Rewinding a submersible motor is a delicate and highly technical process that requires precision, methodical steps, and a clear understanding of how electric motors operate under water-immersed conditions. Unlike surface motors, submersible motors withstand high hydrostatic pressure, variable temperatures, and continuous operation. When the motor fails due to short circuits, overheating, or insulation breakdown, rewinding becomes a cost-effective method to restore its performance. This guide provides an in-depth explanation of the rewinding procedure, focusing on essential aspects such as stator winding, coil insulation, slot alignment, and the crucial final stage: testing and calibration.
1. Introduction to Submersible Motor Rewinding
A submersible motor is typically a water-filled or oil-filled induction motor designed to operate while completely submerged. Over time, the windings can burn due to voltage fluctuation, dry-run conditions, pump blockage, or insulation failure. Rewinding restores the motor to its original electrical and mechanical integrity, extending its lifespan and improving efficiency.
Rewinding is not simply about replacing old wires. It involves analyzing the original winding pattern, ensuring precise slot alignment, selecting the correct wire gauge, applying top-grade coil insulation, and reassembling the motor with careful attention to detail. Any mistake—no matter how small—can cause the motor to fail again or underperform.
2. Tools and Materials Needed
Before starting, gather all the necessary tools to ensure a smooth process:
- Micrometer or vernier caliper
- Coil winding machine
- Measuring gauge for wire diameter
- Insulation paper (slot liners)
- Varnish or epoxy resin
- Heating oven or drying chamber
- Multimeter or megohmmeter
- Puller and mechanical tools for disassembly
- Cutting tools for removing burnt windings
- Number tags for organizing coil groups
3. Disassembly of the Motor
The process begins with a careful and systematic disassembly. This involves:
3.1 Removing the Motor Housing
Submersible motors often have tightly sealed housings to prevent water entry. Remove the top and bottom covers using appropriate tools while ensuring you do not damage the seals.
3.2 Extracting the Stator
The stator is the stationary part of the motor that contains the windings. Use a puller to gently remove the stator from the housing. Avoid hammering, as this can deform the laminations.
3.3 Inspection
Inspect the stator and note:
- Burn marks
- Broken insulation
- Melted copper
- Slot damage
- Moisture or corrosion
These observations help in selecting the right rewinding method and insulation class.
4. Removing the Old Windings
Once the stator is removed, the next task is stripping the damaged windings.
4.1 Cutting and Heating
Cut one end of the winding carefully. Heat the stator in an oven to soften varnish and insulation, making wire removal easier.
4.2 Extracting the Coils
Pull out the burnt wires using pliers. Make sure you count the number of turns, coil groups, and phases before disposal. Document everything or take photos.
This information is vital for reconstructing the correct stator winding pattern.
5. Understanding the Original Stator Winding Pattern
Submersible motors usually employ a distributed winding pattern. Before proceeding, analyze:
- Number of slots
- Pole configuration
- Number of turns per coil
- Coil pitch
- Parallel branches
Accurate reconstruction is essential. Incorrect patterns can cause high current draw, overheating, or reduced torque.
6. Preparing for the New Windings
6.1 Cleaning the Stator
Remove all ash, varnish residue, and debris from the slots. A clean slot ensures proper coil insulation and heat dissipation.
6.2 Installing Slot Liners
Insert high-temperature, waterproof insulation paper into each slot. This step is critical for preventing short circuits between the stator core and winding wires.
7. The Rewinding Procedure
This is the core of the operation. Precision here determines the motor’s long-term performance.
7.1 Choosing the Correct Wire
The enamel copper wire must match the original diameter, insulation class, and temperature rating. Submersible motors typically use Class F or H insulation.
7.2 Winding the Coils
Use a coil winding machine to form coils with the exact number of turns recorded earlier. Consistency is crucial because unequal coil size can lead to vibration, losses, and phase imbalance.
7.3 Inserting the Coils into the Slots
Insert coils into the stator slots following the recorded pattern. Ensure perfect slot alignment, meaning:
- Coils lie neatly and fully inside their respective slots
- No wire bulges out
- Lines of flow follow the original direction
Proper alignment prevents wire-to-wire abrasion, which can lead to insulation failure.
7.4 Interconnecting the Coils
Once coils are placed:
- Connect phase groups properly
- Ensure correct coil pitch
- Maintain proper polarity
Use heat-shrink sleeves to cover interconnections for additional coil insulation.
8. Securing and Insulating the Windings
8.1 Wedging the Slots
Insert wooden or fiber wedges into the top of each slot to tightly secure the windings. This prevents movement when the motor vibrates at high speed.
8.2 Applying Insulation Varnish
Dip the entire stator in high-grade varnish or epoxy. The varnish serves multiple functions:
- Strengthens coil insulation
- Binds the wires together
- Improves moisture resistance
- Enhances heat dissipation
8.3 Oven Drying
Place the varnished stator in an oven at the manufacturer’s recommended temperature. This cures the varnish, making the winding rigid and waterproof.
9. Reassembling the Motor
After the stator has dried:
- Install the stator back into the housing
- Replace or inspect bearings
- Refit rotor and end bells
- Seal the housing properly to ensure waterproof integrity
Make sure all gaskets, O-rings, and seals are intact and properly positioned.
10. Testing and Calibration
Before putting the motor back into use, perform thorough testing and calibration. This step guarantees the rewinding quality and ensures safety.
10.1 Insulation Resistance Test
Use a megohmmeter to measure insulation resistance between:
- Phase to phase
- Phase to ground
Values below acceptable limits indicate moisture or insulation failure.
10.2 Continuity Test
Check each winding for continuity using a digital multimeter. Look for any open circuits.
10.3 Phase Balancing
All three phases must have equal resistance. If not, it means:
- Coil count mismatch
- Poor connection
- Wire length inconsistency
Correct the issue before proceeding.
10.4 No-Load Test
Run the motor outside the water without any load:
- Check current on each phase
- Listen for unusual sounds
- Monitor temperature rise
An efficient rewind should produce smooth rotation, low vibration, and uniform current flow.
10.5 Full Calibration
Finally, install the motor in a test tank or well and perform a full operation test. Monitor:
- Starting current
- Running current
- Vibration
- Temperature
- Seal integrity
This completes the restoration of the submersible motor.
11. Common Mistakes to Avoid
- Incorrect slot alignment, leading to coil rubbing and shorts
- Using low-grade insulation materials
- Miscounting coil turns
- Poor varnish curing
- Incorrect wire gauge
- Loose interconnections
Avoiding these mistakes ensures a long-lasting rewind and reliable motor operation.
12. Conclusion
Rewinding a submersible motor is a complex but rewarding process. By carefully following the rewinding procedure, ensuring accurate stator winding, applying high-quality coil insulation, maintaining precise slot alignment, and performing thorough testing and calibration, you can restore a damaged submersible motor to full operational efficiency.
This method not only saves money but also extends the life of the equipment—making rewinding an essential skill for technicians, engineers, and anyone involved in pump maintenance or electric motor repair.