The stator windings of a 3-phase AC motor will need to be rewound several times during its entire service life, and the timing to do so varies according to the application and operating environment it is in. Unexpected rotor failure can also stem from mechanical failure, requiring motor repair to get it to operate again.
Once the stator windings inevitably fail, restoring the stator to its original performance or better requires addressing many items in the motor. The manufacturer may design the torque, horsepower, and kVA code to its limits when developing the coils and how much ground wall protection they can absorb with a limited amount of slot width. Other manufacturers often put enough ground wall insulation to cover the motor’s specific temperature range while having ample slot width to do so.
1. Check the winding data
It is vital to first determine the motor’s winding data before starting the rewinding process. Achieving this first step starts with heating the motor’s VPI stator windings in a burnout oven at high temperatures. This helps burn away the varnish still on the coils and soften them enough for easy extracting.
However, since varnish flare-ups could potentially damage the iron core, the oven must come equipped with water sprayers to reduce excess fire that could potentially arise during the process. Supplemental data can then be gathered after completing the burnout process, with the turns per coil, wire size, groups, and type of connection being especially important.
Data on the iron core slots should also be included, with a pre-core test prior to the burn cycle as the means to achieve it. The data gathered helps determine if the iron core succumbed to the burning process. After stripping the coils from the iron core, the latter is washed clean and put into a bake cycle between 120°C-140°C for several hours to prevent further flash rust and remove any remaining residual water. The iron core will then undergo another core test post-burn to confirm that it is still usable and undamaged from the burn cycle.
After verifying the coil data, a full set of coils is created and will come with the insulation not impregnated with varnish. At this stage, the stator will undergo sandblasting (ideally with walnut shell media instead of abrasive alternatives like silica sand) to prepare it for the winding process.
2. Prepare the stator for the insertion of new coils
The next step starts with investigating the stator slots for sharp edges and filing them as needed before inserting the coils. This is followed by applying insulation via a glastic filler to the bottom of the slots to ground the bottom side of the coil. An additional filler is added after the coils reach the span between the bottom and top sides of the coils.
Some motors may need magnetic wedges, while others can do without them. Manufacturers typically place wedges in their motor designs for several reasons. In the case of form-wound induction stators, using these wedges is key to sealing the open-slot design and reducing magnetising current, air-gap flux distortion, temperature rise, and starting/in-rush current. The benefits of having magnetic wedges include improvements in power factor, an increase in the motor’s efficiency, and cost savings by reducing the active material or frame size.
That said, magnetic wedges are known to come loose and cause issues like coil vibration or movement that may often lead to a ground wall short circuit. Another potential issue that could arise is if the coil in the slot vibrates, compromising its insulation. This means it is not always recommended to put magnetic wedges instead of the usual glastic-type wedges.
3. Insert the coils
During the insertion process, the non-varnished coils must be properly handled to prevent any damage to the coil insulation. Preparing the stator slots first helps ensure this.
Upon inserting the coils, the next step will be to define the connection for the groups. Placing the right amount of glass/mica insulation over the silver-soldered connections is paramount. All these connections are then secured using a resin-rich fibreglass tape that hardens in the curing process. Finally, the external leads are added to the winding, and the stator is put through the VPI or Vacuum Pressure Impregnation process, which is important for motor windings.
For this procedure, the stator is heated completely at 135°C within a bake oven, placed on a VPI vessel once done, and is followed by a vacuum pressure cycle. At the same time, the varnish is moved from the holding tanks and filled into the dipping vessel at the right level to fully cover the non-varnished coil windings.
After the VPI cycle is complete, the stator unit is then put back into the oven for curing, which can last up to 15 hours. Once cured, the stator is given time to cool, any excess cured varnish is discarded, and the newly rewound stator is all set for assembly.
Conclusion
Motor rewinding is an important process that prolongs the service life of an electric motor by replacing or repairing any of its damaged or worn-out components. For 3-phase electric AC motors, the steps mentioned in the article is the general procedure used by companies that offer services in mechanical and electrical engineering in Singapore to restore the equipment to improve its performance or at least restore it to its original state.
