1. Introduction
In modern industrial motor control systems, the choice between a soft starter and a variable frequency drive (VFD) is critical to ensuring the reliability of production processes. Asynchronous motors, when started directly from the network, consume a current that is 6-8 times higher than the nominal current, which creates a significant mechanical load on couplings, reducers and the motor shaft. In addition, it causes the voltage to sag in the network, which can adversely affect the operation of other sensitive equipment.
For mechanical engineers and reliability specialists, the choice between PPD and FRP is based not only on cost, but on the technical requirements of the process. The purpose of this material is to provide clear selection criteria based on physical operating principles and industry standards.
2. Fundamental principles of work
The soft start device (PPP) is based on the use of power semiconductor elements - thyristors (simistors). By phase control of the opening moment of the thyristors, the PPP limits the effective voltage applied to the motor stator during start-up. This allows you to smoothly increase the engine torque. Once rated speed is reached, the PPT is usually shunted by a contactor to reduce energy losses.
The variable frequency drive (VFD) provides complete control over the speed and torque of the motor by changing the frequency and voltage (U/f law). The CHP converts the alternating current of the network into direct current (direct current link), and then, with the help of pulse width modulation (PWM) on IGBT transistors, forms an alternating voltage with an adjustable frequency at the output. This provides full control over the dynamics of the engine during the entire operating cycle.
3. Technical specifications and standards
The design and selection of devices must meet international standards:
- IEC 60947-4-2: Low-voltage switchgear - Part 4-2: Contactors and motor starters - AC semiconductor controllers and starters.
- IEC 61800-2 / 61800-5-1: Variable speed electric drives - Safety requirements and specifications.
- IEEE 519: Recommended Practice and Requirements for Controlling Harmonics in Electrical Systems (Critical for PSCs).
When choosing devices, it is necessary to take into account the start-up class (class 10, 20 or 30 for IEC 60947-4-2), which determines the permissible start-up time and load capacity.
4. Selection and design criteria
The choice of device depends on the type of load and the required control flexibility.
| Criterion | Soft start device (PPP) | Frequency-controlled drive (CRP) |
|---|---|---|
| Speed control | Impossible (start/stop only) | Full adjustment |
| Starting moment | Limited (up to 50-70% of nominal) | High (up to 150-200% of nominal) |
| Harmonic distortions | Low | High (need chokes) |
| Energy efficiency | High (during operation) | Highest (due to speed control) |
| Dimensions | Compact | Larger (requires cooling) |
5. Installation and commissioning
Ensuring sufficient heat exchange is critical for PPPs, as thyristors generate a significant amount of heat during startup. The use of an external bypass contactor is mandatory for long-term operation.
Critical aspects for PRC are:
- Protection against electromagnetic interference (EMC). Use of shielded motor cables.
- Installation of output chokes or sine filters to protect the insulation of the motor windings from voltage peaks (dv/dt).
- Consideration of harmonics in the power supply network and installation of input reactors or filters (LCL filters).
6. Typical failures and analysis of reasons
PPP refusals:
- Overheating of thyristors due to frequent starts (start class higher than calculated).
- Failure due to incorrectly selected shunt contactors.
- Breakdown of thyristors due to switching overvoltages in the network.
ChRP refusals:
- Degradation of electrolytic capacitors in the DC circuit (typical service life 5-8 years).
- Failure of IGBT modules due to short circuits or overheating.
- Insulation breakdown of motor windings (caused by long unfiltered cables and high dv/dt).
7. Projected maintenance
Maintenance includes:
- Thermographic control: Checking contact connections and cooling radiators.
- Analysis of harmonics: Use of power quality analyzers (according to IEEE 519) to control the impact of PRC on the network.
- Vibration diagnostics of the engine: Checking the absence of mechanical resonances during the operation of the CHRP at low frequencies.
- Monitoring the state of capacitors: Checking the capacity and leakage current in the PRC.
8. Comparative matrix
| Parameter | PPP (Soft Starter) | VFD |
|---|---|---|
| Typical application | Pumps (direct start), conveyors, fans | Extruders, presses, dispensers, adjustable pumps |
| Difficulty setting up | low | High |
| Cost (at 45 kW) | Basic | High (x2.5 - x3) |
| Functional | Smooth start/stop | Full dynamics, PID control, communication |
9. Conclusion
PPP is the optimal choice for systems where only a soft start is needed to reduce shock loads on the mechanical part, and the engine speed remains constant. ChRP is indispensable for technological processes that require dynamic speed control, energy efficiency improvement, or precise adjustment of parameters (pressure, flow). UNITEC-D GmbH provides the supply of certified PPPs and PRPs from leading manufacturers, and also conducts engineering audits to optimize your equipment.
To select the optimal solution, visit our electronic catalog: https://www.unitecd.com/e-catalog/
10. List of references
- IEC 60947-4-2: Low-voltage switchgear and controlgear - Part 4-2.
- IEC 61800-5-1: Adjustable speed electrical power drive systems - Part 5-1.
- IEEE 519-2014: IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems.
- UNITEC-D Operating Instructions: "Drive Control Systems in Industry".
