Energy audits for electric motors: increasing efficiency and potential savings through frequency converter retrofitting

1. Introduction: The engineering challenge

Electric motors are the backbone of industrial manufacturing processes and convert electrical energy into mechanical work. They consume around 45% of all electrical energy worldwide. In the DACH region, industry accounts for an estimated 60-70% of electricity consumption. The efficient use of these engines is therefore not only a question of economic efficiency, but also of ecological responsibility. Outdated or inefficiently operated engines represent a significant cost item, especially given rising energy prices.

An energy audit for electric motors systematically identifies weak points in the drive train, quantifies energy losses and suggests concrete measures for optimization. The aim is to reduce operating costs, increase system availability and reduce the CO2 footprint. For maintenance engineers, reliability engineers, and operations managers, understanding this methodology is critical to ensuring asset reliability according to VDI 2067 Sheet 1 and minimizing total cost of ownership (TCO). UNITEC-D, as your partner for high-quality industrial components, supports you in selecting and purchasing suitable drive solutions.

2. Fundamental principles of engine efficiency

2.1 Physical basics

The efficiency of an electric motor (η) is defined as the ratio of the mechanical power delivered (P_mech) to the electrical power consumed (P_elektr):

η = P_mech / P_elektr

Losses come in different forms:

• Copper losses (I²R losses): Occur from the current flow in the windings of the stator and rotor. These are proportional to the square of the current.
• Iron losses: Result from reversal magnetization losses (hysteresis) and eddy currents in the iron core. They are frequency and flux density dependent.
• Friction losses: Mechanical losses due to bearing friction and air resistance (ventilation).
• Additional losses: Stray field losses that cannot be clearly assigned to the main types of loss.

The power factor (cos φ) describes the ratio of active power to apparent power. A low power factor results in higher currents, which increases network load and causes additional copper losses. Compensation systems can improve the power factor and reduce network losses.

2.2 Functionality and energy saving through frequency converters (FU)

Variable Frequency Drives (VFDs) control the speed and torque of an electric motor by varying the frequency and voltage of the power supply. The energy savings result primarily from the affinity laws for pumps, fans and compressors:

• Volume flow (Q): Proportional to the speed (n). Q₂/Q₁ = n₂/n₁
• Pressure/head (H): Proportional to the square of the speed. H₂/H₁ = (n₂/n₁)²
• Power consumption (P): Proportional to the cube of the speed. P₂/P₁ = (n₂/n₁ )³

For example, if the speed of a fan is reduced by 20% (n₂ = 0.8 * n₁), the volume flow drops by 20%, the pressure by 36% and the power consumption by almost 50% (0.8³ = 0.512). This illustrates the enormous savings potential when operating at partial load.

3. Technical Specifications & Standards

3.1 Efficiency classes of electric motors

The international standard IEC 60034-30-1 classifies the energy efficiency of low-voltage three-phase asynchronous motors. This standard is implemented in Germany by DIN EN 60034-30-1 and is mandatory for motors with a nominal output of 0.75 kW to 375 kW. As of July 2021, the following minimum requirements apply:

• IE3 (Premium Efficiency): For motors from 0.75 kW to 1000 kW.
• IE2 (High Efficiency): For motors from 0.12 kW to 0.75 kW.

As of July 2023, IE4 requirements apply to 8-pole motors and motors from 75 kW to 200 kW.

3.2 Protection types and insulation classes

• Protection class (IP code): According to DIN EN 60529 (IEC 60529) classifies the protection against the ingress of foreign bodies and water. For industrial applications, IP54, IP55 or higher protection classes (e.g. IP65 for water jet protection) are often required.
• Insulation class: According to DIN EN 60034-1 it indicates the maximum permissible winding temperature. Classes F (max. 155 °C) and H (max. 180 °C) are common. Exceeding these temperatures halves the lifespan of the insulation for every 10K increase in temperature.

3.3 Frequency converter standards

• IEC 61800-3: Determination of electromagnetic compatibility (EMC) for frequency converters.
• IEC 60947-2: Covers circuit breakers and their use with frequency converters.
• Harmonische: VDE AR-N 4105 und IEC 61000-3-x regeln die Grenzwerte für harmonische Oberschwingungen, die von Frequenzumrichtern verursacht werden können. Filter measures are often required.

4. Selection & Sizing Guide

4.1 When is an engine change economical?

Replacing a motor with a higher efficiency class is usually economical if the motor:

• Has a long annual operating time (> 2,000 hours).
• Is operated in the upper load range (75-100% of the nominal power).
• Has low efficiency (e.g. IE1 or older).
• Is susceptible to repair.

The amortization period (payback period) can be estimated as follows:

Payback period (years) = (investment costs) / (annual savings)

Annual savings can be calculated by:

Annual savings (€) = P_nominal (kW) × (1/η_old - 1/η_new) × operating hours (h/year) × electricity price (€/kWh)

Example: An 11 kW motor (IE1, η=89.4%) is replaced by an IE3 motor (η=92.1%). Operating hours: 6,000 h/year, electricity price: €0.25/kWh. Investment costs: €1,500.

Annual savings = 11 kW × (1/0.894 - 1/0.921) × 6000 h × 0.25 €/kWh ≈ 490 €/year.

Payback period = €1,500 / €490/year ≈ 3.06 years.

4.2 Sizing of frequency converters

The correct dimensioning of a frequency converter is crucial for its efficiency and service life. The main criteria are:

• Nominal power of the motor: The drive must cover at least the nominal power of the motor. Consider overload reserves for starting currents.
• Load profile of the application: Constant torque (e.g. conveyor belts) or quadratic torque (e.g. fans, pumps). Frequency converters are often optimized for one of the two types.
• Ambient conditions: Temperature, humidity, protection class (IP class) of the drive according to DIN EN 60529. Typical drive protection classes are IP20 (switch cabinet installation) to IP66 (direct field use).
• Network quality: Consideration of harmonics and network feedback. Active front-end converters (AFE) can minimize network disturbances.

5. Installation & Commissioning Best Practices

5.1 Motor installation

• Foundation and alignment: A stable, vibration-damped foundation is crucial according to DIN ISO 10816-3. Precise alignment of the motor and driven machine (using laser alignment) is essential to avoid bearing and coupling damage. A misalignment of 0.1mm can reduce bearing life by 50%.
• Cable cross-section: The cable cross-section must be dimensioned according to VDE 0298-4 for the rated current and the cable length in order to minimize voltage drops and additional losses.
• Cooling and ventilation: The ambient temperature and ventilation must correspond to the insulation class of the motor. An operating temperature of 40 °C is often standard.

5.2 Frequency converter installation

• EMC-compliant installation: According to IEC 61800-3, an EMC-compliant installation with shielded motor cables, correct grounding and avoidance of loop formation is mandatory. Cable lengths should be minimized to reduce capacitive effects.
• Line filters and chokes: For long motor cables (> 50 m) or EMC-sensitive environments, output chokes or sine-wave filters may be required to reduce the load on the motor insulation and the radiation of interference.
• Commissioning: Detailed parameterization of the frequency inverter to the motor and the application is essential. This includes motor parameters (rated current, rated voltage, frequency), PID controller settings for process applications and protection functions (overcurrent, overtemperature). An auto-tuning function on many FUs can simplify parameterization.

6. Error images & root cause analysis

6.1 Engine defects

• Bearing damage: Most common cause (up to 40% of failures). Causes: lack of lubrication (DIN 51825), overload, misalignment, vibrations (DIN ISO 20816). Visual indicators: increased temperature, loud noises, excessive play.
• Winding defects: Short circuits between phases or turns, earth faults. Causes: aging of the insulation, overvoltage, excess temperature. Visual indicators: burnt smell, discoloration of the winding, smoke development.
• Insulation defects: Degradation of the insulation materials due to thermal, electrical or mechanical stress. Causes: Excessive temperature (> insulation class), moisture, aggressive chemicals. Detectable by measuring insulation resistance (VDE 0701-0702).

6.2 Frequency converter defects

• Power semiconductor defects (IGBTs): Overcurrent, overvoltage, overtemperature are common causes. An IP20 drive operating in a cabinet with an internal temperature of 50°C can see its lifespan drastically reduced if it is only designed for 40°C ambient temperature.
• Capacitor failures: Electrolytic capacitors age due to temperature and voltage ripple. They are often the limiting factor for the lifespan of a drive.
• Control failures: Software errors, communication problems or defects in the control electronics can lead to malfunctions.

7. Predictive Maintenance & Condition Monitoring

Predictive maintenance using condition monitoring is crucial to avoid unplanned downtime and maximize the service life of motors and frequency converters. According to VDI 2067, this is an integral part of system efficiency.

• Vibration analysis (DIN ISO 10816): Detects bearing damage, imbalance, misalignment and loose components at an early stage. Monitoring vibration amplitudes and frequencies provides information about the condition of rotating components.
• Thermography (DIN EN 13187): Measures the surface temperature of motors, bearings, windings and frequency converters. Overheating indicates overload, poor cooling or electrical problems. A temperature increase of just 10°C can reduce the lifespan of electrical components by 50%.
• Motor Current Signature Analysis (MCSA): Analyzes the motor's current spectrum to detect mechanical problems (rotor rod breakage, bearing damage) and electrical problems (winding faults).
• Oil analysis: For larger engines with oil-lubricated bearings. Analyzes particle wear, viscosity and chemical composition of the lubricant.
• Partial discharge measurement: In high-voltage motors, it detects insulation faults that can lead to breakdown.

8. Comparison matrix: Motor and FU technologies

Selecting the right motor and frequency converter depends on the specific application and economic goals. UNITEC-D offers a wide range of certified components that meet the highest standards (CE, TUV, ATEX).

9. Conclusion

The implementation of energy audits and the consistent implementation of efficiency measures for electric motors and drives is a central component for the economic and ecological success of manufacturing companies in the DACH region. By replacing old motors with models with efficiency classes IE3 or IE4, retrofitting with frequency converters and implementing condition monitoring, significant energy savings of 20-50% can be achieved. This leads to a quick amortization of investments, increased system reliability and a significant reduction in maintenance costs. UNITEC-D is your reliable partner for the procurement and advice of high-quality industrial components that make your systems future-proof.

Discover our comprehensive range of certified electric motors and frequency converters in our e-catalogue:

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10. References

1. IEC 60034-30-1: Three-phase motors - Part 30-1: Efficiency classes of mains-fed three-phase motors (IE Code).
2. DIN EN 60529 (VDE 0470-1): Degrees of protection through housing (IP code).
3. VDI 2067 Sheet 1: Economic efficiency of building technology systems – basics and definitions.
4. DIN ISO 10816-3: Mechanical vibrations – measurement and evaluation of vibration strength on machines with non-rotating parts.
5. ABB White Paper: The total cost of ownership of electric motors.