Boiler water circulating pumps are critical-duty motors, so winding quality directly affects boiler safety, efficiency, and uptime. Below is a clear, field-oriented explanation useful for rewinding, inspection, and maintenance.
1. Motor Type Commonly Used
- 3-Phase Squirrel Cage Induction Motor
- Voltage: 415 V / 6.6 kV / 11 kV (plant dependent)
- Speed: 1450 RPM or 2900 RPM
- Duty: S1 (Continuous)
- Insulation Class: F or H
- Protection: IP55 / IP65
- Cooling: IC 411 / IC 416
2. Importance of Proper Winding
Boiler circulating pumps work at:
- High temperature
- Continuous load
- High moisture environment
Poor winding can cause:
- Overheating
- Frequent tripping
- Reduced pump flow
- Boiler shutdown
3. Winding Specification (Typical)
| Parameter | Details |
|---|---|
| Stator Core | Silicon steel laminations |
| Winding Type | Double layer, distributed winding |
| Conductor | Electrolytic copper |
| Insulation | Class F / H varnish |
| Slot Insulation | DMD / Nomex |
| Phase Insulation | Polyester / Nomex paper |
| Impregnation | VPI / Dip & Bake |
4. Rewinding Procedure (Step-by-Step)
Step 1: Pre-Inspection
- Record original winding data
- Measure IR & PI values
- Check bearing and shaft condition
Step 2: Stripping
- Controlled burning (≤ 350°C)
- Clean slots thoroughly
- Check core for hot spots
Step 3: Coil Manufacturing
- Same turns, pitch & wire size
- Use high-temperature enamel wire
- Proper coil forming to avoid stress
Step 4: Winding & Lacing
- Correct slot filling
- Proper phase separation
- Strong end-winding support (critical)
Step 5: Varnish Impregnation
- Prefer VPI
- Otherwise Dip–Bake (2 cycles)
Step 6: Testing
- IR & PI test
- HV test
- No-load current test
- Vibration check after assembly
5. Common Winding Failures & Causes
| Failure | Cause |
|---|---|
| Insulation breakdown | High temperature |
| Phase-to-phase fault | Loose end winding |
| Earth fault | Moisture ingress |
| Overheating | Overloading / poor cooling |
6. Recommended Improvements During Rewinding
✔ Upgrade insulation to Class H
✔ Improve end-winding bracing
✔ Use RTD / PTC sensors
✔ Ensure correct bearing clearances
✔ Balance rotor dynamically
7. Best Practices for Long Life
- Maintain proper boiler water chemistry
- Ensure pump alignment
- Avoid dry running
- Periodic IR testing
- Keep motor ventilation clean
Boiler Water Circulating Pump Motor Rewinding – Complete Practical Checklist
A. Before Rewinding (Incoming Inspection)
☐ Record motor nameplate details (kW, HP, V, A, RPM, Insulation class)
☐ Note winding connection (Star / Delta)
☐ Measure & record IR values (Phase–Phase, Phase–Earth)
☐ Conduct PI test (for HT motors mandatory)
☐ Check bearing condition & clearances
☐ Inspect shaft run-out & coupling alignment
☐ Verify cooling method (IC 411 / IC 416)
☐ Photograph & note original winding data
B. During Stripping & Core Inspection
☐ Controlled de-varnishing (burning temp ≤ 350°C)
☐ Remove old coils carefully – no lamination damage
☐ Clean stator slots thoroughly
☐ Inspect for core hot spots / shorted laminations
☐ Check stator bore ovality
☐ Perform core loss test (recommended for critical motors)
C. Winding Data Confirmation
☐ Confirm number of slots
☐ Coil pitch & span verified
☐ Turns per coil confirmed
☐ Conductor size & parallel paths verified
☐ Winding type (single / double layer) confirmed
☐ Correct phase sequence planned
D. Insulation Material Checklist
☐ Slot insulation – DMD / Nomex (Class F or H)
☐ Phase separators – Polyester / Nomex
☐ Wedge material – Glass fiber / Epoxy wedges
☐ End winding support – Glass cord / banding tape
☐ Enamel copper wire – Class F / H
☐ Varnish – Thermal Class F / H
E. Winding & Lacing
☐ Uniform coil insertion – no enamel damage
☐ Correct slot fill factor maintained
☐ Phase separation properly maintained
☐ Tight end-winding lacing done
☐ No sharp edges touching insulation
☐ Leads sleeved with fiberglass tubes
F. Impregnation & Drying
☐ VPI preferred for boiler pump motors
☐ If dip-bake:
- ☐ First dip & bake completed
- ☐ Second dip & bake completed
☐ Oven temperature maintained as per varnish spec
☐ Complete curing ensured
G. Electrical Testing After Rewinding
☐ IR test after baking
☐ PI test satisfactory
☐ High Voltage (HV) test passed
☐ Resistance balance test within limits
☐ Phase sequence verified
H. Mechanical Assembly & Checks
☐ Bearings replaced (if required)
☐ Correct bearing greasing done
☐ Rotor dynamic balancing done
☐ Fan & cooling ducts cleaned
☐ Proper air gap maintained
I. Final Trial Run
☐ No-load current within limit
☐ No abnormal noise or vibration
☐ Temperature rise normal
☐ Direction of rotation correct
☐ Cooling airflow verified
J. Documentation & Handover
☐ Test reports attached
☐ Rewinding data sheet prepared
☐ Warranty / guarantee mentioned
☐ Maintenance recommendations given