Root Cause Analysis: Servo motor overheating due to sizing, duty cycle, and cooling system failure

1. Introduction: Symptoms of refusal and initiation of investigation

A production line shutdown often begins with a sudden alarm. In this case, the control system (PLC) detected a critical error of the drive (Error code: Ovt / ERR 14 - Thermal overload of the motor). The protection tripped during the execution of a standard milling cycle on a 3-axis CNC machine. A visual and tactile inspection revealed a surface temperature of the servo motor body of more than 105°C, which was accompanied by a characteristic smell of degradation of the insulating varnish.

Overheating of industrial servomotors is a critical issue, driving mean time between failures (MTBF) down from an expected 40,000 hours to less than 5,000 hours. Every 10°C overshoot of the nominal operating temperature cuts the life of the insulation in half. The purpose of this analysis is to identify the technical causes of overheating, exclude symptomatic treatment of the problem, and implement long-term corrective actions based on the standards of DSTU EN 60034-1.

2. Overview of the component: Functional purpose and operating conditions

The object of analysis is a permanent magnet synchronous servo motor (PMSM) designed for highly dynamic applications. The motor is integrated into the X-axis feed system. Its main task is to provide precise positioning with high acceleration and braking.

• Nominal power (P_n): 4.5 kW
• Nominal torque (M_0): 15 Nm
• Peak torque (M_max): 45 Nm
• Nominal speed: 3000 rpm
• Insulation class: F (maximum temperature 155°C)
• Degree of protection: IP65

The servomotor is equipped with a high-resolution absolute encoder and a system of seals to protect against coolants. In particular, the SKF 1015633 radial shaft seal is used, which ensures the tightness of the front bearing assembly. Working conditions include an ambient temperature of up to 40°C. According to the specifications, the engine is designed to work in the S3 mode (repeated short-term mode) with a duty cycle (DU) of 60%.

3. Actual failure data: Results of technical review

The engineering group conducted a series of diagnostic tests directly at the site of operation before dismantling the unit.

Thermographic analysis

The use of an infrared camera showed localized heating zones. The stator core temperature reached 118°C, while the front flange temperature was 95°C. The back cover, where the encoder is located, heated up to 102°C, which exceeds the permissible limit for the electronic components of the sensor (normally 85°C).

Electrical measurements

Measurement of the insulation resistance with a megohmmeter (test voltage 500 VDC) showed a value of 0.8 MΩ between the phases and the case. According to DSTU EN 60034-27-4, the minimum permissible value for safe operation is 5 MΩ. This indicates the initial stage of insulation breakdown due to thermal aging. The resistance of the windings (R-S, S-T, T-R) remained symmetrical (1.2 ohms), which excludes inter-turn shorting as the root cause.

Mechanical condition and vibration

A vibration spectral analysis (corresponding to ISO 20816-1) revealed an increase in amplitude at 1X frequency (rotating frequency) to 4.5 mm/s (RMS), which is a consequence of thermal expansion and temporary bending of the rotor. During disassembly, it was found that the SKF 1015633 radial shaft seal lost its elasticity, the material (elastomer) hardened and had microcracks. This is a typical sign of exposure to temperatures above 120°C for a long time. There was a slight leakage of grease from the front bearing due to seal degradation.

4. Root cause investigation: System analysis

To determine the root cause, the method "5 Whys" (5 Whys) is applied in combination with the analysis of the work cycle.

Problem: Servo motor stopped due to thermal overload error.

Why 1: Why did thermal protection work?
Because the PTC thermistor in the stator winding recorded a temperature of over 130°C.

Why 2: Why did the winding temperature exceed 130°C?
Because the heat release (I²R losses) far exceeded the system's ability to dissipate heat.

Why 3: Why was the heat release excessive?
Because the rms current (I_rms) during the duty cycle was 12.5 A, while the rated current of the motor (I_0) is 10.2 A.

Why 4: Why did the rms current exceed the nominal?
Because the technological process was accelerated three months ago. Cycle time decreased from 4.5 seconds to 3.2 seconds, which required higher accelerations and reduced dwell time.

Why 5: Why didn't the engine handle the new cycle?
Because the equivalent moment (M_rms) was not recalculated when the process parameters were changed. The engine turned out to be undersized for the new motion profile. In addition, the cabinet cooling fan filter was clogged, raising the ambient temperature to 48°C.

5. Identified root causes

Based on the collected data, a list of root causes was formed with an assessment of their impact on failure:

1. Duty cycle calculation error (65% probability): The change in motion profile caused the equivalent torque (M_rms) to exceed the rated motor torque (M_0). Servo motors can produce peak torque for a short time (up to 3x M_0), but the average value per cycle must remain below the nominal value. Exceeding M_rms causes an exponential increase in copper losses.
2. Cooling system failure and violation of operating conditions (Probability 25%): The temperature in the engine operating area and in the control cabinet reached 48°C (with a norm of 40°C). A decrease in the temperature gradient between the engine casing and the air reduced the effectiveness of convective cooling.
3. Moment of inertia mismatch (10% probability): The ratio of load inertia to rotor inertia (J_load / J_motor) was 8:1. For high-dynamic applications, the recommended ratio is between 3:1 and 5:1. High inertia requires more energy for acceleration and braking, which further loads the current circuit of the drive.

6. Corrective actions

Immediate actions (Immediate Fix)

• Component Replacement: Dismantling the damaged motor and installing an identical spare servo motor to resume production.
• Replacement of seals: Installation of a new radial shaft seal SKF 1015633 on the seat with a check of shaft runout (tolerance no more than 0.02 mm).
• Cleaning of cooling systems: Replacement of filters on the control cabinets and cleaning of the cooling fins on the body of the mechanism.

Long-term solutions (Long-term Prevention)

• Optimization of motion profile: Reduction of acceleration (jerk) in PLC by 15%. This will increase the cycle time by 0.2 seconds, but reduce the peak currents by 25%, bringing M_rms back into the safe zone.
• Cooling upgrade: Installation of an active air conditioning system for the control cabinet (instead of passive fans) to maintain a stable 35°C.
• Revaluation of motor size (Sizing): In the next planned modernization of the line, replace the current motor with a model with a higher rated torque (M_0 = 20 Nm) and a larger rotor to improve the inertia ratio.

7. Quick diagnostic checklist for technical personnel

This checklist is designed to be used on tablets during equipment walkthroughs. It allows you to detect early signs of overheating before a critical failure occurs.

8. Strategy of prevention and condition monitoring

Preventing overheating requires a shift from reactive maintenance to proactive methods. The basis of this strategy is the correct calculation of kinematics. The equivalent moment is calculated by the formula:

M_rms = √ ( (M_1²*t_1 + M_2²*t_2 + ... + M_n²*t_n) / T_total )

Where M_i is the moment at each stage of the cycle, t_i is the duration of the stage, T_total is the total time of the cycle. If the calculated M_rms approaches 90% of the motor's nominal torque, it is necessary to review the motion profile or select a larger motor.

Condition Monitoring

• Current Signature Analysis (MCSA): Modern servo drives allow continuous analysis of the current spectrum. The appearance of harmonics can indicate mechanical problems (such as worn bearings or damaged SKF 1015633 seals) that create additional resistance and lead to heat.
• Integration of thermal models: The use of internal mathematical models of the drive (I²t protection) must be configured taking into account the real ambient temperature. If the shop temperature is 45°C, the trigger threshold I²t should be lowered.

Maintenance intervals

The maintenance schedule should include checking and replacing the control cabinet filters every 2,000 operating hours. Thermal imaging control of servomotors and cable assemblies should be carried out once a quarter. Sealing elements operating under severe conditions should be replaced every 8,000 hours or when the first signs of elastomer hardening are detected.

9. Conclusions

Overheating of a servo motor is rarely the result of a manufacturing defect in the component itself. In most cases, this is the result of a change in duty cycles without an appropriate calculation of the load (M_rms), deterioration of the cooling conditions or mechanical wear of the related nodes. A systematic approach to diagnostics, including current, vibration and thermography analysis, allows you to accurately identify the root cause. Adherence to design standards and regular inspection of the condition of seals and bearings guarantee stable operation of the equipment and minimize downtime.

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10. Literature and regulatory documents

• DSTU EN 60034-1: Electric rotating machines. Part 1. Nominal data and operating characteristics.
• ISO 20816-1: Vibration is mechanical. Measurement and assessment of machine vibration.
• DSTU EN 60034-27-4: Measurement of insulation resistance and polarization index of windings of electric machines.
• Technical recommendations of servo drive manufacturers regarding the setting of current circuits and the calculation of heat losses.