Increasing the efficiency of three-phase asynchronous motors: energy audit and frequency converter retrofitting

1. Introduction

In industrial plants in the DACH region, an estimated 70 percent of electricity consumption is accounted for by electric drive systems (EAS). With energy costs continually rising, identifying savings potential through a systematic energy assessment is essential. A loss-making operation of asynchronous motors, often due to oversizing or throttle control, places a significant burden on operating costs (OPEX). This reference document describes the procedure for conducting an energy audit and the technical implementation of efficiency upgrades and frequency converter retrofits.

2. Basic principles

The power balance of a three-phase asynchronous motor is defined by: P_input = P_output + P_losses. The losses (P_losses) consist of stator and rotor winding losses (I²R), iron losses, mechanical friction losses and additional losses due to harmonics.

The efficiency is primarily determined by the quality of the magnetic circuit, the winding geometry and the material properties (e.g. electrical sheet). With throttle control (e.g. pumps or fans), the motor remains close to its rated speed while the mechanical resistance increases - this is thermodynamically inefficient. Speed ​​control using a frequency converter (FU) allows adaptation to the actual power requirement in accordance with the affinity laws (P₂ ~ n³), which leads to massive savings at partial load.

3. Technical specifications & standards

The motor efficiency is classified according to DIN EN 60034-30-1. The international standard IEC 60034-2-1 defines the measurement methods for determining losses.

The DIN EN 50598-2 standard is relevant for drive systems (Power Drive Systems, PDS), which evaluates the energy efficiency of frequency converters and their interaction with motors.

4. Selection & Interpretation: Guide

The selection must be based on the real load profile. A point-by-point measurement is inadequate.

• Data acquisition: Measurement of voltage (U), current (I), power factor (cos φ), frequency and load condition over at least one complete operating cycle.
• Calculation: Determination of the operating hours per load point.
• Profitability assessment: Calculation of the payback period taking into account the investment costs (CAPEX) and the annual energy cost savings.

5. Installation & commissioning

When retrofitting frequency converters, the EMC guidelines (DIN EN 61800-3) must be adhered to. Frequency converters generate harmonics that can cause network disturbances. The use of throttles or filters is often necessary.

Particular attention is paid to the cabling: The use of shielded motor cables and low-resistance grounding are essential to minimize electromagnetic interference and limit transient voltage peaks (dv/dt) at the motor connection.

6. Causes of failure & error analysis

A common problem with FC powered motors is induced bearing voltage. This discharges through the bearing, causing electrical erosion (EDM) and drastically shortening bearing life. According to ISO 10816, vibration monitoring is an integral part of condition monitoring.

• Winding damage: Due to transient overvoltages as a result of high dv/dt values ​​on long cable routes.
• Bearing damage: Due to EDM currents (Protection: Insulated bearings or shaft grounding).
• Overheating: When operating at very low speeds without an external fan.

7. Predictive maintenance

Condition monitoring today includes:

• Vibration analysis: According to ISO 18436.
• Thermography: For identifying local heating on terminal boards or housings.
• Current signature analysis (MCSA): For early detection of rotor bar breaks or bearing errors.

8. Comparison matrix: drive concepts

9. Summary & CTA

The energetic optimization of drive systems requires a precise measurement basis and the correct selection of components in accordance with DIN and IEC standards. UNITEC-D supports you in selecting highly efficient motors and suitable frequency converters for your specific application. Visit our e-catalogue for technical specifications and expert advice: https://www.unitecd.com/e-catalog/

10. References

1. DIN EN 60034-30-1: Rotating electrical machines - Part 30-1: Efficiency classification.
2. DIN EN 50598-2: Ecodesign for drive systems, electric drives, motor starters and their electronics.
3. IEC 60034-2-1: Standard methods for determining losses and efficiency from tests.
4. ISO 10816: Mechanical vibrations - evaluation of the vibrations of machines.