Soft starters vs. Frequency Converters: The Right Choice for Motor Control in Industrial Applications

1. Introduction: The Essence of Motor Control for Operational Reliability

The industrial sector in the Benelux, characterized by continuity and efficiency, relies heavily on the reliability of electric motors. Induction motors are the workhorse of many manufacturing processes, but their direct start-up can cause significant mechanical and electrical stress. Uncontrolled starting currents lead to mains disruptions, accelerated wear of mechanical components and a shortened motor life. Adequate motor control is therefore not an option, but a critical necessity for a reliable installation in accordance with NEN-EN 50110-1.

Two primary technologies dominate the field of controlled motor control: the soft starter and the frequency converter (VFD, Variable Frequency Drive). Both offer advantages for engine starting, but their functionalities, application areas and complexity differ significantly. This article analyzes the technical principles, specifications and operational considerations to inform the right choice for your specific application, contributing to optimized operational reliability and energy efficiency within your production environment.

2. Fundamental Principles: The Technology Behind Controlled Engine Start and Speed Regulation

2.1. The Softstarter: Controlled Start-up

A soft starter is an electronic device that controls the starting of a three-phase asynchronous motor. It works by gradually increasing the voltage sent to the motor. This is achieved using thyristors (SCRs – Silicon Controlled Rectifiers), which are connected in anti-parallel per phase. By progressively increasing the conduction angle of the thyristors at the beginning of the start phasing, the effective voltage on the motor is slowly built up.

The primary purpose is to reduce the starting current and starting torque. A direct start (DOL) can lead to starting currents of 6 to 8 times the nominal current (I_n). A soft starter typically reduces this to 3 to 4 times I_n, which significantly reduces the thermal and mechanical load on the motor and the connected mechanics (gearboxes, couplings, pumps). The build-up time of the voltage and torque is adjustable, often from 1 to 30 seconds, depending on the application. Many soft starters have a bypass contactor, which bypasses the thyristors after completion of the starting phase for maximum energy efficiency and heat dissipation.

2.2. The Frequency Converter (VFD): Variable Speed and Torque Control

A frequency converter, also known as Variable Speed Drive (VSD), offers much more extensive functionality than a soft starter. The basic principle involves three steps:

1. Rectification: The incoming AC supply voltage is converted into a DC voltage with a rectifier (often a diode bridge).
2. DC bus: The rectified voltage is stored in a DC bus, consisting of capacitors, which provide a stable DC voltage and partially smooth harmonic currents.
3. Conversion: The DC voltage is converted back into a variable AC voltage and frequency by an inverter (usually using IGBTs – Insulated Gate Bipolar Transistors). This is done via Pulse Width Modulation (PWM).

By varying both frequency and voltage, a VFD can precisely control the speed and torque of a standard three-phase induction motor over a wide range (typically 0 to 400 Hz). This not only enables controlled ramp-up and ramp-down, similar to a soft starter, but also continuous speed control, positioning and energy savings in variable load applications, such as fans and pumps. Modern VFDs offer advanced control modes such as vector control (Field Oriented Control) for high dynamics and accuracy, and sensorless vector control for cost-effectiveness.

3. Technical Specifications & Standards

The selection and implementation of motor controls require compliance with relevant industry standards and specifications.

3.1. Softstarters: Compliance and Performance

• IEC 60947-4-2: This International Standard specifies the requirements for AC solid-state motor starters and controllers. It regulates performance, testing methods and safety aspects.
• Starting current reduction: Soft starters can typically reduce the starting current to 3-4 I_n, which is crucial for reducing thermal stress on the motor windings and mechanical shock to the drive. This contributes to a longer service life of the motor (MTBF increase of 10-20% compared to DOL start) and connected components.
• Start time and profile: Adjustable start and stop times (e.g. 0.5 to 60 seconds) and start profiles (constant current, voltage ramp) provide flexibility.
• Bypass functionality: An internal or external bypass contactor reduces heat development in the thyristors after starting, which increases efficiency (power losses of thyristors are approximately 2 W per ampere) and extends the life of the soft starter.
• Protection functions: Built-in protection against overcurrent, undercurrent, phase loss, motor overtemperature (via PTC or thermistor), and excessively long ramp-up times in accordance with IEC 60947-4-2.

3.2. Frequency Converters: Comprehensive Regulation and Efficiency

• IEC 61800-3: This standard addresses the EMC requirements for variable speed electric propulsion systems, including limits for harmonic emissions and immunity. Compliance is crucial to avoid interfering with other electrical equipment.
• IEC 61800-5-1: Specifies the safety requirements for electric propulsion systems.
• EN 50598: The eco-design standard for drive systems, which provides guidelines for energy efficiency and ecological aspects of VFDs and motors. The efficiency of VFDs can reach 98-99%.
• Control Range and Accuracy: VFDs provide a wide speed control range (e.g. 0.1 Hz to 400 Hz) with high accuracy (typically ±0.1% in vector control), essential for precision processes.
• Harmonic Distortion (THD): The use of VFDs introduces harmonic currents and voltages into the power supply. Limits for this are defined in standards such as IEEE 519-2014. Passive or active filters (e.g. active front-end VFDs) are often necessary to limit THD to below 5-8%.
• Built-in Functionality: Many VFDs integrate advanced PLC functionality, PID controllers and communication protocols (Modbus, Profibus, Ethernet/IP) according to IEC 664-1, for seamless system integration.

4. Selection & Sizing Guide

The choice between a soft starter and a frequency converter depends on specific application requirements. A structured approach is necessary.

4.1. Important Considerations

1. Speed control: Is it necessary to continuously adjust the engine speed? If the answer is yes, then a VFD is the only option.
2. Starting characteristics: Is it purely about reducing starting current and mechanical shocks at a fixed speed? Then a soft starter is often sufficient.
3. Energy savings: In variable torque applications (fans, pumps), speed reduction with a VFD can lead to significant energy savings (cubic law: power ~ (speed)³). In constant torque applications the savings are less pronounced.
4. Cost: Soft starters are generally cheaper to purchase and install than VFDs, especially for capacities above 110 kW.
5. Complexity: VFDs are more complex in programming and commissioning.
6. Grid disturbances: VFDs can cause harmonic disturbances. This may incur additional filter costs.
7. Motor Protection: Both provide protection, but VFDs measure continuous current and provide more advanced motor monitoring.

4.2. Decision Matrix for Motor Control

The following table provides structured guidance for selecting the appropriate motor control technology.

5. Installation & Commissioning Best Practices

Correct installation and commissioning are essential for the optimal operation and service life of motor controls. NEN 3140 and EN 60204-1 provide guidelines for electrical safety and machine equipment.

5.1. General Principles

• Earthing: Correct earthing in accordance with IEC 60364-4-41 is fundamental for safety and EMC performance. Use short, thick ground conductors.
• Cabling: Segregate power cables from control and communications cables to minimize inductive coupling and EMC problems.
• Cooling: Provide adequate ventilation. The permitted ambient temperature for many components is 40°C. A temperature increase of 10°C halves the lifespan of electrolytic capacitors.

5.2. Specific Guidelines

Soft starters:

• Wiring: Follow the manufacturer's diagram carefully. Note the bypass connection; in delta configuration, a 6-wire connection can relieve the soft starter.
• Parameter setting: Correctly set the ramp-up and ramp-down times, starting voltage/current limit and motor protections (overcurrent, thermal) based on the motor board data and application requirements.
• Thermal Capacity: Check the rated current and duty cycle of the soft starter. A soft starter that is too small will overheat during heavy or frequent starts.

Frequency converters (VFDs):

• Motor wiring: Use shielded, symmetrical cables between VFD and motor to limit EMC emissions and prevent bearing damage. The maximum cable length is crucial (often up to 100-200 meters without additional filters).
• Filters: If necessary, install EMC filters (RFI filters) on the input side according to IEC 61800-3 category C2 or C3. Du/dt filters or sine filters may be required to protect motor winding insulation over long cable lengths.
• Parameter setting: Always enter the exact motor plate data. Run an autotune function for optimal performance. Configure all protection functions (overcurrent, overvoltage, undervoltage, earth fault, motor overtemperature via PTC).
• Cooling: VFDs generate significant heat. Ensure sufficient airflow and consider VFDs with through-cooling (push-through cooling) for installation in smaller cabinets.

6. Malfunctions & Cause Analysis

Quickly and correctly identifying failure modes is essential for minimizing downtime. FMEA (Failure Mode and Effects Analysis) is a structured method to analyze potential failures.

6.1. Common Failure Modes Soft Starters

• Thyristor failure: Can occur due to overvoltage (e.g. lightning strike, switching surges) or overcurrent (short circuit, motor blocking). Visual indication: burnt or cracked thyristor housing.
• Insufficient cooling: Leads to overheating of the semiconductors, shortened lifespan. The cause could be clogged fans or insufficient cabinet ventilation.
• Motor overheating: Often a result of too long ramp-up times or too frequent starts, where the soft starter exceeds the thermal capacity of the motor.
• Bypass contactor failure: Wear of contacts during frequent starts, resulting in contact resistance and heat generation.

6.2. Common Frequency Converter Failure Modes

• IGBT failure: The power modules (IGBTs) are sensitive to overvoltage (regenerative voltages), overcurrent (short circuit), and overtemperature. Visual indication: burned IGBT ports or packets.
• DC bus problems: Overvoltage (e.g. due to regenerative energy when braking without braking resistor) or undervoltage (due to insufficient mains voltage). Capacitor swelling in the DC bus is an indicator of aging or thermal stress.
• Harmonic overload: Too many harmonics in the network can cause resonance and overheating of transformers and cables. This can affect the VFD and lead to malfunctions.
• Motor bearing damage: Circulating bearing currents, caused by common mode voltages from the VFD, can lead to electrical erosion of the bearing surfaces. Use insulated bearings or ground brushes (according to NEMA MG1-2016, Part 31).
• Cooling fan defective: The fan is a critical component. Failure will result in overheating and shutdown of the VFD. Periodic inspection and replacement are necessary (MTBF fans: typically 50,000 hours).

7. Predictive Maintenance & Condition Monitoring

Predictive maintenance focuses on detecting potential failures before they occur, resulting in planned downtime and cost savings. Condition monitoring strategies in accordance with ISO 17359.

7.1. Soft starters

• Thermal monitoring: Periodic thermographic inspection with an IR camera to identify hot spots on thyristors, heat sinks and wiring connections. A temperature difference of >10°C between phases is an indication of a problem.
• Current profile analysis: Analysis of the inrush current and time can identify changes in load or motor condition. Trends in the inrush current may indicate increasing mechanical resistance.
• Contactor inspection: Visual inspection for wear of the contacts of the bypass contactor, measurement of contact resistance.

7.2. Frequency converters

• Harmonic analysis: Regular measurement of the total harmonic distortion (THD) in the supply current and voltage with a power quality analyzer. Deviations from IEEE 519-2014 limits indicate problems.
• Vibration analysis: Condition monitoring of motor bearings via vibration measurements in accordance with ISO 10816. High frequency spikes can indicate electrical erosion by VFDs.
• Temperature monitoring: Monitoring the temperature of IGBT modules, heat sinks and DC bus capacitors. Many VFDs have internal sensors that make this data available via communication buses.
• DC bus voltage analysis: Trend analysis of the DC bus voltage can provide insight into the condition of the DC bus capacitors. An increase in ripple voltage indicates loss of capacity.
• Cooling fan check: Regular inspection for contamination and function. Replacement according to manufacturer guidelines.
• Partial Discharge (Partial Discharge): At higher voltages (>1000V), PD measurements can detect insulation faults in motor windings, often exacerbated by the high dU/dt of VFDs.

8. Comparison Matrix: Detailed Analysis

The following table provides a detailed comparison between soft starters and VFDs based on various technical and economic factors, crucial for an informed decision.

9. Conclusion

The choice between a soft starter and a frequency converter is not a matter of 'better', but of 'correct' for the specific application. Where the main objective is controlled, jerk-free starting and coasting at a fixed engine speed, soft starters provide a cost-effective and reliable solution, which also provides protection for the engine and mechanical drivetrain. For applications requiring continuous speed control, precise torque control, advanced positioning capabilities and significant energy savings under variable loads, the frequency converter is the undisputed choice.

Accurately evaluating operational requirements, required control functionality and total cost of ownership (TCO) is crucial. UNITEC-D GmbH is a reliable partner and supplier of a wide range of high-quality motor control components, including soft starters and frequency converters, from certified manufacturers. With our expertise we support you in selecting and implementing the most suitable technology for your industrial processes.

Visit the UNITEC-D e-catalogue for a comprehensive range of motor controls and related components.

10. References

1. IEC 60947-4-2: Low-voltage switchgear and controlgear - Part 4-2: Contactors and motor-starters - AC semiconductor motor controllers and starters.
2. IEC 61800-3: Adjustable speed electrical power drive systems - Part 3: EMC requirements and specific test methods.
3. IEEE 519-2014: IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems.
4. NEMA MG1-2016: Motors and Generators - Part 31: Definite Purpose Inverter-Fed Polyphase Motors.
5. Key information for Energy Efficiency in Industrial Motors, Agoria, 2018.