Maintenance departments usually have a wide range of diagnostic tools at their disposal, but for advanced servo drives, standard verification methods often prove insufficient. Common practice relies on static measurements, i.e., verifying winding resistance, phase symmetry, and insulation resistance testing. However, service experience shows that a correct result of these measurements does not guarantee device functionality. A motor can have ideal insulation and symmetrical resistance, yet still generate control errors, lose position, or cause an immediate inverter shutdown after startup.
The reason for this is that a multimeter only verifies the continuity of electrical circuits, not the condition of the magnetic system or the precision of feedback necessary for FOC algorithms. To effectively diagnose the cause of a servo drive failure, it is necessary to perform dynamic tests and analyze parameters that only become apparent during rotor operation. Below, we present a technical analysis of three key areas where standard workshop diagnostics fall short.
Degradation of permanent magnets causes instability of the control loop and motor errors
Servomotors generate torque through the interaction of the stator's magnetic field with strong permanent magnets located on the rotor. The key parameter determining the efficiency of this system is the electromotive force, which is directly proportional to the strength of the rotor's magnetic flux and rotational speed.
In industrial conditions, magnets degrade due to long-term operation at high temperatures or as a result of current surges that generate a strong, opposing magnetic field. The phenomenon of demagnetization leads to a permanent reduction in the motor's voltage constant. The servo drive controller, striving to maintain the set torque, must compensate for the weaker magnetic flux by drastically increasing the current intensity.
The consequence of this phenomenon is rarely physical overheating of the windings, as modern inverters have advanced thermal models. Instead, control system instability occurs. The inverter detects a discrepancy between the mathematical model of the motor and its actual behavior, resulting in errors. At PLE Service, we verify this parameter through a load test.
Commutation angle error after bearing replacement prevents drive startup in vector mode
In servo motors, the encoder performs a function beyond simply counting shaft positions. Its most important task is to provide commutation information, which is essential for vector control. For the inverter to precisely control torque, the signal
from the encoder must be perfectly synchronized with the physical position of the rotor's magnetic poles.
Commutation problems are one of the most common reasons for devices being sent for service, especially after self-repair attempts. Disassembly of the encoder and its reassembly without a zeroing procedure results in an angular shift of the feedback signal. Even a minimal deviation causes the stator current vector not to be perpendicular to the rotor flux.
The effect of a commutation error is often an immediate shutdown of the drive. The controller, trying to force movement, supplies maximum current, which with an incorrect angle does not generate torque, but leads to an immediate overcurrent error or shaft lock. Diagnostics at PLE Service include verifying the correctness of feedback signals using dedicated interfaces, which allows precise setting of the physical position of the encoder relative to the magnetic field, restoring full drive efficiency.
Wear of holding brake linings indicates errors in the control sequence
Electromagnetic brakes in servo drives are designed for static operation, to lock the shaft after the motor has completely stopped. Unlike service brakes, they are not intended to dissipate the kinetic energy of a rotating shaft.
If service verification shows mechanical wear of the friction linings and an increased air gap, the cause usually lies in improper operation or errors in the higher-level control. Damage most often occurs as a result of closing the brake too early (before the PLC controller reaches zero speed) or frequent emergency stops under full load. Another cause of torque loss is oil contamination of the linings due to motor seal failure.
A symptom of such a fault can be the dropping of vertical axes after power-off, despite correct coil resistance and audible electromagnet armature action. The only reliable verification method is a holding torque test. At PLE Service, we perform load tests, checking whether the brake is able to maintain the rated torque specified in the technical specification, which eliminates the risk of installing a mechanically faulty component.
Advanced technical verification eliminates downtime risk
The analysis of the above cases leads to the conclusion that relying solely on basic measuring instruments in servo drive diagnostics carries a high risk of error. Replacing inverter components or cables based on the assumption that the motor is functional generates unnecessary costs and extends machine downtime.
Effective maintenance requires an approach based on hard data. Entrusting diagnostics to a specialized service, equipped with technology for testing, calibration, and load tests, allows for unequivocal identification of the problem source. Thanks to this, repair decisions are made based on the actual technical condition of the device, and not just on its basic electrical parameters.
