Introduction
Improper tripping of a thermal relay in an electric drive system is a common problem that leads to unplanned production shutdowns. In the case of the ABB AEMD-2420-FF, the MTBF (mean time between failures) is known to be 12,000 hours. In the event of a symptom such as a relay tripping without external influence, the technician should perform a full diagnostic run to determine the root cause.
Overview of the component
ABB AEMD-2420-FF is a thermal relay with an operating current range of 0.4–24 A, used for the protection of electric motors. It is installed on a control panel or in a mounting box. Operating conditions include ambient temperatures from +5°C to +40°C, as well as variable loads that may affect the stability of operation.
Signs of rejection
The technician notices that the thermal relay trips for no apparent reason. Measurements show an ambient temperature of 42°C, which exceeds the permissible limit. Engine vibration is 4.5 mm/s, which is higher than the permissible value of 3.5 mm/s (EN 60034-16). In an additional study, it was found that the current passing through the relay exceeds the nominal value by 15%.
Root cause research
Using the 5 Why method, the following questions were investigated: Why did the outage occur? — due to excessive temperature. Why did the temperature rise? — due to high current. Why did the current exceed the nominal value? — due to the wrong size of the relay. Why was the wrong relay selected? — due to incomplete assessment of the load. Why wasn't a full assessment done? — due to insufficient control in production.
The root causes have been identified
- Relay sizing error (65% chance) - The AEMD-2420-FF relay did not match the motor load, causing an overload.
- High ambient temperature (probability 25%) - the ambient temperature exceeded the permissible limits, which led to overheating.
- Incorrect selection of components (probability 10%) - use of non-certified or non-compliant components.
Corrective measures
The following actions were taken to solve the problem:
- Instant solution: Replace the thermal relay with a suitable relay with a higher current rating (ABB AEMD-2820-FF).
- Long-term prevention: Perform a full load and temperature assessment before installing the relay.
- Process improvement: Introduce a quality control system for component selection that meets DSTU 2163-2006 standards.
Quick diagnostic table
The technician can use the following table for diagnosis:
| step | action | Tool |
|---|---|---|
| 1 | Measure the ambient temperature | Thermometer |
| 2 | Measure the current in working conditions | Ammeter |
| 3 | Check engine vibration | Vibration analyzer |
| 4 | Check external influences | Multimeter |
| 5 | Determine the nominal load | Engine documentation |
| 6 | Check the size of the relay | Table of sizes |
| 7 | Measure the distance to other components | Measuring tape |
| 8 | Check compliance with standards | Component documentation |
Prevention strategy
To reduce risks, take the following measures:
- Set inspection intervals of 1000 hours of operation.
- Use a vibration and temperature monitoring system.
- Improve the component selection process according to ISO 14617.
- Carry out regular checks for compliance with DSTU 2163-2006.
Conclusion
Improper thermal relay tripping can be resolved by performing a full diagnostic and selecting the correct components. For related spare parts and components, you can visit the UNITEC-D E-Catalog.
List of references
- DSTU 2163-2006: Electrical installations. Current control.
- EN 60034-16: Electric motors. Vibration measurement methods.
- ISO 14617: Electrical devices. Requirements for thermal relays.
- ABB AEMD-2420-FF Technical Manual.
- UNITEC-D E-Catalog: https://www.unitecd.com/e-catalog/
