1. Introduction
The reliability of electric drives is a critical condition for the continuity of technological processes in industry. Failure of an electric motor due to overload, phase asymmetry or rotor locking leads to significant economic losses associated with line downtime. Understanding the principles of operation of engine protection devices allows you to minimize these risks and increase the operational resource of the equipment. This guide focuses on the technical characteristics and methods of selecting modern means of protection.
2. Fundamental principles
Motor protection is based on limiting the thermal effect of the current on the stator windings. According to the Joule-Lenz law, the release of heat is proportional to the square of the current: Q = I²Rt. When overloaded, the temperature of the insulation rises, which leads to its degradation. The time-current characteristic (I²t) is the basis for the selection of the protection device, as it must match the heating curve of the motor, allowing inrush currents, but disconnecting the supply in the event of a sustained overshoot of the rated current (FLC).
3. Technical standards
The design and selection of components must meet international standards:
- IEC 60947-4-1: Requirements for contactors and motor starters.
- IEC 60947-8: Requirements for overheating protection devices (thermistor protection).
- IEC 60034-1: Nominal data and operating characteristics of rotating electric machines.
Compliance with these standards ensures that the protection device will provide safe operation within the design parameters.
4. Selection and selection of type size
The choice of the relay depends on the decoupling class, which determines the operating time at a current of 7.2xIr. The table below helps to determine the required class for different types of load.
| Uncoupling class | Type of load | Typical application |
|---|---|---|
| 5 | Easy start | Fans, pumps |
| 10 | Standard | Conveyors, compressors |
| 20 | Difficult launch | Centrifuges, mills |
| 30 | Overweight | Crushers, mixers |
5. Installation and commissioning practices
To ensure type 2 coordination (according to IEC 60947-4-1), it is necessary to correctly select the automatic motor protection switch (MCB) and the contactor. In the event of a ground fault or short circuit, the protective device must guarantee the absence of damage to the contactor and the possibility of its further operation. It is recommended to check the tightness of the contact connections after 24 hours of operation under load to prevent overheating of the terminals.
6. Typical malfunctions and root cause analysis
The most common causes of outages:
- Asymmetry of phases: A voltage skew of more than 5% causes a significant increase in current in one of the phases, which leads to local overheating.
- Frequent starts: Exceeding the number of starts per hour does not allow the engine to cool down.
- Rotor lock: Causes a current surge that requires immediate shutdown.
7. Projected maintenance
Modern intelligent relays allow protection to be integrated into condition monitoring systems. Analysis of current consumption (current signature analysis) makes it possible to detect degradation of bearings or problems in mechanical transmission before an emergency situation occurs. The data is transmitted via field buses (Modbus, Profinet) to the PLC for further processing.
8. Comparative matrix of protection devices
| Characteristics | Thermal relay | Electronic relay | Intelligent relay |
|---|---|---|---|
| The principle of action | Bimetallic plate | Microprocessor | Microprocessor |
| Accuracy | average | High | Very high |
| Phase asymmetry | Limited | Yes | Yes |
| Communication | Ні | Ні | Yes (Fieldbus) |
| Price | low | average | High |
9. Conclusion
The choice between thermal, electronic and intelligent relays depends on the criticality of the process and the diagnostic requirements. For simple drives, bimetallic relays are enough, while for responsible mechanisms, the use of intelligent control systems is a justified tool for increasing reliability. UNITEC-D GmbH offers a wide range of certified components for engine protection, compatible with the requirements of Ukrainian production standards. Check out the full range in our catalog: https://www.unitecd.com/e-catalog/.
10. List of references
- IEC 60947-4-1, Low-voltage switchgear and controlgear - Part 4-1: Contactors and motor-starters.
- IEEE Std 141-1993, IEEE Recommended Practice for Electric Power Distribution for Industrial Plants.
- NEMA ICS 2-2000, Industrial Control and Systems: Controllers, Contactors, and Overload Relays.
