PDMA Test Equipment 2

[electricpete]

Does anyone have experience with PDMA motor test equipment (on-line or off-line versions).

I'm interested to hear whether this equipment has been easy to use, has proven useful in detecting problems, has generated false alarms, etc.

 

[jbartos]

Suggestion: Visit

http://www.pdma.com/

(check the downloads for useful software)

http://www.electricmotorservicenter.com/

etc. for more info

 

[GordS]

We have been using the off-line version of PdMA for about 4 yrs and the on-line version for about 2 yrs. We track about 100 critical process motors about every 6 months. It has picked up a few items that could have led to motor failures. The big problem is interpretation of the results. Some of our technicians have a hard time using something unless the answer obvious. With PdMa, I think the value is more in being able to interpret trends as opposed to interpreting individual tests. For a while there, we were finding the PdMA was correctly indicating a problem which we missed util we did the failure investigation and looked back at the PdMA data. We are seeing more benefit as time goes on and our technicians start to know what to look for in the results.

 

[electricpete]

Thanks GordS... that's great info. Can you share what type of problem it was that PDMA showed that eventually resulted in a failure?

 

[rhatcher]

I have some experience with both online and offline testing using the PDMA. As well, I have been involved in repairs of problems detected by the unit. It is very powerful, but like anything else is only as good as the person operating it. We do not own one but use a consultant for this testing when required by our customers. He was one of the guys involved in it's development, so his understanding of it's capabilities and limitations is extensive.

A lot of our customers are interested in testing performed with this unit, but there has also been a lot of frustration with the unit among the same group since, as pointed out by GordS, this unit is not a "crystal ball".

Specifically, it cannot predict every failure and the results obtained from any testing must be evaluated by someone who understands what the test is measuring, how it is measured, and what the results mean with respect to the specific type of motor under test. As well, trending is necessary to get the true picture offered by any type of periodic testing

As an example of one instance where we worked with this unit, the PDMA tech detected high resistance in one leg of the rotor of a 3 phase wound rotor motor. The testing is usually done from the motor controller, so this included the leads, brushes, and slip rings as well as the winding itself. The PDMA tech wanted the slip ring cleaned and the brushes re-seated. When I asked for the basis of that determination, he pointed out that although the resistance was high, the inductance was in line with the other legs so that meant that the problem was not a winding fault. That being the case, he was guessing (an educated guess that was correct)that the brushes were the most likely culprit. You can't argue with that logic...

The slip rings and brushes were not perfect, but also were not visibly "bad" either. I had a tech perform the required repairs and the follow up PDMA test confirmed them as good.

Unfortunately, the $50,000 price tag for the model that includes on-line and off-line testing is prohibitive for us right now, so we hire the aforementioned consultant to do our testing when required. It is not a bad deal for us because of his experience with the unit and his extensive understanding of motors. In fact, we tried to hire him outright, but he prefers his independence (can't blame him...).

Anyway, I hope this helps. If you have any specific questions let me know.

 

[electricpete]

Good info, rhatcher.

Going off on a little tangent, have you ever seen the unit used to find rotor bar problems? (Rotor Influence Check)

 

[rhatcher]

That is not a tangent at all. To answer your question, I have seen the rotor influence check performed but have never seen it detect a broken bar. Specifically, that is not to say that it failed to detect a broken bar when it was present, but to say that I haven't been fortunate enough to see it used on a motor with a broken bar. (Yes, in the repair business a broken rotor bar is a good thing...)

However, based on the explanation given me about how it works and seeing it in action, I am confident that it is a valid test. You can actually count the # of bars on the rotor by performing this test!

Another useful feature was the online plotting of current and voltage over time. In one case, we were plotting the current waveforms of a 3 phase motor on a belt driven 2 cylinder compressor. The current waveforms were seen to "pulse" with a period equal to about 5 or 6 electrical cycles. At first we thought it was the belts "tensioning" as they could be seen to pulsate with some regularity. However, we soon realized that the belts were not the cause but another symptom of the same cause. It turns out that we were able to correlate the current pulses with the compression strokes of the cylinders by taking into consideration the pulley ratios and assuming that the motor was a 1-delta winding and that the cylinder strokes were 180 out ( in the reference frame of compressor rotation). That was pretty neat! (for me at least, you know what they say: "simple pleasures for simple minds....&quot. This implies that this can be useful for evaluating loads as well as motor perfomance. For example, knowing this to be true we could now check for a weak or dead cylinder based on the current pulses during the compression strokes.

Another useful (and neat!) online tool was the plotting of the starting current. Really, the list goes on.

Have you had a chance to evaluate one? When we were considering purchasing one and the rep wanted to do a show and tell, we did so by testing a few motors in the shop, both offline and online. After all, the proof is in the pudding. If you haven't tried one out yet, contact them and set up an in-plant demo.

If you are interested, I may be able to put you in touch with someone who can set that up for you. As well, I am open to discussing these features or others if you want to know more. I will say that if you get really technical I will probably have to call our PDMA consultant for help, but that is no problem.

Have a nice weekend and a happy holiday.

 

[electricpete]

rhatcher - thanks that's some more good info.

Actually I have attended a one-day seminar by PDMA and I have had them on-site for a demo. They checked a motor where vibration analysis indicated eccentricity. Their conclusion was the same, but it didn't give me anything beyond what the vibration analysis told me.

So I've heard the vendors side and I'm pretty familiar with the tests. Overall I was impressed but I'm still skeptical of whether I've been duped by clever presentations. I'm waffling a little and hoping to firm up my position if I can identify some clearcut positive examples of the benefits from other users.

Here's my general impression of the unit based almost soley on my limited exposure from sales seminar and demo:
(I have cut and paste from a message I posted at reliability-magazine.com)

The off-line unit seems like a good replacement for traditional megger IR/PI test. In literally only a few minutes more than it takes to run the traditional IR/PI test, the unit will collect a mountain of data....plot of insulation resistance vs time (10 minutes), winding resistance for each phase, resistive imbalance, winding inductance for each phase, inductive imbalance, and capacitance to ground.... all tests are automatically performed without changing connections and data is electronically recorded. Clearly you get a lot more data in approx the same time as the old ir/pi test. You have a better chance of detecting internal or external high resistance connections. Certain winding contamination problems might show up in the capacitance tests or in the "shape" of the insulation resistance vs time curve (a jagged plot may indicate a problem even though IR and PI are fine). Upward trends in inductance or excessive inductive imbalance might be a tipoff of possible rotor problems which could be investigated with the same test box by doing a rotor influence check (takes a little more time... requires rotating the shaft).

Returning to my original point, you get a lot more data in approx the same measurement time as an IR/PI test.... but there clearly will be a lot more analysis time. It appears there is not yet much guidance to assist in interpretting these results. So there will have to be a lot of trending, comparison among sister units... and comparison with alternative test methods.

I should mention that the rotor influence test is DIFFERENT than the single phase test I described in my previous post. Single phase test uses a relatively high current to cause transformer action which interacts with the rotor. In contrast the rotor influence test relies on residual magnetism in the rotor and uses low test current (hence it is easily incorporated into the relatively small test set). The two tests aim to detect the same types of problems in different ways.

The above discussion applied to PDMA's offline testing unit called the MCE unit. They also have an on-line unit called EMax. The Emax measures all currents and voltages (on the secondary of instrument transformers for higher voltages). This will require a little bit of thought to find the proper locations and a little care in making the connections. Once hooked up, the unit takes voltage and current waveforms which are used to calculate motor input power, motor current imbalance, system voltage distortion and system voltage imbalance. Also the current FFT spectrum can be displayed and checked for 1 - pole pass sidebands around 60hz 2 - rotor bar pass frequency with sidebands of 2*FL... both of which may be used to help diagnose rotor bar problems or eccentricity problems. High 120hz might indicate stator core problem or overexcitation. And the current is rich in spectral information which might hold other mysterious secrets.... an interesting thing for engineers to ponder but not a lot of practical value ... yet (other than the specific symptoms mentioned above).

One thing the vendor admits is there clearly is a steep learning curve for these units (in terms of data interpretation). But taking the data is fairly easy.

For $50-$55K you get a unit which includes both the on-line and off-line functions discussed above. This also includes a laptop which controls the tests and which provides long-term storage of the data from different types of tests organized by machine. From a brief look, it seemed like they had some good data handling and reporting options.

I havent yet used the machine and we won't be buying one in the immediate future. If you get the chance to go to one of the free seminars, I recommend you go to it. They are offered periodically at many major metripolitan areas. See the pdma website.
[.i]

 

[rhatcher]

electricpete,
Based on your eval, I would say that you have a pretty fair understanding of the unit. I believe in it as being everything that they claim it to be and then some. As you alluded, the capability of this unit as a whole is much greater than the sum of it's parts, especially in the hands of someone who can understand it enough to see everything that it can show you, a lot of which is beyond the basic capabilities of the unit as advrtised. However, I will admit that my opinion is influenced by working with a PDMA consultant who is very knowledgeable on motors and very knowledgeable about the unit.

Perhaps you can rent a unit or talk the rep into providing a demo unit for a week or so to allow you to become more familiar with it. Or, you may consider as a trial hiring a consultant to do testing on your larger motors. That would give you a better idea of the capabilites and an opportunity to make a case for purchasing one based on your own findings and actual costs saved. Of course, at that point, you may realize that the consultant is the way to go.

I hope this helps.

 

[GordS]

electricpete...the problem I referenced where the PdMa identified a problem after the fact was several months ago so memory is a bit foggy. We has two identical systems fed by VFD's. The motor on one system failed about 6 mo after start-up. The motor vendor immediately called it a "VFD failure". When we took the motor apart, the appearance of the failure did not look like what we figured a VFD failure would look like. We then checked our commissioning records for the motor anf found the motor that failed had a noticeably higher resistive imbalance than the motor that did not fail.
The PdMa even red flagged the test but our people did not respond. It seems obvious that someone should respond when a test instrument red flags something - I guess they were told to test the motor and that was exactly what they did.

Regards the posts on rotor tests, we had a case there a conveyor drive motor was indicating broken rotor bars on an online test ( not PdMa - we were experimenting with a competitor at that time but I think the theory behind every on-line test is basically the same). However, it was a false alarm because the "broken bars" disappeared when we took the motor to a rewind shop and tested it with the PdMa in the shop. We even stalled the motor for 10 sec to see if the "broken bars" would show up after some thermal stress on the bars. Still nothing. We concluded the problem was mechanical - even though the mechanical people have never been able to isolate the problem.

 

[rhatcher]

GordS and electricpete,

GordS:

I would love to know more about the high resistance failure or perhaps see pics of the winding. I am wondering if the failure was in an internal joint or connection in the winding. A bad joint would cause high resistance readings in an induction motor stator while not affecting the inductance readings. I have seen this before and can say that this is about the only way that I know of to get unbalanced resistance w/ good inductance for an AC stator. Also, the life expectancy of 6 months is reasonable for this type of fault. I have seen that before too. Sounds like a warranty to me.

With respect to the other tester, are you referring to the Baker unit? I evaluated that one as well but have no experience with it beyond that and remember little of it as I was not impressed. The similarity between Baker and PDMA ends once you get beyond the looks. Don't get me wrong though, we use other Baker instruments in our shop and they are the industry standard for surge testers.

Anyway, the rotor influence test for the PDMA unit is offline and has results independent of the load or the power system. I believe that both units can see rotor effects online, but these are not as reliable (in my opinion) as the offline test.


electricpete:

First, I meant to ask you this before, but when you got your in house demo of the PDMA unit, did the rep doing the demo know that there was a known problem with eccentricity before you did the test, or did he do it blind and come up with the same conclusion independently? Just curious..

Second, I hope GordS doesn't mind me saying so, but his case makes a point that I meant to bring up earlier. Technologies like vibration analysis, thermography, and the PDMA tester have really come into the mainstream over the past few years. It is natural to want to incorporate them into your maintenance program by purchasing the equipment and performing the testing and analysis in house. Even with the high cost of the equipment, the consulting rates of $1000-1500 per day make it cost justifiable for bigger facilities.

In principle this is fine, but in practice there are problems that I have seen. GordS' story has highlighted two common ones. First, the personnel performing the test must be trained on the tester and must be very knowledgeable on motors in order to properly evaluate the results. Second, the results must be analyzed and acted on by someone in authority to take action on the findings, not just filed for future reference. These are exagerated by the fact that most maintenance departments are overtasked to begin with. This type of program is usually the first to fall to the wayside because of it's demand for the top people and the fact that it does take some time to truly implement all of the analysis and trending that is required to do this right. That is why I really believe that with a consultant you can have much better results. Of course, if your facility can have truly dedicated personnel for this type of work, then go for it.

 

[jbartos]

Suggestion: The predictive nature techniques seem to be clue. E.g. if one has a new motor with new bearings, one can precisely measure the center of the shaft with respect to the reference. After some time, one can measure how much lower is the shaft center with respect to the reference. Then, one continues and can plot the distance of the shaft to the reference point with respect to time. The plot then can indicate when the motor will need a new bearing. The motor manufacturer may support this, so that the prediction of the motor bearing replacement is accurately predicted.
Similarly, one may obtain the motor starting current signatures and deduce from those signatures in time how the motor is, including the shaft load deteriorating in time. This is done for motor operating valves (MOVs).
Etc.

 

[GordS]

rhatcher...Sorry I can't send pics of our motor that had the high resistance fault. Every once in a while I clean up my computer by deleting files I think I won't need again so I have no pictures. However, thanks for you analysis of what the problem might have been - it sounds reasonable. I must admit I was stumped.

I definitely don't mind your comments on the new maintenance technologies and incorporating them into maintenance programs. We have expereinced all of the drawbacks you have mentioned. Fortunately, our plant manager came from a maintenance background and is very supportive of these new technologies. We took an electrician from our manintenance department who was dedicated to getting the system functional on our company LAN system, getting people trained, and always reinforcing the need for performing a PdMa on regular intervals. It took several years but now our technicians treat a PdMa almost like a meggar - it is just a part of normal business. This supports your last statement - it helps if you have dedicated personnel.

My bias is I don't like to depend on consutants for maninteance work. I think it is beneficial to increase the knowledge base of the maintenance technicians. Also, if something goes wrong, I think it is advantageous to have the person able to fix the problem nearby, not several hours / days away from me. However, there is a learning curve that has to be endured. I am hoping the long term benifit justifies the short term pain.

Incidentally, the competitor on-line tester that we tried was Liberty. I thought they were comparable to PdMa at the time.

 

[electricpete]

I took a break from the board over the Holiday weekend. Was pleasantly surprised to see a lot of good discussion by rhatcher and GordS when I checked back.

rhatcher - We didn't tell the PDMA rep what our vibration diagnosis had found. Vibration had shown very high RBPF +/-k*7200 k=0,1,2,3,4 pattern and 7200 which we diagnosed as eccentricity. Current signature analysis also showed a sidebands around RBPF +/-k*7200, k=1,3,5 pattern which also means eccentricity.

I think that looking for dynamic eccentricity at these very high frequencies in current spectra is relatively new. (All along we've been looking for pole-pass sidebands around 60hz... using our Entek vibration data collector with current probe input). GordS - you mentioned all seem the same... does your Liberty box measure high frequency in the neighborhood of RBPF. I think that takes special signal processing.

GordS - I have had a similar experience where we saw pole pass sidebands around 1x vibration and pole pass sideabands around 60hz current which was greater than 1% of the fundamental. The motor vendor is adamant that it's a false alarm. And this particular motor vendor has quite a bit of credibility on this subject (it's a Westinghouse motor used for Reactor Coolant Pump... Westinghouse has tracked virtually all of in-service motor failures/problems on RCP motors for 30 years and published bulletins and summaries).

I agree with rhatcher most likely scenario if we had seen resistive imbalance followed by motor failure would be high resistance at an internal brazed connection. In my limited experience I have heard of one other instance of failure of a brazed connection... it was written up by a utility and presented at an EPRI conference. There's another scenario that the test-box people will tell you resistance checks are useful for finding... and that is shorted turns. But that would have also resulted in inductive imbalance... and more importantly motors don't operate more than a few minutes with a shorted turn before they fail from what I've heard.