Tim Browne
Industrial Electric Motor Service, Inc.
I suspect that just about everyone in our industry at one time or another has had the joy of repairing a “purpose-built” motor. This kind of motor is built for a specific purpose and has characteristics that may allow it to operate under non-standard conditions. Due to the limited information that some of them display on the nameplate, the repair of these motors can be somewhat of a challenge.
Sometimes these motors possess differences such as the color of paint, the shaft size, the bearing size, or type. It can be the operating temperature and at times it can be the motor in its entirety. Following are a few useful tips we use when repairing a motor with so many question marks.
Tip One
Follow EASA’s Recommended Practice for the Repair of Rotating Electrical Apparatus (ANSI/EASA AR100) to the best of your ability. This document is full of useful information and will guide you in many ways. When you’re repairing a purpose-built motor, you need to be aware and observant of the motor’s differences when compared to standard electric motor designs.
Tip Two
Know your limits and the risks you’re willing to take. These should include the limits and capabilities of your equipment, your physical and financial resources, and your staff. If you’re unsure, why take the risk?
Tip Three
Contact the purpose-built motor manufacturer for any information they might be willing to share. Some are willing to share information to help out the customer. Some will want to protect their design (who can blame them) and request that you send them the motor to be repaired. This may be a good option if there is a time constraint for the customer and the risk of repair is too great. Repairing a purpose-built motor back to general industry specs may end up costing you in the end.
Tip Four
Research the original equipment to get a better understanding of the application. If you have a good idea on the motor’s operating conditions and potential, it is easier to identify signs of what is unique about the motor before it is taken apart.
Tips for disassembling a motor
Prior to taking the motor apart, document the motor’s physical attributes by taking several photos of the motor when assembled. If a motor has nonstandard or modified features, it is easy to overlook things.
The next step after this documentation is to locate the nameplate and retrieve any data that this may have regarding the potential manufacturer of the purpose-built motor. The Internet is a very useful resource in locating motor information because there is a good chance that other people have attempted to find this motor as well. The Internet can also help in identifying and contacting purpose-built motor manufacturers overseas. At times, just having the parts breakdown can be helpful with disassembly and assembly issues.
Next, observe the motor for signs of external body corrosion. Then, identify if there is deterioration in places such as the cooling system or output shaft. Also, assess for overheating of electrical leads, shaft and key way attrition. Finally, assess the functionality of the junction box, the mounting feet and the mounting face of the motor.
Signs of wear
Once you have the motor disassembled, look for signs of internal wear. Signs of wear should be identified in places such as the bearing housings, shaft fits, electrical system and all internal mechanical fits. Instead of using standard bearing housing and shaft measurements, look for signs of wear and fretting first. If a purpose-built motor has special bearing fits and you do not know what they are, you can be jeopardizing it by having machine work done. If an electric motor is designed to run at a higher temperature, it is the differential temperature between shaft and housing that may require a different bearing internal clearance. Also, don’t assume the bearings are standard. They might be made of a different material or may be a different design. For example, just like a shaker motor, a purpose-built machine might require bearings with C4 internal clearance as opposed to the more common C3 internal clearance. In fact, several manufacturers use C4 internal clearance bearings for close-coupled compressor motors. (That is a common factor in bearing failure of these motors.)
When it comes to rewinding a purpose-built motor, look for signs of difference in material, winding design and winding layout. If the design information is not on the nameplate, we have to identify it on our own. Compare the insulation to what you use and what your supplier offers. If the motor has a permanent magnet (PM) rotor, don’t look to redesign the winding, as you might change the operating characteristics. And, if powered by a variable frequency drive (VFD), the drive may not operate the motor. Try to put the winding back exactly the way you took it out. This will help prevent any unexpected problems in the repair process.
Purpose-built motors were designed for a purpose by someone who has direct knowledge of the equipment and job it is required to perform. It is our job to repair electric motors — not redesign them. So don’t try to reengineer their design. Look for the signs of degradation and fix those parts to the best of your abilities.
I’m sure there are many more tips that others use. These are ones we’ve used. I hope they’re beneficial to you.
Learn from our experience
Our service center repaired a 75 hp (55 kW) motor on a purpose-built compressor motor a few years ago. It had lost a bearing and burned up the motor. We rewound it back with what we took out of it. We went back with standard machine specs on the bearing shaft and housing. We test ran the motor and saw no signs of problems. The motor was reinstalled, but only lasted a few hours.
After calming down the customer, we went through the motor to find out what went wrong. When we took it apart, we found the bearings had been destroyed by what looked like preload. After a series of questions, we found that the motor specs had called for a special surface finish of 250 microinches on the bearing housings to allow for thermal expansion. We assumed the housings had a standard finish of 16 microinches and by doing so we destroyed the motor. The customer had to replace the motor and we ended up with a very expensive lesson on how to handle this kind of repair. Most mistakes in life (and a service center) have already been made, so learn from others and save yourself time and frustration.
AVAILABLE IN SPANISH
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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