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
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.
The problem: We recently rebuilt a 2-pole motor and the centrifugal blower it drives. When the customer reinstalled them, he reported high vibration levels. Everything runs smoothly for 10-15 minutes after a cold startup. Then the vibration starts to climb. We balanced the rotor and blower to G 1.0 tolerances. We even balanced each of the 7 blower impellers separately using a balancing mandrel. Shaft runout was less than 0.0002" on the motor and blower when we finished the job. The customer uses laser alignment. He is convinced that something is loose and wants us to rebuild the blower again. What did we do wrong? The solution: First, you probably did nothing wrong. The precision balance was a smart move, because the relatively small diameter shaft on this type of blower tends to be flexible. With several impellers stacked on a common shaft, multiple planes of imbalance exert radial force on the shaft in several directions at once.With the impellers shouldered against each other, it is possible to deflect the shaft when tightening the clamping nuts. "Stacked tolerances" (when several mating surfaces have slight imperfections) can add up to unacceptable total deviations. Your final shaft runout indicator readings make it unlikely that this is the case. The cause of the vibration is probably misalignment.
Shaft couplings are devices that connect two rotating shafts together. They efficiently transfer motion and power from the drive unit to the driven unit without adversely impacting either piece of rotating equipment. Under ideal conditions, both shafts should function as a continuous unit. The design of a flexible coupling is to accommodate small amounts of shaft misalignment. Coupling manufacturers have designed their couplings to withstand the forces resulting from excessive shaft misalignment. Unfortunately, shaft alignment tolerances have sometimes been governed by the coupling manufacturers' design specifications.These are maximum values that are dimensionally possible for a specific coupling. The coupling misalignment tolerances reported by coupling manufacturers apply ONLY to the coupling.
When vibration problems occur, the magnitude and direction of the vibration can give a good indication of where to look for the cause. When vibration is higher in the vertical plane, one of the first things we should examine is the base/foundation of the motor. If the high vertical readings are compounded by indications of an eccentric airgap, such as high axial vibration and a predominant twice-line-frequency vibration, a "soft foot" or twisted frame is often to blame.
End play in an electric motor is the amount of axial movement allowed by the motor's construction. This end play is limited by the motor's bearing design. The bearing's primary purpose is to locate the shaft radially so it can be aligned to the driven equipment shaft and efficiently transmit torque to the load. It is also important that the axial location be controlled such that the motor and driven equipment bearings are not subjected to excessive thrust or vibration and still have room for thermal growth of the shaft as it heats up during operation. This can be accomplished by a number of ways depending on the design of the motor. If the motor has sleeve bearings, axial movement is expected within the limits of the bearing design. Most rolling element bearings have much less axial clearance but must be contained in the bearing housing to control the end play. This article looks at:
- Sleeve bearings
- Axial hunting
- Ball bearings