Servo motors in OTC series welding robots are the core drive units for axis movement, path reproduction, and orientation adjustment. Their operating condition directly determines welding path accuracy and the stability of coordinated robot motion. In production, a servo motor that stops completely and cannot start represents a typical severe fault. Unlike less severe wear-related issues such as vibration, unusual noise, or drift at low speed, this fault can lock an individual axis, prevent coordinated robot motion, and bring the entire production line to a halt. Possible causes include mechanical seizure, electrical circuit failure, servo drive protection, and loss of encoder signals. Many maintenance technicians immediately conclude that the motor itself has burned out, but replacing components without identifying the cause can lead to unnecessary maintenance costs. A systematic investigation based on the model's electrical logic and mechanical design can accurately distinguish fault locations and support efficient, precise OTC robot repair.
Mechanical binding is a common physical cause of an OTC servo motor failing to rotate, usually resulting from drivetrain wear or abnormal external forces. During prolonged welding operations, wear debris and carbonized grease contaminants can accumulate inside joint reducers, causing gears to bind and sharply increasing transmission resistance. When the load exceeds the servo motor's rated capacity, the equipment triggers its protection mechanism, locking the motor output and stopping rotation. Mechanical displacement caused by welding torch collisions, compression from workpieces, or overload impacts on an axis can alter drivetrain clearances, leading to faults such as axis binding or rotor seizure. Typical signs include a motor that seizes under load, noticeable resistance and binding points when the joint is turned manually, and normal electrical test readings. Adjusting electrical parameters alone cannot resolve these faults.
Motor stoppage caused by electrical circuit faults is a hidden fault type that is particularly easy to misdiagnose on site. OTC robot servo motors rely on stable power and signal circuits to start and stop. Broken power cables, oxidized or burned terminals, and loose connections can interrupt the motor's power supply, preventing operating current from being established and leaving the motor unable to start. Prolonged equipment vibration and repeated cable bending can break internal conductors while leaving the outer sheath intact, resulting in electrical transmission failure with no visible external damage. Deteriorated drive module contacts inside the control cabinet, abnormal fuse failures on the power board, and excessive voltage drops in the power supply circuit can also directly interrupt motor output commands. The system may report only a servo fault without identifying the wiring fault precisely, making troubleshooting substantially more difficult.
Abnormal brake locking is a frequent fault cause specific to welding robots and is easily overlooked during OTC robot repair. The servo motor has an integrated brake unit that engages when the equipment stops to hold the axis in position and releases when welding starts so the motor can rotate normally. A burned-out brake coil, an open brake circuit, or insufficient release voltage can prevent the brake friction elements from releasing properly. The mechanical locking mechanism then remains engaged, and the energized motor cannot overcome the braking resistance, so it remains stationary. Unlike other forms of mechanical seizure, this fault leaves the axis completely locked with no play when an attempt is made to turn it manually, while the motor's no-load current is abnormally high. It is a characteristic cause of axis stoppage in OTC robots.
A protective servo shutdown caused by loss of encoder signals is a fault state triggered by the equipment's intelligent protection mechanism. The encoder provides real-time feedback on motor speed, angle, and position. Signal interruptions or corrupted data prevent the servo system from determining the axis position. To avoid positioning errors, collisions, and overtravel, the system disables motor output and prevents axis movement. Prolonged operation at high temperatures and the ingress of welding fumes and dust into the encoder assembly at the rear of the motor can contaminate the optical grating, degrade circuitry, and disrupt signal reception. This can produce an unusual condition in which no fault is reported at power-up, yet the motor does not rotate at all. Accurate diagnosis requires dedicated equipment to read the servo feedback data.
Targeted, staged troubleshooting and repair can efficiently address different causes of motor stoppage and avoid unnecessary disassembly and reassembly. Begin by checking the mechanical condition and manually turning each axis to distinguish mechanical seizure from electrically induced locking. Remove contaminants from the reducer, correct displaced drivetrain components, and replace degraded lubricant to eliminate physical binding. For brake faults, measure brake coil resistance and release voltage, replace damaged brake components, and restore normal brake release and engagement. Perform continuity and insulation tests on wiring harnesses, terminals, and power supply circuits, and repair broken conductors, loose connections, and oxidized contact points to restore stable power and signal transmission. Encoder faults require cleaning the optical grating or replacing compatible components, then realigning the axis position parameters to restore signal acquisition.
After all faults have been repaired, perform servo matching across the robot and verify operation under working conditions. Recalibrate the motor home position, servo gain, and load-matching parameters to eliminate parameter deviations introduced during disassembly and reassembly for DAIHEN robot repair. Run each axis individually without load and check for smooth motor starting and stopping, stable current, and consistent speed. Once there is no binding, unusual noise, or fault indication, perform coordinated multi-axis tests and trial runs along the actual welding path to verify positioning accuracy, motion coordination, and operation under load. Accurate fault identification and targeted OTC robot repair can resolve servo motor stoppage, restore the robot's full operating capability, reduce production line downtime, and support stable operation of automated welding and assembly lines.