Motor Rewind Basics
Electric motors are a necessity but, unfortunately, they wear out over time. Electric motors can often be given a second chance at life through an electric motor rewind. With a rewind, an electric motor is refurbished to like new condition and can be put back into service. So, what are the basic steps of an electric motor rewind?
Can any Motor be Rewound?
While many motors are great candidates for a rewind, what caused the failure in the first place will determine whether the motor will get a second chance at life. For an electric motor, the stator core is one of the most important elements.
Consisting of insulated and stacked iron laminations and windings of copper wire, the stator core insulation must be in good condition in order for the motor to be repaired. If the stator core is physically damaged, say from extreme heat or a bearing failure, a rewind will not be sufficient to restore the motor to serviceable condition. Common motor failures that are solved by a rewind include:
- Faulty motor
- Controls or outside electrical issues
Before beginning an electric motor rewind, it’s important to assess the extent of the damage. EASA certified repair shops follow strict inspection protocols in order to maintain the motor’s efficiency and successfully return the motor to serviceable condition.
It is important to collect all relevant information from the motor’s nameplate. This should be the first step in a motor’s preliminary inspection. The technician should then do a thorough inspection and failure analysis to assess the condition of the motor. Any signs of wear, discoloration, or missing parts should be noted. The final step of the information gathering process is talking with the customer. Any information the customer can provide about the application or runtime of the motor will be useful information in diagnosing the cause of the motor’s failure.
Complete lamination removal should be avoided but lamination material can be restacked if missing in small sections. Minor missing core material should not exceed 2% of the length of the core or not affect more than 10% of the number of teeth
Old Winding Removal
Before the new windings can be installed, the old ones have to go. Depending on the size of the motor, this can happen in a couple of different ways. For smaller motors, old windings can often be removed mechanically by the hands of a skilled technician.
For larger motors, removing the old windings can take a bit more finesse. To remove the windings from the stator, the stator is typically placed into a large oven where the components are heated to around 700°F. After around 10 hours at this temperature, the insulation on the windings will start to turn to ash.
Cleaning the Stator Core
Slot insulation and debris must be cleaned from the stator core to prepare the stator for new windings. EASA/AEMT recommended methods for stator cleaning include:
- Careful scraping
- High-pressure washing
- Abrasive blasting with mild abrasive
- Wire brushing
Regardless of the technique chosen for cleaning, a gentle and patient approach is key to a thorough cleaning without causing stator laminations to short out.
Getting the motor’s “vital signs” is key to correctly diagnosing the problem before rewinding. Watts loss and temperature rise of the core should both be within specified parameters before a repair is performed. Excessive overheating or mechanical damage to the core typically prevent a motor from being repaired.
Make the New Windings
Either by hand or with the help of a coil winding machine, the new windings for the motor must be prepared. Key parameters for the winding include:
- Wire size
- Number of turns on the coil
- Winding connections
Keeping these parameters consistent between windings is critical to a proper working motor. Technicians will either duplicate the original motor winding pattern or move to a new pattern that is equivalent or better than the original winding. Minimizing I2R losses is critical during the rewind process.
Installing the New Windings
After making the new windings, they must be carefully inserted into the stator core. Coils and leads must be connected to match the original connections. Once the windings are physically installed and electrically connected, testing must be done to ensure a proper repair. The tests most commonly performed are:
- Winding resistance tests
- Surge test
- Hipot test
- Polarization index test
- Megger test
VPI and Bake
Once wound and tested, the windings must undergo a vacuum pressure impregnation (VPI) process. Following the VPI process the windings are then baked at about 300°F for a minimum of four hours in a process that is often referred to as a “VPI and bake”.
With the motor repaired and tested, the final steps are reassembling the motor and restoring the exterior to good condition. Bearing installation lubrication and fan placement must all be done properly to ensure the rewound motor will have a long operating life.
The rotor or stator air gap surface must also be protected from rough handling. Any damage to this surface during reassembly will result in reduced motor efficiency. The final step of reassembly is painting. Ventilation openings must be protected during painting to ensure the motor will be able to operate once again as designed.
Trust the Pros when it Comes to Motor Rewinds
While many businesses offer electric motor rewind services, only EASA Accredited AR-100 repair facilities are certified to follow EASA best practices during the motor rewind process. For the peace of mind that comes with having an electric motor rewound professionally by certified experts, trust the pros.
B+M Industrial is one of less than 160 companies across the world with this accreditation and their decades of experience ensure your electric motor will be rewound correctly. To learn more about how B+M Industrial can help you with your electric motor rewind needs, contact our team of experts today.