Is it worth replacing a working IE2 motor with an IE3 motor?

Usually only for motors that run many hours at high load. A 55 kW motor at 75% load, 6,000 hours a year, saves about 3,100 kWh going from IE2 to IE3, around 370 a year at 0.12 per kWh, so replacing a working motor rarely pays back quickly. The case is much stronger when the motor has failed and you are choosing between rewinding and buying new.

Should I rewind or replace a failed motor?

Compare the extra cost of a new high-efficiency motor over the rewind cost with the energy it saves. A rewound motor often loses some efficiency, so the saving is larger than the nameplate difference suggests. For motors that run long hours, replacing is often paid back within a few years.

Motor Efficiency Upgrade Calculator: IE2 to IE3 or IE4 Payback

See what a higher-efficiency motor saves each year, and how long it takes to pay back, whether you are replacing a working motor or deciding against a rewind.

How the calculation works

The shaft power the driven machine needs stays the same whichever motor drives it. A more efficient motor simply draws less electrical power to deliver it:

P_shaft = rated power × load
kW saved = P_shaft × (1 ÷ η_existing − 1 ÷ η_new)
Annual saving = kW saved × hours × price per kWh

For a working motor, the payback is the full installed cost of the new motor divided by the annual saving. For a failed motor, you are paying either way, so only the extra cost of buying new over rewinding counts.

Worked example

A 55 kW, 4-pole pump motor runs at 75% load for 6,000 hours a year, at 0.12 per kWh. Shaft power is 41.25 kW.

Replacing a working IE2 motor (93.5%) with IE3 (94.6%): 0.51 kW saved, 3,080 kWh and about 370 a year. At 5,000 installed, payback is about 13.5 years. Not worth it on energy alone.
Same motor, but it has failed: the rewound motor would be about 93.0% efficient. Buying IE3 saves 0.75 kW, 4,500 kWh and about 540 a year. The extra cost over a 2,500 rewind is 2,500, so payback is about 4.6 years.
IE4 (95.7%) instead of IE2: 1.01 kW saved, about 730 a year from the same working motor.

The lesson most plants learn: decide your motor policy before motors fail, so that the high-efficiency replacement is ready when the decision has to be made in a hurry.

IEC 60034-30-1 minimum efficiencies, 4-pole, 50 Hz

Rated power	IE2 (%)	IE3 (%)	IE4 (%)

Values at full load. Check the standard or the manufacturer's data sheet for 2, 6 and 8-pole motors, 60 Hz motors, and exact values.

Assumptions and limits

Efficiency is taken as constant at the average load. Most motors keep close to their rated efficiency between about 50% and 100% load and fall off below that.
A new high-efficiency motor often runs slightly faster (lower slip). On centrifugal pumps and fans this can increase the load and eat into the saving unless the impeller or speed is adjusted.
Check that a replacement motor matches frame size, mounting, starting current and starting torque, especially on IE4 and above.
Simple payback ignores maintenance, downtime, financing and energy price changes.

Questions

Is it worth replacing a working IE2 motor with IE3?

Usually only for motors that run many hours at high load and where a larger efficiency gap exists, for example an old motor below IE1. Otherwise the stronger case is at failure, when you are paying for a rewind or a new motor anyway.

How much efficiency does a rewind lose?

A good rewind to recognised repair practice can keep efficiency close to the original. Poor practice, especially burning out the old winding at excessive temperature, commonly costs 0.5 to 1 percentage point or more, and repeated rewinds add up.

Where do I find the existing motor's efficiency?

On the nameplate for most motors built in the last fifteen years. For older motors, use the manufacturer's data sheet or assume a value below IE1 for that rating.