The best proof is the high inductances measured with such testers, nowhere near the real inductances measured according to IEEE 112.
The magnetic asymmetry of the above rotor is nothing unique. All closed slot rotors are non-symmetrical to certain extent and produce an inductive imbalance.
So if you find a motor with large inductive imbalance performing the low voltage “testing”, do not forget to do some testing before you condemn the motor.
jank

asymmetrical_rotor.ppt (116 Kb, 19 downloads) The magnetic asymmetry of the surface of the rotor