1. Introduction
In August 2023, a critical situation arose at one of the workshops of a Ukrainian food industry enterprise: the main drive of the packaging line stopped every 4-6 hours due to the tripping of a protective thermal relay. The technician replaced the starter assembly three times during the week, but the problem persisted. Such shutdowns not only reduce overall equipment effectiveness (OEE), but also lead to thermal cycling of the motor windings, which shortens their life. Our task as engineers at UNITEC-D GmbH is to carry out a systematic root cause investigation according to the failure analysis methodology.
2. Overview of the component and operating conditions
The thermal relay is designed to protect asynchronous electric motors from overload, phase interruption and asymmetry of currents. It is located in the power control cabinet, directly after the AC contactor. In this system, a motor with a nominal power of 15 kW (400 V, 28.5 A) is used, which works in the repeated short-term S3 mode (PV 40%).
The mechanical part is controlled through a high-precision gearbox and spindle assembly, where the key elements of rotation are precision radial thrust ball bearings, such as SKF 7007 CD/P4ADGA, designed for high axial and radial loads at maximum speeds of up to 12,000 rpm. Any deviation in the electrical or mechanical part is instantly reflected in the consumption current profile.
3. Opt-Out Evidence and Data Collection
During the visit to the object, the specialists of our service department carried out a set of diagnostic measures. The following parameters are fixed:
- Actual operating current in stable mode: 26.2 A (below the nominal value of the relay 29.0 A).
- Air temperature inside the distribution cabinet: +52 °C (at the standard relay calibration temperature of +20 °C according to EN 60947-4-1).
- Vibration amplitude on the engine bearing shield (ISO 10816-3): 1.8 mm/s (within the norm for zone II class).
- Thermogram of bimetallic relay plates: local overheating of contact groups up to +78 °C was recorded.
Technical director's note: "Protection devices often respond not to motor overload, but to a change in the physical environment inside compact control cabinets during the summer."
4. Systematic analysis of root causes (Method 5 Why)
- Why does the thermal relay trip? Because the bimetallic plates are bent to the critical opening position due to excessive heat generation.
- Why is too much heat generated? Due to high inrush current and increased ambient temperature in the control cabinet.
- Why does the temperature in the cabinet exceed the design +35 °C? The forced ventilation system (filter fans) has insufficient performance due to dust clogging.
- Why did the filters clog before the schedule? The service schedule did not take into account the seasonal increase in the concentration of loose materials in the workshop.
- Why did the selection of the equipment not take into account the temperature coefficient? During the design of the control cabinet, the correction for the deratization of the rated current of the relay according to EN 60947-4-1 at temperatures above +20 °C was not applied.
5. Identified root causes (Ranked list)
| The root cause | Probability | The evidence |
|---|---|---|
| 1. Thermal drift due to heating of the cabinet | 65% | The temperature in the cabinet is +52 °C; lack of temperature compensation in the selected series of relays. |
| 2. Inconsistency of the starting profile (opening class) | 20% | Frequent starts in the S3 mode cause heat to accumulate faster than the bimetal cools. |
| 3. Incorrect calculation of the denomination (selection error) | 15% | Selection of the setting without taking into account the starting characteristics of the mechanism with the spindle assembly. |
6. Corrective and preventive actions
To eliminate the current malfunction and prevent similar incidents in the future, an action plan has been developed:
- Immediate remediation: An additional control cabinet air conditioning module was installed and the internal temperature was reduced to +28 °C. The thermal relay setting has been recalculated taking into account the new DSTU requirements EN 60947-4-1.
- Long-term prevention: Switched over to electronic overload relays with extended functionality and independence from ambient temperature in the range from -20 °C to +60 °C.
7. Quick diagnostic checklist for technical personnel
Use this checklist directly in the shop on a tablet when checking the control cabinet:
- [ ] Measure the air temperature inside the control cabinet (standard: up to +35 °C).
- [ ] Check the condition of the forced ventilation filters and, if necessary, blow with compressed air.
- [ ] Take the indicators of phase currents with the help of current-measuring clamps under load.
- [ ] Check the tightening torque of the power terminals (the recommended torque according to the passport is 2.5 Nm).
- [ ] Compare the actual setting of the relay with the passport data of the electric motor.
- [ ] Assess the condition of the contact surfaces for signs of oxidation or burning.
- [ ] Check the integrity of the insulation of cable lines and the absence of microcracks.
- [ ] Record the presence of extraneous vibrations on the rotation transmission mechanism.
8. Strategy to prevent refusals
To achieve an MTBF of more than 50,000 hours of uninterrupted operation, the following must be implemented:
- Regular thermal imaging control of contact connections of switching equipment (every 3 months).
- Checking the cleanliness of the air filters of the control cabinets every month.
- The use of original components and proven rotation units (for example, SKF 7007 CD/P4ADGA bearings to ensure the accuracy of the spindles and the absence of unnecessary mechanical resistance).
9. Conclusion
False activations of thermal relays are almost always the result of shortcomings in the design of the cooling system of the control cabinet or errors in the selection of temperature corrections. A systematic approach to the analysis of the environment and current parameters allows you to completely eliminate idle equipment. A full range of certified switchgear, electrical components and precision parts is kept in stock in our warehouse. Get acquainted with the technical characteristics and place an order through the UNITEC-D E-Catalog.
10. Links
- DSTU EN 60947-4-1:2014 Low-voltage distribution and control equipment. Part 4-1. Contactors and motor starters.
- ISO 10816-3:2009 Vibration. Evaluation of machine vibration by measurements on non-rotating parts.
- SKF General Catalogue: Precision Bearings Technical Handbook.
- IEEE Std 141-1993 IEEE Recommended Practice for Electric Power Distribution for Industrial Plants.
