It’s been a while since we featured tips from PdMA. PdMA is an excellent source for your predictive maintenance questions and needs.
L&S Electric has partnered up with PdMA for some seminars in the past and hopefully, more to come in the future.
More Training, Better Results
From the PdMA Intro to MCEMAX Training Manual we find that during a Rotor Influence Check (RIC) test if the rotor’s residual magnetic field produces a RIC test graph that is sinusoidal in shape, the rotor is referred to as a Rotor With Influence. If the sinusoidal shape is smooth with a consistent amplitude between phases it is given a normal status. If the inductance values have little change as the rotor is positioned it produces a flat graph and hence is referred to as a Low Influence Rotor (LIR). A Low Influence Rotor on a baseline test does not indicate a bad rotor. Instead it commonly indicates a higher quality rotor possibly designed with lower retentive steel, copper bars, or no rotor defects. With or without influence, base-lining a RIC test as soon as possible and being educated on the effects of rotor reflected impedance is critical.
Having a well-trained staff will improve test data quality, ensure proper analysis, and increase the benefit of the motor testing program overall. In the end, more training equals better results.
Destructive Versus Non-destructive Testing
A Potentially Destructive Test has the “Potential” to destroy (although not purposefully destroy) the Device Under Test (DUT), which if there is a failure, the specimen may be rendered useless for its intended purpose. Dielectric Insulation Tests that fall into this category include maintenance, acceptance, and proof tests (whether performed in the field or in the shop) such as Surge, Hipot, and Step Voltage. These tests are all performed above the rated voltage of the insulation system, thus, during the test, stress the insulation system and have the potential to render the DUT useless for its intended purpose.
Non-destructive testing evaluates the properties of the DUT without causing or
having the potential to cause damage. Tests that fall into this category include Insulation Resistance-to-Ground (RTG) commonly called a Megger® test, Polarization Index, Insulation Resistance Profile, and Capacitance-to-Ground (CTG). These tests are intended to be performed within the rated specifications of the DUT, thus, during the test, do not stress the insulation system beyond its rated capability. Should there be a failure during a non-destructive test the insulation system was not in sufficient condition to perform its intended function prior to the test.
Troubleshoot a Tripping Motor
Analyzing peak transient in rush current is a standard approach to troubleshooting a tripping motor. An instantaneous trip would indicate a stator or trip set point anomaly. An overload/time delay trip would indicate more of a mechanical load/rotor driven anomaly. The In-Rush/Start-Up test from PdMA records an RMS enveloped value of current throughout the start-up of the electric motor, which is an ideal tool for trending condition variables such as mechanical or electrical anomalies. However, following an instantaneous trip, quantifying the peak current seen by the starter during an in rush transient would require the current signal in a time domain format.
What’s the Difference?
Preventative, predictive, proactive, so what’s the difference? Your facility practices may fall into one, none, or more than one of these categories, but do you have the best approach for your application? Preventative maintenance is about allowing for time-based maintenance to prevent equipment failures. This includes statistical failure data, routine inspection and condition monitoring, basic filter and lube care, and calibration.
Predictive maintenance is about knowing the machinery condition at all times. A good predictive maintenance program uses the application of vibration, oil, infrared, ultrasonic, motor current, and trending practices to ensure maximum reliability and up-time Comprehensive communication between maintenance and technologies, along with training, planning, and eliminating root causes are the major areas of focus in applying a predictive maintenance aspect of reliability.
Proactive maintenance is considered the ultimate step in reliability. A proactive maintenance program seeks the root cause of issues, has active communication with departments, and employs various methods and tools, which may include those used for preventative or predictive maintenance, for extending equipment life and eliminating the cause of failure. Proactive maintenance is more a mindset than a methodology. It is more about eliminating defects that result in the failures that require maintenance.
Galvanic Corrosion and High-Resistance Connections
Have you ever had a light bulb corrode within the socket, making it difficult to replace the light bulb or worse the glass breaking free of the threads? Galvanic corrosion may be the cause. Connecting dissimilar metal (brass light bulb threads into aluminum socket) without an approved antioxidant solution will result in a galvanic corrosion and eventually a deteriorating high resistance connection. Galvanic corrosion of dissimilar metal lugs is an electro-chemical process resulting in the transfer of metal from one lug to the other. This corrosion is worsened in a moist environment. The most common scenario is connecting aluminum lugs to copper or brass. If a high resistance connection is seen through resistive imbalance at the starter, but disappears when testing the motor only it may be a result of galvanic corrosion at the lugs. The act of disconnecting and reconnecting the
lugs may clear the problem, but it will only be temporary as the metal transfer will start again as soon as they are reconnected.
What kind of lug are you putting on your cables?
You are invited to submit an Electric Motor Testing Tip of your own and receive a free PdMA mug or hat if they publish it! Contact Lou at 813-621-6463 ext. 126 or firstname.lastname@example.org.