Service centers routinely check the shaft extension runout of motors and generators. When there are issues associated with them, or when applicable, the coplanarity of the mounting feet and the amount of end foat of horizontal sleeve bearing motors and generators are checked. A common point about all three of these dimen-sions is that they are checked with the machine assembled; that is, no disas-sembly is required. There are many other mechanical tolerances associated with motors and generators, such as bearing fits. However, the focus of this article will be the three tolerances just mentioned. Rather than referring to both electric motors and generators, for brevity the term “machine” will be used.
This book covers low- and medium-voltage horizontal and vertical squirrel-cage induction motors in the 300 to 5000 horsepower range. Many of the principles discussed also apply to motors of all sizes and technologies. Although the focus is on motors built to standards of the National Electrical Manufacturers Association (NEMA), much of the material applies to motors designed according to standards of the International Electrotechnical Commission (IEC).
This book is designed primarily as a training manual for EASA’s seminar Principles of Large AC Motors. Since it contains reference materials, graphics and visual aids for both the instructor and the student, it also makes an excellent “on the job” shop reference and training guide.
As shown below, the manual is organized in 24 sections or modules, complete with a table of contents to guide users to appropriate sections. The material is configured for use with students having various levels of knowledge and experience, from technicians to engineers. The entire course can be presented in one or two days, depending on the time available. Individual sections can also be presented as stand-alone training modules.
Table of Contents
- Basic Motor Theory
- Typical Large Motor Applications
- Large Motor Enclosures
- Motor Manufacturers
- Large Motor Standards
- Safety Considerations
- Root Cause Failure Analysis
- Test and Inspection Procedures
- Winding Connections and Starting Methods
- Motor Accessories
- Motor Nomenclature
- Stator Construction
- Rotor Construction
- Bearing Systems for Horizontal Motors
- Bearing Systems for Vertical Motoros
- Lubrication Systems
- Shaft Construction
- Motor Geometry and Alignment
- Vibration and Noise
- Outlet Box Construction
- Cleaning and Reconditioning
- Storage Procedures
- Repair Tips
Fundamental to every good mechanical repair is the ability to disassemble, repair and reassemble the motor correctly without unnecessary damage to any of the motor parts. This sounds simple, and yet too many costly mistakes are made in this process of taking things apart. If every motor repaired was in “as new” condition, the task would be much simpler. But this would be no guarantee that the reassembly would be correct.
There is usually an easy way and a hard way to remove and install parts. Brute force is seldom the easiest or the correct way. The old saying of “don’t force it, get a bigger hammer” is seldom the best way.
When a service center is paid to repair equipment, the service center wants it to stay in operation. If the equipment fails again—within the warranty period—the service center pays to repair it again. It makes sense to repair it correctly the first time.
In order to improve equipment, it is important to know how and where it operates. Without understanding why a motor fails, it is impossible to deliberately improve its mean time between failures.
To do this, there must be communication between the service center and the motor user. Not only does this help the repairer decide the best course of action, but it helps the user appreciate the professionalism of the service center.
Repair procedures, like motors themselves, are affected by changes in technology. This book attempts to include the latest proven technologies. Time-honored methods of repair, in many cases, may still be the most practical option. Options presented throughout this book are intended to help the technician select the appropriate repair method, recognizing that the ultimate decision rests with the equipment owner.
Repair methods sometimes fall into disfavor, not because better methods are introduced, but because of poor techniques. Other repair methods are well-suited to some applications but not to others. It is the job of the repairer to decide what is the best method for each case.
This book is divided into sections for basic motor components with repair methods and tips dispersed throughout. Where practical, reasons for failures are also discussed. These will aid the technician in selecting the most appropriate method of repair for each unique application.
The information presented draws from EASA publications, IEEE publications, technical journals and literature supplied by vendors, motor manufacturers and established service centers.
This book contains many suggestions on how to correctly handle the various parts of a motor during the repair process so as to minimize damage. However, it is impossible to develop an all-inclusive list. Instead, the basic principle of taking the time to use the correct tool and correct procedure will usually lead the technician down the right path. Always remember, if it has to be forced beyond reason, it might be that neither the proper tool or procedure is being used or something is wrong with the parts. Step back and ask “What am I overlooking?”
Table of Contents
- Motor Nomenclature
- Motor Applications and Enclosures
- Test and Inspection Procedures
- Motor Disassembly Tips
- Bearing Housing Repair, Shaft Openings, Seals and Fits
- Motor Assembly
- Motor Accessories and Terminal Boxes
- Motor Dynamics
- Vibration and Motor Geometry
- Shaft/Bearing Currents
- Special Considerations for Explosion-Proof Motors
- Failures in Mechanical Components
- Miscellaneous Repairs
This book is available as part of EASA's Fundamentals of Pump Repair seminar.
This book was developed to help electric motor technicians and engineers prevent repeated failures because the root cause of failure was never determined. There are numerous reasons for not pursuing the actual cause of failure including:
- A lack of time.
- Failure to understand the total cost.
- A lack of experience.
- A lack of useful facts needed to determine the root cause.
The purpose of this book is to address the lack of experience in identifying the root cause of motor failures. By using a proven methodology combined with extensive lists of known causes of failures, one can identify the actual cause of failure without being an “industry expert.” In fact, when properly used, this material, will polish one’s diagnostic skills that would qualify one as an industry expert.
The book is divided into the various components of an electric motor. In addition to a brief explanation of the function of each component and the stresses that act upon them, numerous examples of the most common causes of failure are also presented.
Since it is not always possible to pinpoint the exact cause of failure, some examples are used more than once. Due to a lack of all the necessary facts associated with the application and history of a given machine, it is only possible to assign the root cause to the most probable scenario.
A reference section is included at the back of this book for those wanting to further research root cause failure analysis.
The book is available only in black & white. Photographs in the CD-ROM version are in color, where available.
Table of Contents - (Download the complete Table of Contents)
- Root Cause Methodology
- Bearing Failures
- Stator Failures
- Shaft Failures
- Rotor Failures
- Mechanical Failures
- DC Motor Failures
- Accessory Failures
- Case Studies
- Reference Materials
This book and it's companion CD-ROM is available as part of EASA's Root Cause Falure Analysis seminar or it may be purchased in EASA online store.
This presentation covers:
- How to recognize the symptoms and determine the presence of damaging currents
- Causes of damaging currents (e.g., machine dissymetry and VFDs)
- Testing to confirm and assess the magnitude of shaft and bearing currents
- Solutions to eliminate or control shaft and bearing currents (e.g., insulators, isolators and ceramic bearings)
"We rebuilt a 75 hp electric motor recently. It ran fine in the service center, but the customer reported high bearing temperatures shortly after installing the motor. The bearings failed after only a few hours at full load." The first response for most of us is to suspect an alignment problem. But there is another possibility that should be considered. An electric motor must have room for thermal expansion of the shaft, or bearing life will be severely reduced. The endplay of a ball bearing motor plays an important role in bearing life.
Have you ever repaired a sleeve bearing motor, only to have the customer complain that it leaks oil? Perhaps the motor had a history of oil leaks, and the windings were oil-saturated when you dismantled it. Two-pole machines are especially notorious as chronic oil leakers. The first step toward correcting an oil leak is to identify the cause. A good place to start is to determine whether the motor has a forced-oil system. If so, check for a metering plate in the oil supply line. The typical metering plate (see figure) has about a 3/32 diameter orifice to meter the volume of oil. Often installed in a pipe union, the metering plate is easily lost when the motor is removed from service. The repairer rarely gets the forced-oil system with the motor. The customer does not recognize that little piece of metal that looks like a conduit knockout, and that tiny hole cant possibly be for oil flow. So it gets thrown away .