Do High-Efficiency Motors Always Save Energy?
Not necessarily.
In a recent fixed speed project, replacing IE2 motors with IE4 motors actually increased energy consumption. The main reasons were:
👉 Motor sizing: Modern motors achieve peak efficiency when operating at around 70-90% load. Oversized motors may not deliver the expected savings.
👉 Motor slip: The new motor ran at 988 rpm compared to the original 980 rpm. Although only an 8 rpm difference, the higher speed increased energy consumption.
In this case, the additional speed delivered no measurable improvement in throughput, product quality, or process performance, meaning the customer simply used more energy. With 50 motors installed, the impact was significant!
To correct the slip difference between the old and new motors we added an ABB Inverter to the installation below.
CASE STUDY: (original motor nameplate as below)
Application: 15 kW milling machine (DOL)
Average load: 9 kW
Peak load: 13.2 kW
Existing IE2 efficiency: 91.2%
New IE4 efficiency: 93.7%
To verify the results, we compared:
Original IE2 motor at 980 rpm (50Hz)
IE4 motor with inverter at 988 rpm
IE4 motor with inverter at 980 rpm
The ABB inverter was configured with flux optimisation to minimise energy consumption at lower loads.
Results
🌟 IE4 motor at 988 rpm achieved a 3% energy saving despite inverter losses.
🌟 IE4 motor at 980 rpm achieved a 9% energy saving.
Key Takeaway
Higher motor efficiency does not automatically mean lower energy consumption.
Before upgrading motors, consider:
 ✅ Motor loading
 ✅ Slip speed differences
 ✅ Process speed requirements
 ✅ Whether a VSD is required
The best results come from optimising the entire system, not just replacing the motor with a higher efficiency model. This is something EDC Scotland and EDC (North East) are renowned for their expertise in.