Servo Motor Overheating: Root Cause Analysis - Sizing, Duty Cycle, and Cooling Failure Calculation Errors

1. Introduction

Servo motor overheating is a critical operational problem that can lead to unplanned production shutdowns, reduced equipment life, and significant financial losses. This technical analysis focuses on the HYDAC 2127408 servo motor in use in an industrial plant and systematically examines the typical root causes of overheating: size mismatch, incorrect duty cycle calculation and cooling system failures.

Ignoring the initial signs of overheating can lead to degradation of winding insulation, mechanical damage to bearings and motor failure, making preventive analysis and timely intervention necessary to maintain operational reliability according to DSTU EN 60034-1. standards

2. Overview of the component

The HYDAC 2127408 servo motor is a key element of precision positioning and dynamic control systems in industrial applications such as metalworking machines, packaging lines and robotic complexes. Its task is to provide precise control of angular position, speed and torque. Typical specifications for this class of servo motors include a rated power of 2.2 kW, a rated speed of 3000 rpm and a rated torque of 7 Nm. The motor has insulation class F, which allows a maximum winding temperature of 155°C, and protection degree IP65, which provides protection against dust and water jets. The design mean time between failures (MTBF) is 50,000 hours.

Servo motors are typically equipped with built-in temperature sensors (such as thermistors or RTDs) to monitor winding and case temperatures, which is critical to meeting the temperature limits defined in EN 60034-1.

3. Evidence of refusal

During the routine inspection of the HYDAC servomotor 2127408, the following signs of overheating were recorded:

• Temperature measurement: The infrared thermometer showed a surface temperature of the motor housing of 85°C, well above the nominal operating temperature of 60°C. The built-in winding temperature sensor reading through the control system was 160°C, exceeding the permissible limit of 155°C for insulation class F.
• Visual signs: Discoloration and paint burning on the engine housing, especially around the ventilation shroud. There was a sharp smell of burnt insulation.
• Vibration Analysis: Vibration measurements with an accelerometer mounted on the engine housing revealed a total vibration level of 9.2 mm/s SWR, which exceeds the upper limit of "acceptable" condition (7.1 mm/s SWR) for small machines according to ISO 10816-3. This indicates bearing degradation or rotor imbalance due to thermal expansion.
• Control System Data: The PLC (Programmable Logic Controller) alarm logs contained frequent "Motor Over Temperature" and "Over Current" warnings. The average current consumption of the motor was 18% higher than the nominal one.
• Degraded performance: Operators have reported intermittent failures in positioning accuracy and reduced maximum speed under load.
• MTBF reduction: The actual engine life before these symptoms occurred was approximately 10,000 hours, while the design MTBF is 50,000 hours.

4. Study of root causes

For a systematic study of the root causes of overheating, the "5 why" methodology and analysis according to the Ishikawa diagram were applied:

1. Why did the motor overheat? The temperature of the windings exceeded the permissible limits.
2. Why did the temperature of the windings exceed the limits? Excessive heat generation or insufficient heat dissipation.
3. Why excessive heat generation/insufficient heat dissipation?
• Option A (Excessive heat generation): The engine is operating at an overload or under conditions that do not correspond to its designed duty cycle.
• Option B (Insufficient heat removal): The cooling system is not working efficiently or external conditions prevent cooling.
4. Why is the motor overloaded/duty cycle incorrect (Option A)?
• Sizing errors: Initial motor selection was made without consideration of peak torques, load inertia, or system dynamic characteristics. The engine turned out to be undersized for the actual needs of the application.
• Incorrect duty cycle calculation: Changes in the manufacturing process resulted in increased continuous operation time, increased acceleration/deceleration frequency, or prolonged load retention for which the motor was not designed.
5. Why is the cooling system inefficient (Option B)?
• Cooling system failure: Contamination of radiators, blocking of ventilation holes, malfunction of the cooling fan or pump (for liquid cooling), clogging of filters.
• Insufficient coolant quality: Low coolant level or contamination (for liquid systems).
• High ambient temperature: Operation of the engine in conditions where the temperature of the ambient air (or coolant) exceeds the permissible limits (for example, >40°C), which reduces the efficiency of heat dissipation.

5. Identified root causes

Based on data analysis and research, the following root causes of HYDAC 2127408 servo motor overheating were identified:

1. Undersized motor (probability 40%):
   
   • Evidence: Increased current consumption by 18% above nominal, frequent activation of overload protection. Calculations showed that the peak torque required to accelerate the load exceeds the rated torque of the motor by 35%.
   • Consequence: The engine is constantly operating in a mode close to overload, which leads to excessive heat generation according to the Joule-Lenz law (Q = I²RT).
2. Miscalculation of duty cycle (35% probability):
   
   • Evidence: Analysis of the PLC data showed that the motor runs continuously for 85% of the operating time, whereas it was originally selected for S3 mode (repeated short-time mode with frequent stops) with a relative duty cycle (RDU) of 60%. This leads to heat build-up.
   • Consequence: The motor does not have enough time to cool down between duty cycles, resulting in an increase in the average temperature of the windings.
3. Inefficiency of the cooling system (probability 25%):
   
   • Evidence: Significant contamination of the cooling radiators with dust and oil deposits was detected, which reduces the efficiency of heat removal by 30%. The ambient air temperature in the engine installation area reached 45°C due to a malfunction of the workshop ventilation system.
   • Consequence: Even at nominal load, the engine cannot effectively remove heat due to deterioration of heat exchange with the environment.

6. Corrective measures

6.1. Undersized engine

• Immediate Solution: Reduce workload and/or process speed by 15% prior to motor replacement to reduce current load and heat generation.
• Long-term prevention: Perform a full engineering recalculation of torque and load inertia requirements. Replace the HYDAC 2127408 servo motor with a model with appropriate characteristics (eg 3.5 kW, 10 Nm) or consider using a gear reducer to reduce the load on the motor. Follow the DSTU EN 60204-1 recommendations regarding the selection of electrical equipment.

6.2. Incorrect duty cycle calculation

• Immediate Solution: Modify the PLC program to introduce short pauses (5-10 seconds) between heavy duty cycles to allow partial motor cooling.
• Long-term prevention: Optimize the motion control algorithm to reduce the time of intense accelerations/decelerations and the time of holding torque. Review the specifications for the equipment, taking into account the actual mode of operation. It is possible to switch to an engine with a higher heat resistance class of insulation (for example, class H) or with a forced cooling system designed for S1-mode (continuous rated mode).

6.3. Inefficiency of the cooling system

• Immediate solution: Thoroughly clean radiators and engine ventilation ducts of dirt. Check and restore the shop ventilation system to reduce the ambient temperature to <40°C.
• Long-term prevention: Implement a regular cleaning schedule for engine cooling systems (for example, quarterly) using compressed air and non-aggressive cleaning agents. Install dust filters on the ventilation openings of the engine and housing. Consider installing additional fans or an air conditioning system to maintain a stable temperature in the operating area in accordance with ISO 13849-1 safety requirements for machines and their components.

7. Express diagnostic checklist for technicians

This checklist is intended for a quick assessment of the condition of the servomotor directly in the workshop. Use it on your tablet.

"Red flags" (early warning signs):

• Localized hot spots (>80°C) on the body detected by the thermal imager.
• Intermittent engine temperature warnings that disappear after a short stop.
• A small but steady increase in current consumption (>5% of nominal) for several days.
• An increase in the noise level of bearings or a small backlash of the shaft during manual inspection.

8. Prevention strategy

An effective strategy to prevent overheating of servomotors requires a comprehensive approach covering design, installation, operation and maintenance:

• Condition Monitoring: Implementation of systems for continuous monitoring of the temperature of windings and bearings, as well as vibration analysis (according to ISO 20816-1 and DSTU ISO 10816-1) for early detection of deviations. Thermographic control using thermal imagers during routine inspections. Monitoring of motor current consumption.
• Regular maintenance: Strict adherence to cleaning schedules of cooling systems (fins, fans, filters) from contaminants that reduce heat dissipation efficiency. Checking the tightness of liquid cooling systems and the quality of the cooling liquid.
• Correct selection and calculation: Detailed analysis of the load profile (torque, speed, inertia, duty cycle) at the design stage. Using software to simulate the operation of the system and the exact selection of the servo motor. Ensuring sufficient power reserve (eg 15-20% torque) to compensate for unforeseen conditions.
• Environmental control: Ensure adequate ventilation and maintain optimal temperature (<40°C) in the work area where the servo motors are installed.
• Staff training: Conducting regular trainings for engineers and technicians on diagnostics, maintenance and optimization of servo systems.

9. Conclusion

Overheating of the HYDAC 2127408 servo motor, like any other critical component, is not simply a technical problem, but an indicator of systemic deficiencies in design, operation, or maintenance. A comprehensive approach to root cause analysis, including thorough evidence gathering, structural analysis, and implementation of both immediate and long-term corrective actions, is necessary to restore reliability and extend equipment life.

Proactive maintenance, constant monitoring of the condition and accurate engineering calculation of parameters are the main keys to efficient and trouble-free operation of industrial servo systems.

To replace components or select new servomotors according to changed operating conditions, please refer to the UNITEC-D E-Catalog for a wide range of compatible solutions and accessories.

10. Links

• DSTU EN 60034-1:2018 (EN 60034-1:2010, IDT; IEC 60034-1:2010, IDT) Electric rotating machines. Part 1. Nominal operating modes and operational characteristics.
• DSTU EN 60204-1:2018 (EN 60204-1:2006, IDT; IEC 60204-1:2005, IDT) Machine safety. Electrical equipment of machines. Part 1. General requirements.
• ISO 10816-3:2009 Mechanical vibration — Evaluation of machine vibration by measurements on non-rotating parts — Part 3: Industrial machines with nominal power above 15 kW and nominal speeds between 120 r/min and 15,000 r/min when measured in situ.
• ISO 20816-1:2016 Mechanical vibration — Measurement and evaluation of machine vibration — Part 1: General guidelines.
• ISO 13849-1:2023 Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design.
• Manufacturer Guidelines for HYDAC Servo Drives and Motors.