Initial Situation and Fault Pattern.

A sporadic alarm A.b1 occurred on an older CNC milling machine. The fault was irregular and therefore particularly critical from a diagnostic perspective. At first, a cable problem in the drag chain was suspected. The wiring was renewed, but the alarm still remained. In addition, it was known that the encoder had been damaged by dirt ingress caused by a defective motor seal.

Incoming Inspection and Initial Diagnosis

During incoming inspection, the condition of the motor, connectors and cables was checked first. The dirt ingress in the encoder area was clearly traceable as a real mechanical defect. Such a defect can cause signal disturbances, corrosion and dropouts. However, the key point was the separation between the actual motor damage and the alarm pattern reported by the customer.

Technical Analysis

Alarm A.b1 does not primarily indicate a classic encoder fault, but a fault in the readout of the speed reference inside the Servopack. This resulted in a two-part cause. On the one hand, there was actual encoder damage in the motor. On the other hand, the reference path in the SGDH-05DE had to be considered as an additional fault source. Especially in older drives, thermally unstable input stages, drifting analog values, contact problems or ageing assemblies are typical causes of sporadic faults.

Repair Measures and Overhaul

The repair was therefore carried out on two levels. On the motor side, cleaning, reworking of the sealing point and assessment of the encoder area were performed. Dirt and residues had to be completely removed in order to prevent renewed damage. On the drive side, the reference input, connectors, signal path and thermally suspicious areas of the Servopack were checked. In addition, shielding, grounding and cable routing were inspected in order to reliably exclude coupled interference.

Final Functional Test

During the functional test, the system was operated for an extended period with a reproducible reference signal. Start stop behavior, speed demand, warm-up and signal stability were checked. The most important point was the behavior in the warm operating state, because sporadic A.b1 faults often only occur under thermal load.

Result and Conclusion

The encoder damage was technically clear as a consequence of the leaking motor seal. However, the alarm A.b1 additionally showed that the reference input or the internal signal processing of the Servopack had to be evaluated separately. Only the combined consideration of motor, encoder, seal and reference path led to a reliable repair statement. That is exactly what matters for operators, because sporadic faults on older machines often result from several overlapping causes.

Safety Notice

Before work: switch off the system, secure it against being switched on again, wait for discharge time and verify absence of voltage. Measurements on live parts may only be carried out by qualified electricians.

Information about the mentioned Servopack and Servomotor:

• Yaskawa SGMAH-03DAA61D-OY Servo Motor
• Encoder Yaskawa TRD-FWAG13, UTSIH-B13DC-E
• Yaskawa Servopack SGDH-05DE-OY

More information about our Yaskawa repairs can be found here.

📞 Feel free to contact us if you have any questions regarding your Yaskawa drive technology.

Technical Information for the Motor According to the Nameplate

The motor nameplate now allows the core technical data to be stated more precisely. It is noticeable that the motor itself is rated for 400 V. The previous general specification for the servo system must still be considered separately for the Servopack.

Technical Classification of the Motor

The present motor is a compact AC servo motor from the Yaskawa SGMAH series with a rated power of 300 W. The rated speed of 3000 r/min with a rated torque of 0.955 N·m is typical for dynamic feed axes in smaller CNC applications. The rated voltage of 400 V specified on the nameplate is relevant for the motor design. For system evaluation, it must always be considered together with the SGDH-05DE Servopack used, the parameterization and the machine application.

Updated Technical Specifications for Motor and System