When a Z-axis servo motor triggers an overload alarm while stationary (holding its position), the main causes can be summarized into the following categories. We list them in order from most to least likely:
1. External Mechanical Load
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Compared with horizontal X/Y axes, the vertical Z-axis is also affected by gravity. When the motor is stationary, gravity pulls the load downward and applies a force to the motor shaft. If the motor relies solely on the servo enable function to hold the shaft in position, an overload alarm can easily occur. For Z-axis applications, a motor with a brake should normally be selected during the initial motor selection stage, so that the motor shaft can be mechanically locked by the external brake when stationary.
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If a brake has already been installed, check whether the brake is working properly. It is possible that the brake remains released while the motor is stationary and therefore does not lock the motor shaft.
You can perform a simple test: while the motor is stationary, place a support under the load to counteract the effect of gravity. If the overload alarm no longer occurs, the problem is related to the brake.
2. Gain Parameter Adjustment
If the position-loop/velocity-loop gains and rigidity level of the servo drive are set too high, the servo tuning may be too aggressive, causing small high-frequency oscillations while the motor is stationary. The motor continuously adjusts its position, generating heat and eventually triggering an overload alarm.
3. Hardware Failure
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Similarly, if the motor brake is controlled by an external relay, burned relay contacts or a damaged switching power supply may cause the brake to malfunction.
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The servo drive itself may have an internal hardware failure. You can cross-test with another servo drive to verify whether the drive is the cause.