1. Introduction
The design and maintenance of industrial electric drives requires a clear understanding of the operation modes of asynchronous motors. Direct start (DOL — Direct On Line) is accompanied by starting currents at the level of 5–8 times the nominal value ($I_n$) and significant electrodynamic forces on windings and mechanical gearboxes. For Ukrainian industrial enterprises, where a large part of the electric motor fleet is operated beyond the standard resource, the reduction of starting loads is a key factor in preventing premature failures. Reliability engineers and power engineers are faced with a choice between two main starting limiting technologies: soft starters and variable frequency drives (VFDs). The wrong choice leads to either premature failure of the mechanical part due to shock loads, or excessive capital costs and power losses due to higher harmonics. In this technical article, we will conduct a detailed comparative analysis of both technologies according to current standards.
2. Fundamental principles
The induction motor with a short-circuited rotor is, by its physical nature, a transformer with a variable air gap and a moving secondary circuit. When applying the nominal voltage $U_n$ during the sliding stop $s = 1$, which causes the maximum consumption of reactive power and a high starting moment $M_p = (1.5 - 2.5) M_n$.
The principle of operation of soft start devices (PPP)
The PPP controls the current value of the voltage supplied to the motor stator by means of thyristors (trimistors or counter-parallel connected thyristors in each phase), changing the opening angle (phase control). The network frequency remains unchanged ($f = 50 ext{Hz}$). Since the starting torque of an induction motor is proportional to the square of the voltage ($M \propto U^2$), reducing the voltage reduces the starting current and torque. However, at the same time, the ability of the engine to overcome the resistance of the mechanism also decreases, which requires a careful calculation of the starting characteristic.
The principle of operation of frequency converters (IF)
Inverters implement a complete conversion of electricity: they rectify the mains voltage through a diode or thyristor rectifier into a constant voltage of the intermediate circuit (filter), and then invert it using IGBT transistors using the Pulse Width Modulation (PWM / PWM) method into an alternating voltage with adjustable amplitude and frequency. This allows you to maintain a constant voltage-to-frequency ratio ($U/f = ext{const}$) or use vector control (with or without a fast feedback sensor) to provide the motor's full rated torque (up to $200\%$) at any frequency from 0 to 50 Hz and above.
3. Technical specifications and standards
The design and selection of equipment must be carried out in strict compliance with international and national standards:
- DSTU EN 60947-4-2: Low-voltage distribution and control equipment. Part 4-2: Contactors and motor starters. Semiconductor alternating current motor starters and regulators (PPP).
- DSTU EN 61800-5-1: Power electric drives with adjustable speed. Part 5-1: Safety requirements. Electrical, thermal and energy.
- IEC 60947-4-1: Electromechanical contactors and starters.
- ISO 9001 / ISO 14001: Quality management standards and environmental management of production processes.
The IEC/EN 60947-4-2 classification of UPS operating modes is determined by use category (eg AC-53a for current-limited starting with shunt contactor and AC-53b without shunt). In the passport data of the PPP, the permissible overload capacity is indicated, for example, $4 \cdot I_n$ for 20 seconds with a duty cycle of 10 starts per hour.
4. Guidance on selection and selection
The choice between the PPP and the inverter is based on the technological requirements of the mechanism for speed, torque and energy efficiency. The following decision matrix systematizes the selection criteria for mechanical and energy engineers.
| Evaluation criterion | Soft start device (PPP) | Frequency converter (IF) |
|---|---|---|
| Speed control | Impossible (start and stop only) | Smooth adjustment in a wide range (0-400 Hz) |
| Starting torque ($M_p$) | Limited by the square of the voltage ($30-70\% M_n$) | Full torque ($100-200\% M_n$) from zero speed |
| Energy efficiency in working mode | High (when using bypass, there are no additional losses) | High at partial loads, but there are losses on the inverter ($2-4\%$) |
| Harmonic Distortion (THD) | Low (short-lived during start-up) | Constant higher harmonics in the network (need filters or chokes) |
| Capital Expenditure (CAPEX) | Low / moderate | High |
| Operating expenses (OPEX) | Low (fewer power electronics components) | Moderate (control of filters, cooling, capacitors) |
| Typical applications | Pumps, fans, conveyors, mills | Extruders, crane mechanisms, presses, dispensers |
The expression for the dissipated energy is used to calculate the thermal load of the PPP during start-up:
E = \int_{0}^{t_p} I(t)^2 \cdot R_{eq} \, dtWhere $t_p$ is the start-up time, $I(t)$ is the instantaneous value of the start-up current, $R_{eq}$ is the equivalent resistance of the circuit. For the supply of reliable industrial automation and power electrical components, it is recommended to refer to the UNITEC-D GmbH catalog.
5. Best Practices for Installation and Commissioning
Correct installation determines the trouble-free operation of the electric drive with a MTBF of more than 100,000 hours.
- Ventilation and thermal regime: PPP and inverter emit significant thermal energy (approximately 2–3% of the nominal power of the equipment). Control layers must provide forced ventilation with calculated air exchange and protection class not lower than IP54 for workshop conditions.
- Electromagnetic compatibility (EMC): The cables from the inverter to the motor must be shielded (asymmetrical symmetrical cable with grounding core). The screen is connected to a circular clamp (360°) at both ends or according to the manufacturer's instructions to minimize high-frequency emissions (according to EN 61800-3).
- Surge protection: Class II (type 2) surge arresters (SPDs) must be installed at the input of the supply lines to protect semiconductor elements from switching impulses in the industrial network 380/400 V (±10%), 50 Hz.
- Parameter settings: Acceleration and deceleration time must be consistent with the swing moment of the mechanism ($GD^2$). It is forbidden to set the acceleration time less than the recommended time to prevent overheating of the PPP thyristors or tripping of the IGBT protection of the inverter.
6. Failure modes and root cause analysis (RCA)
Maintenance statistics of industrial enterprises show the typical reasons for failure of starting equipment:
- PPP thyristor breakdown: Occurs as a result of frequent starts (exceeding the number of starts per hour according to the AC-53a category) or overvoltages in the network. Visual indicator: phase error, activation of the input fuse type $aR$ (high-speed semiconductor protection).
- Failure of power IGBT modules of the inverter: The reason is switching overvoltages, failure of cooling systems (radiators clogged with dust, fan failure) or short circuits in the cable line to the motor. Visual indicator: Overcurrent (OC — Overcurrent) error code, activation of the drive's internal protection.
- Leakage of capacitors of the intermediate circuit of the IF: Consequence of long-term operation at high temperatures (>40 °C). A decrease in capacity leads to the appearance of rectified voltage ripples and an emergency stop with a DC bus undervoltage (UV — Undervoltage) code.
7. Predictive maintenance and condition monitoring
Modern concepts of Industry 4.0 require a transition from planned and preventive repairs to maintenance according to the technical condition (PdM — Predictive Maintenance). The following methods are used for semiconductor control systems:
- Thermographic control (Thermal vision): Periodic scanning of contact connections, power terminals and cooling radiators using infrared cameras. Exceeding the temperature of the contacts above 75 °C indicates loosening of the bolted connections or oxidation.
- Monitoring of harmonics and power consumption: Power quality analyzers record the level of total harmonic distortion of current and voltage ($THD_i$, $THD_u$), which allows timely detection of filter element degradation.
- Control of the motor insulation resistance: A megaohmmeter or built-in systems for checking the insulation resistance of the motor windings before each start-up warn of insulation breakdown on the housing ($R_{is} < 0.5 ext{ MΩ}$ at operating voltage).
8. Comparative matrix of technologies
| Technical parameter | Direct Start (DOL) | Soft start device (PPP) | Frequency converter (IF) |
|---|---|---|---|
| Starting current ($I_p / I_n$) | 5.0 - 8.0 | 2.5 - 4.0 | 1.0 - 1.5 |
| Speed control range | Fixed (100%) | Fixed (start/stop smooth) | 0 – 100%+ (smooth adjustment) |
| Mechanical shock at start-up | Maximum (high gear wear) | Minimum (adjustable torque) | Absent (soft acceleration) |
| Difficulty of installation | low | average | High (requires EMC measures) |
| Estimated resource (MTBF, h) | 50,000 | 80,000 - 100,000 | 60,000 - 90,000 |
9. Conclusion
The choice between a soft starter and a frequency converter should be based on a careful engineering calculation of the technological requirements of the mechanism, energy parameters of the network and economic feasibility. For non-regulated mechanisms with a constant rotation speed (centrifugal pumps, general purpose fans), the PPP is an optimal, reliable and cost-effective solution. For processes that require precise speed regulation, torque maintenance at low speeds or optimization of energy consumption in variable modes (conveyors with different loads, extruders, mixers), the only correct choice remains a frequency converter.
To ensure the uninterrupted operation of your enterprise and the selection of quality industrial components, equipment for soft start and frequency regulation, use our product catalog: https://www.unitecd.com/e-catalog/.
10. Links
- DSTU EN 60947-4-2:2015. Low-voltage distribution and control equipment. Part 4-2: Contactors and motor starters.
- DSTU EN 61800-5-1:2015. Power electric drives with adjustable speed. Part 5-1: Safety requirements.
- IEC 60947-4-1:2018. Low-voltage switchgear and controlgear - Part 4-1: Contactors and motor-starters.
- IEEE Std 519-2014. IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems.
- Technical Whitepapers on Industrial Motor Control, Siemens AG & Schneider Electric, 2021-2023.
