Qualitrol Blog
SAG Mill Motor Programmed Rewind
During a visual inspection and condition assessment on the SAG mill, an insulation anomaly was identified in the rotor. To remove the risk of unexpected failure, the team planned and executed a programmed rewind within a 10-day shutdown window.
Scope & Timeline (10-Day Program)
| Phase | Days | Key Activities |
|---|---|---|
| Pre-outage preparation | Weeks prior | Specification preparation; global tendering; tender evaluation; vendor audits (Dalian, China & Auckland, NZ); witness testing of coils; logistics planning; HSE and QA plans. |
| De-energize & disassemble | Day 1–2 | Lockout/tagout; rotor removal from stator; clean and inspect core, wedges, ventilation; verify fits and clearances; confirm work fronts. |
| Rewind execution | Day 3–7 | Strip old windings; install new coils; connections and brazing; slot cell/liners as required; blocking and bracing; varnish/curing (per spec and vendor procedure). |
| Electrical & mechanical tests | Day 8–9 | IR/PI, winding resistance balance, surge/hi-pot per spec, shaft/runout checks, vibration baseline, thermography. Owner’s rep signs QA records. |
| Commissioning | Day 10 | Cold and hot commissioning, protection checks, no-load to load ramp, post-start inspection; handover package with as-built drawings and test results. |
Why On-Site vs Off-Site?
| On-Site Planned Rewind | Off-Site Shop Rewind |
|---|---|
| Minimal transport risk and no cranage/shipping delays; shortest outage path when space and utilities allow. | Access to full shop tooling/fixtures and ovens; useful when site access is restricted or major machining is required. |
| Tighter schedule control with the owner’s superintendent on the floor; easier integration with plant HSE and permits. | Extra time for disconnect, packaging, transport, re-install, and alignment can extend lost-production days. |
| Lower total cost when downtime is the driver; this project finished on time and on budget (~$250k) and avoided about $700k versus a traditional off-site path. | Higher logistics and outage duration typically raise total cost even if the shop rate is competitive. |
Quality Assurance & Verification
- Witness testing of coils before delivery; vendor audits completed in China and New Zealand.
- Electrical acceptance tests: IR/PI, winding resistance balance, surge/hi-pot to spec, partial discharge checks as applicable, and thermography.
- Mechanical acceptance: runout/alignment, end-winding support checks, vibration baseline at no-load and load.
Our Recommended Monitoring & Program Tools
- PDA-IV (portable PD testing) for periodic post-rewind checks using installed 80 pF EMC sensors; analysis in PDView.
- GuardII+ or PDTracII 4208 for continuous PD trending on critical motors; alarms support early intervention between shutdowns.
- FluxTrac (if synchronous) rotor magnetic-flux monitoring to detect shorted turns early and verify rotor health after rewind.
Outcome
The programmed on-site rewind returned the SAG mill motor to reliable service within the planned 10-day window. The project was delivered on time and on budget (~$250k) with an estimated net saving of about $700k versus an off-site repair, while strengthening QA rigor and future monitoring.
