Large Motor Maintenance: Basics For Machine Reliability
Kathy | September 1, 2008
If your operations rely on these workhorse machines, you may want to brush up on their care and feeding.
The true workhorses of industry, electric motors, provide the means to convert electrical energy into a meaningful and measurable output. Because they are so prevalent and critical to industry, the ability to accurately diagnose, predict and efficiently deal with motor problems is essential to maintenance, engineering and operations personnel.
One of the bigger challenges is being able to recognize, diagnose and remedy an evolving motor problem—to the point that you can prevent an unexpected catastrophic event. Understanding the basic visual, mechanical and electrical maintenance techniques will help you in this quest to keep large electric motors on line and producing.
The U.S. Department of Energy estimates that approximately 63% of all electricity used in industry is for process motor system energy.
(Refer to Fig. 1 for a breakdown of the percent averages by equipment type.) Industrial motor use accounts for approximately 25% of the total electricity usage in the United States. Thus, it makes sense that one would take all the necessary steps and precautions to assure long-term health and reliability of these vital machines, especially when they are of a larger horsepower.
Using guidance found within InterNational Electrical Testing Association (NETA) field-testing specifications, the ANSI/NETA MTS-2007 Standard for Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems, this article will discuss some of the available maintenance trends and techniques that can assist owners and users of large electric motors. The focus will be primarily on medium-voltage AC induction (2.3 kV – 13.2 kV) machines.
First and foremost: understand the hazards!
Working on or near electrical equipment is by its very nature a hazardous task. Before any equipment is inspected or any maintenance is done, the person performing the tasks must be qualified and able to assess all hazards associated with the scope of work to be performed. If the person performing the work is not qualified, the end results could be significant equipment damage or possibly serious injury or death to personnel.
Some maintenance tasks require the work to be performed on energized equipment while it is in normal operation. Personnel should be familiar with—and comply with—the applicable OSHA, NFPA 70E, plant-specific and other electrical safety rules and regulations.
Whenever performing de-energized motor maintenance—and before physically touching the motor—one should be sure that the unit in question does not present a shock hazard to anyone that will be working on that particular motor. At a minimum, the affected parties should be notified, the machine should be locked out/tagged out, tried out, checked for the absence of voltage (live-dead-live), and the proper personnel protective grounds applied. Again, the key is to have a qualified worker perform these tasks to assure those in the area that an electrically safe condition has been obtained.
Electric motor basics
Basic components and their elements of failure…
The Electrical Apparatus Service Association (EASA) has several publications that can help users understand electric motor construction, repair and operation; for more information, visit www.easa.com. EASA’s Principles of Large AC Motors, Version 1.0, details nine key stresses that lead to failures across four key components of an electric motor. Understanding these elements of stress and the components they apply to is a key factor in being able to properly apply and prioritize the necessary maintenance decisions for large motors. The chart in Fig. 2, on the next page, illustrates the relationship between types of stresses and the four basic components of an electric motor.
An Electric Power Research Institute (EPRI) study of electric motor failures indicated that 53% of electric motor failures are related to mechanical components and 47% to electrical faults (not including new and repaired equipment defects). See Fig. 3 for an example of a mechanical failure of a 1000 hp machine applied to a fan at an industrial facility. Mechanical defects have traditionally been detected using vibration analysis and infrared thermography, while electrical defects have been detected with resistance tests, insulation tests, high-potential tests, surge comparison tests and partial discharge testing. The failure in Fig. 3 was attributed to excessive vibration and heat from a defective fan shaft bearing.
Four basic electric motor components…
While materials and insulation systems have changed, the basic principles and operation of an electric motor have not changed very much over the last 100+ years. An electric motor is made up of four basic components:
- Stator winding
- Rotor assembly
these components are exposed to stress conditions, failure of the motor can occur. The nine key elements of stress that can lead to motor failure are:
To counteract these stress elements, one of the key parameters to an effective motor maintenance program is to establish test and inspection procedures that allow the owner to trend data over time. It is this trended data that helps in diagnosing the overall health of the machine. Let’s look at the visual and mechanical tests/inspections and electrical tests that can be performed on large machines.
Visual and mechanical inspections
An important aspect of large machine maintenance is the visual and mechanical inspection.
- Inspect the machine’s physical and mechanical condition.
- Check for signs of oil or water leakage.
- Verify that air inlets are not plugged.
- Check for abnormal sounds or smells.
- Check the water and oil supply piping.
- Check the drain piping.
- Look at the condition of the foundation, grout, bed plates, anchor bolts, shaft extensions, couplings and guards.
- Check the surroundings for any environmental issues that may affect performance or service life.
- Inspect anchorage, alignment and grounding of the motor, driven equipment and base.
- Inspect air baffles, filter media, cooling fans, slip rings, brushes and brush rigging.
- Inspect bolted electrical connections for high resistance.
- While the unit is under full load, perform a thermographic survey.
- Perform special tests such as air-gap spacing and machine alignment, if applicable.
- Verify the application of appropriate lubrication and lubrication systems.
- Verify the bearing oil level.
- Check for improper lubrication, oil of the wrong type, viscosity that is too heavy or too light.
- Verify that there is sufficient oil in bearing bracket to cover bottom of rings.
- Look for dirty oil or old oil (should be replaced and/or tested).
- Verify that the oil rings are turning (especially at low temperatures).
- Check for water or other contamination within the lubrication system.
- Verify that the feed oil is connected to the correct ports When the bearing and seals are inspected the following should be considered:
- Check for excessive bearing clearance.
- Verify seal clearance and condition.
- Make sure there is not improper seating of shaft journal in bearing or a bent shaft.
- Verify the absence of unusual mechanical or electrical noise or signs of overheating.
- Check for pitting of bearing and journal surfaces due to bearing currents.
- Verify integrity of bearing insulation.
- Make sure there are no rough bearing surfaces due to corrosion or careless handling.
- Verify that there is not excessive end thrust from the mechanical load.
- Check for poor alignment.
- Make sure that the bearing Babbitt has not been fractured or damaged due to impact or shock loading of the bearing journal.
- Verify that resistance temperature detector (RTD) circuits conform to drawings and are functioning properly.
Electrical tests for AC induction motors
As mentioned previously, the collection of valid test data and the trending of that data are vital if overall machinery health is to be determined. Electrical tests performed on large motors can yield significant information as to the overall health of the machine.
Some of the more common electrical tests and procedures include:
- Resistance measurements taken through bolted connections with a low-resistance ohmmeter;
- Insulation-resistance tests in accordance with ANSI/IEEE Standard 43;
- DC overpotential tests on machines rated at 2300 volts and greater in accordance with ANSI/IEEE Standard 95;
- Phase-to-phase stator resistance tests on machines 2300 volts and greater;
- Insulation power-factor or dissipation-factor tests;
- Power-factor tip-up tests;
- Surge comparison tests;
- Insulation-resistance tests on insulated bearings;
- Testing and inspection of surge protection devices;
- Testing and inspection of motor starters;
- Resistance tests on resistance temperature detector (RTD) circuits;
- Verification of machine space heater operation, if applicable;
- Vibration testing of motor after it has started running.
Following a prescribed set of visual, mechanical and electrical tests such as the ANSI/NETA MTS-2007 standard can help a company keep its large rotating assets producing and reliable. The key is to perform the maintenance tests and inspections with qualified personnel who understand the safety considerations, as well as the collected data that will be used for trending in upcoming maintenance cycles. MT
Ron Widup is the executive vice president and general manager of Shermco Industries, Inc., in Dallas, TX. Shermco provides testing, repair, professional training, maintenance and analysis of rotating apparatus and electrical power distribution systems and related equipment for the light, medium and heavy industrial base nationwide. Widup is a NETA Certified Level IV Senior Test Technician, State of Texas Journeyman Electrician, a member of the IEEE Standards Association and an Inspector Member of the International Association of Electrical Inspectors. E-mail: rwidup@ shermco.com