Rewind or Replace a 75 kW Irrigation Pump Motor
A 75 kW, 4-pole, 415 V IE2 induction motor drives a vertical turbine pump for an irrigation scheme. It runs about 6000 hours a year. A recent measurement shows 58 kW input at 0.78 power factor, with an estimated output of 53 kW. The motor has failed insulation on one phase and the winder has quoted ₹70,000 for a rewind, warning that a rewound motor typically loses 1.5–2 percentage points of efficiency. Two new motors are available: a 55 kW IE3 at ₹2.6 lakh and a 75 kW IE3 at ₹3.1 lakh. The utility tariff is ₹7.40/kWh, and a power factor penalty of 0.5% of energy charges applies for every 0.01 below 0.90. The pump duty is fixed by the irrigation schedule and head; the measured maximum demand at the shaft has never exceeded 53 kW in three seasons of records. Supply is from a 100 kVA transformer with limited starting capacity.
1. Compare the three options - rewind, 55 kW IE3, and 75 kW IE3 - on annual energy cost and simple payback, using incremental cost over the rewind.
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Baseline: input 58 kW × 6000 h = 348,000 kWh/year, costing ₹25.75 lakh. Output ≈ 58 × 0.915 ≈ 53 kW.
Option 1 - rewind. Efficiency falls to roughly 89.5%, so input becomes 53/0.895 = 59.2 kW, i.e. 1.2 kW worse than today: +7,200 kWh/year, about ₹53,000 a year extra. No payback; this option costs money every year.
Option 2 - 55 kW IE3. At 53 kW output the motor is at 96% load, where IE3 efficiency is about 94.0%. Input = 53/0.940 = 56.4 kW. Saving = 58 − 56.4 = 1.6 kW → 9,600 kWh/year → ₹71,040/year. Incremental cost over rewind = 2.6 − 0.70 = ₹1.90 lakh. Payback = 1.90/0.7104 ≈ 2.7 years.
Option 3 - 75 kW IE3. At 53 kW the motor is only 71% loaded, where efficiency is about 93.8%. Input = 53/0.938 = 56.5 kW. Saving = 1.5 kW → 9,000 kWh/year → ₹66,600/year. Incremental cost = 3.1 − 0.70 = ₹2.40 lakh. Payback ≈ 3.6 years.
Ranking: 55 kW IE3 (2.7 years) beats 75 kW IE3 (3.6 years), and both beat rewinding, which increases consumption. The efficiency gain is worth more than the cheaper capital cost of the rewind within three years.
2. Is the 55 kW motor technically the right choice at 96% loading? Discuss the risk and the conditions you would attach before recommending it.
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At 96% of rated output the 55 kW motor is inside its service factor but has almost no margin. That is acceptable only if the duty is genuinely fixed. Evidence in favour: three seasons of records with shaft demand never above 53 kW, and an irrigation schedule that fixes head and flow.
Conditions I would attach:
- Confirm the 53 kW figure by a calibrated input-power measurement at maximum flow, not by nameplate or by an assumed efficiency. If the true peak is 55–56 kW, the 55 kW motor is overloaded and the 75 kW unit becomes the correct choice despite the longer payback.
- Check supply voltage unbalance. A 2% unbalance can raise winding temperature by 8–10 °C; at 96% loading that margin matters.
- Check ambient and altitude derating at the pump house; derating factors reduce available output.
- Check starting. A 55 kW direct-on-line start on a 100 kVA transformer will cause a severe dip; specify a soft starter or star-delta starter and confirm the transformer can carry the starting current, especially with other loads connected.
- If the pump may be upsized or the head increased later, the 75 kW motor's 3.6-year payback is the safer engineering choice - the cheapest kWh is not worth a burnt winding in year two.
Power factor also improves from 0.78 to about 0.86, which reduces the kVA demand and the penalty exposure, but that is a secondary benefit.
3. The finance team asks why not simply rewind, since the motor 'still works'. How would you present the lifecycle case, and what monitoring would you put in place after replacement?
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Frame it as a lifecycle cost, not a repair bill. Rewinding is ₹70,000 today but locks in about 59.2 kW of input instead of 56.4 kW for the remaining life - roughly ₹53,000 a year of avoidable energy cost, and more if tariffs rise. Over a typical 10-year remaining life the rewind costs about ₹5.3 lakh in wasted energy on top of its capital cost, which dwarfs the ₹1.9 lakh incremental price of the IE3 unit.
Also note that a rewind restores the motor to its original IE2 efficiency minus core and copper losses added by the burnout and strip-out; it does not restore the lost efficiency. The second failure would then be a second rewind on an already-degraded stator.
Monitoring after replacement:
- Log input kW, current and power factor monthly and compare against the 56.4 kW baseline; a 5% drift signals impeller wear or voltage problems.
- Record running hours and schedule bearing and insulation resistance checks annually.
- Track energy per unit of water delivered, not just total kWh, so that pump efficiency is separated from motor efficiency.
- Verify the power factor penalty disappears from the bill within two billing cycles.
If capital is genuinely constrained this year, the honest interim answer is to rewind and plan the replacement for the next financial year - but the decision should be recorded as a deferral with a cost, not presented as a saving.