January 18 2016 MotorDoc Update

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Presently I am enjoying the latest MotorDoc tour – about a month on the road testing, consulting, training and troubleshooting.

As I have mentioned in my last newsletter, I have obtained a new technology that covers an area that I have had limited need in the field – high voltage field testing. The Electrom iTIG II D12 fully loaded includes all of the tests in most field high voltage testers plus low voltage tests of Capacitance, Impedance, and Inductance. In the MotorDoc blog I will have a discussion related to the differences between this tester and the ALL-TEST PRO 5 motor circuit analyzer based upon larger (4160V and 13.8kV) machines. In February, when I have my custom tester in hand (I am presently using a loaner), I will be running comparisons between the Electrom and AWA, especially with some of the concerns raised by the repair shop.

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Presently, the life of hotels and laundromats has been exciting with no lack of excitement in the various projects that we have been involved in. The MotorDoc LLC selected subcontractors have been working extremely well, expanding our capabilities and ability to service all of our clients. The change in direction from the original SUCCESS by DESIGN Consulting model to more of a service and training company with consulting support in reliability versus a reliability company that does some service has been allowing for excellent opportunities for expansion. So far the MotorDoc LLC model has been extremely successful.

The March class is on for Electrical Motor Diagnostics (March 1-4, 2016) to be held at Dreisilker Electric Motors in Glen Ellyn, Illinois. We will have the full suite of ALL-TEST Pro LLC equipment, the AWA IV and the Electrom iTIG II D-12, and any additional instrumentation that is brought along. In addition, we will relate vibration and infrared to what we are seeing with these instruments with the expansion of the class focused on more than 50% hands-on and the remainder being data interpretation and the leap from theory to what you will really see in the field. http://www.motordoc.org/training/electrical-motor-diagnostics/ for more information.

We look forward to working with you. Contact us at info@motordoc.com for more information!


Howard W Penrose, Ph.D., CMRP

ALL-TEST Pro 5 and Electrom iTIG II D12*

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A client asks for it, we are capable, we make the investment.

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At the end of 2015 with a project coming up fast we were asked to perform testing using an AWA IV 12kV unit and made the attempt to lease one. After a number of attempts where they were damaged/broken, we contacted Electrom instruments to see what they had available. To our surprise they had a system that performed more tests at a greater published accuracy plus a 50-60Hz surge test versus a 5kHz surge test in exactly the same price range (with all the additional tests we ordered in a customized system). This was a concern on the one that I have used in the past where obvious issues were missed possibly because the 5 Hz allowed the ionized gas (air) to dissipate after each impulse. We put a fully loaded system on order and were sent a loaner that the first time we put it in action was the first day of testing – very easy to use plus we were walked through the operation via phone. We’ve found their response is pretty much 24/7 while we were waiting for a response (which we never received) from the other company over the holidays.

In a recent set of testing and during a motor diagnostics class the accuracy had been a concern. The ALL-TEST PRO 5 (AT5) was consistently finding faults that we would have expected the AWA to catch including some very obvious problems (visibly shorted windings). To be perfectly fair, many of the faulted motors that were found over time (~60%) had insulation to ground issues that did not proceed to the surge test portion. For some of these insulation to ground problems we use a high voltage insulation resistance meter in order to obtain a polarization index, insulation resistance profile and dielectric discharge (reference IEEE Std 43-2013) alongside the AT5. For winding shorts, however, we found the AT5 was performing far better.

In February we are going to do some direct comparison studies between the AT5, Electrom and AWA which we will publish.

For field work, in which my comparison of the AT5 and Electrom system involved moving around a large plant with difficult to access areas and some challenges with finding good outlets. I used a rolling file case to move the instrument plus carry other instruments and a 50ft extension cord, which was very convenient for longer walks. Moving the tester, which is the lightest of the equivalent field surge testers of this type, was simple on straight aways but required effort when moving through tight spaces. Setup and teardown between tests kept the full testing time to just under an hour as we were performing step high potential tests and full ten minute polarization index tests. To give an example, I was testing ten motors in one area, four of which were de-energized and set up for testing. I performed AT5 and vibration tests on the six running motors by the time I had finished the first machine which gave me time to investigate a high potential trip on one of the motors tested with the Amprobe high voltage tester while testing the other machines.

What I did find, in general, is that the technologies are complimenting each other significantly versus my experiences with other mixes. A combined ATP5, with all options, and a 5kV Amprobe high voltage insulation resistance tester runs under $20k USD where both the Electrom fully loaded and AWA fully loaded run about $40k USD each.

In the attached video of the ATP5 and Electrom instrument, we were performing other work so did not point the iPhone at the tester the full time, so I put my watch in front of the camera from the start (after setup) until completion of all tests. This includes some really cool testing capabilities that the manufacturer requested I not disclose until we have an opportunity to play with it some more. I will state that the beta of the special testing performed exceedingly well.


*Note: MotorDoc LLC is not a distributor of Electrom Instruments, just a user.

Classic Rotor Bar Fault with ESA

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There have been a number of cases that have been brought to our attention related to rotor bar problems that turn out to be something else. It is extremely important that you do not rely just on what the ‘expert’ software associated with your technology states and confirm that the sidebands around line frequency are related to the pole pass frequency. When you have machines that have belts or gears attached, as well as compressor, air and pump systems that have a little cavitation or turbulence, the signature will show up around the line frequency.

Pole pass frequency is calculated as twice the slip frequency. There are several ways of calculating this value with the common being the number of poles times the slip frequency. So, for instance, in the example figure with this post, the operating speed is 1791.1 RPM, or 29.852 Hz, in a 4-pole motor, of which the synchronous speed is 30 Hz.   The difference is 0.148 Hz in a 4-pole motor which results in 0.592 Hz as the slip frequency. The current ripple shown in the top of the figure is at this frequency.

rotor barsFigure: Click to Enlarge


You will also note the harmonic of this frequency around the line frequency. This occurs when there is at least one good rotor bar between groups of bad rotor bars and will usually show as a harmonic for each group of broken bars. In this case there were two large groups of broken rotor bars.

The values of the sidebands were down a bit in this image because the motor is a 4160 Vac machine and data was taken through CTs. This results in dampening of the signatures and sometimes the frequency. This means that the alarms should be set a little lower for these types of machines. When possible, direct current measurements should be taken from cable or leads to confirm frequency and amplitude.

What Happens in Extreme Motor Overvoltage

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During final testing of an 800 horsepower, 900 RPM, 4160 Volt motor, it was determined that we should perform an electrical operating stress test.  The motor connections allowed us to connect the motor for 2600 Volts and apply 4160 Volts.  Full electrical and mechanical inspection was performed on the motor and all components were found in good condition.  Rotor bar testing using a core loss tester was performed, including infrared testing, as discussed in the http://www.motordoc.org/2015/12/21/repair-shop-rotor-testing/ post.

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Figure 1: Room Temperature Inrush

The motor was operated at high voltage for about 5 minutes with data collected using an ALL-TEST PRO OL Electrical Signature Analyzer (ESA) and evaluated.

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Figure 2: ESA During Overvoltage

The nameplate current on the motor was 103 Amps.  Under over Voltage conditions the current was 122.2 Amps no load.  There were multiple sidebands of pole pass frequency that would relate to rotor bars and the current also rippled in such a way as would be expected if there were rotor bar related conditions.  In this case, it is believed that the core and rotor bars were saturated which caused the motor windings and rotor to exceed full load temperature within the five minutes.

The motor was allowed to cool until the rotor and stator temperatures approached operating temperatures and the insulation resistance and polarization index were satisfactory.

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Figure 3: Inrush and Time at Operating Temperature

The inrush and starting time was reduced.

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Figure 4: Operating Temperature No Load Current

The operating current when connected and operating at 4160 Vac was 24.5 Amps.

The operating speed in both conditions was 889.9 RPM.