Efficiency Impact of Rewinding
A poorly executed rewind can reduce motor efficiency by 1–3%. The main causes of efficiency reduction:
- Core burn-off temperature too high: Stripping old windings by burning off insulation in an oven at >350°C degrades the lamination steel, increasing core losses. EASA recommends <315°C and time-limited cycles.
- Copper fill reduction: Rewinders may use slightly smaller wire to fit the groove, reducing conductor cross-section and increasing resistance losses.
- Insulation quality: Using lower-grade insulation materials reduces dielectric strength and can reduce cooling efficiency.
A quality rewind following EASA/IEC 60034-23 guidelines should preserve original efficiency within ±0.5%. Ask the rewinder for their documented rewind procedure and what oven temperature and time they use for core strip.
Cost Analysis: Rewind vs Replace
Key variables: rewind cost, replacement motor cost, energy cost (£/kWh), operating hours/year, efficiency difference.
| Motor power | Typical rewind cost | IE3 replacement cost | Efficiency gain (IE1→IE3) | Annual saving (6,000 h) | Simple payback for replacement |
|---|---|---|---|---|---|
| 7.5 kW | £300–500 | £400–600 | ~4.6% | ~£48 | ~4–6 years over rewind |
| 15 kW | £500–700 | £700–1,000 | ~3.9% | ~£74 | ~3–4 years |
| 37 kW | £800–1,200 | £1,500–2,200 | ~2.8% | ~£150 | ~3–5 years |
| 75 kW | £1,500–2,500 | £3,000–5,000 | ~2.4% | ~£260 | ~4–6 years |
| 200 kW | £3,000–6,000 | £9,000–15,000 | ~2.0% | ~£540 | ~8–12 years |
Based on £0.15/kWh, full-load operation, IE1 original motor
Decision Criteria
Replace with IE3 when:
- Motor is <37 kW and runs >4,000 h/year on continuous duty
- Original motor is IE1 (standard efficiency) — the efficiency gap is largest
- Motor has failed multiple times (rewind history exists) — indicates underlying problem
- Replacement availability is assured (stock item vs 6-week delivery for a non-standard frame)
- Regulatory compliance: replacing an IE1 with a rewind now requires the rewind to meet IE3 levels in some jurisdictions
Rewind when:
- Motor is >75 kW and replacement lead time would cause unacceptable downtime
- Motor is a special design (vertical shaft, marine duty, non-standard frame) where replacement is expensive or unavailable
- Motor is IE2 or higher — efficiency gain from replacement is marginal
- Rewind cost is less than 50% of replacement cost and annual operating hours are low (<2,000 h/year)
Specifying a Quality Rewind
If rewinding, specify to the rewinder:
- Core strip method: chemical strip or oven at <315°C maximum, for minimum time
- Core loss test before and after strip — losses must not increase by >20%
- Same wire gauge and turns per coil as original design (provide original test report if available)
- Class F or H insulation system (even if original was Class B)
- Full load test after rewind — compare efficiency to original nameplate data
- EASA Q certification or equivalent — indicates rewinder follows the AC Motor Repair Standard