Introduction and Technical Analysis: Critical Test Points When Buying Used Servo Motors

Servo motors, serving as the heart of industrial automation systems, are indispensable for applications requiring precise positioning, speed, and torque control. Utilized across a wide spectrum from CNC machines and robotic arms to packaging systems and printing presses, these devices play a critical role in production efficiency and quality. The cost of a new servo motor, especially for high-power and specialized models, can represent a significant burden on budgets. This situation has increased interest in the used servo motor market within the industry. However, purchasing a used servo motor comes with certain risks. A motor that appears to be in good visual condition may have hidden defects in its internal structure or electronic components. These hidden defects can lead to breakdowns shortly after purchase, causing unexpected downtime, production losses, and additional repair costs. Therefore, comprehensively and systematically testing a used servo motor before purchase is vital to ensure return on investment and guarantee operational reliability. This guide provides industrial automation professionals with critical test points to consider when acquiring used servo motors, offering a technical perspective enriched with field experience. Our aim is to provide a detailed checklist and analysis method to help potential buyers make informed decisions, ensuring a reliable choice without compromising cost advantages.

Operating Principles and Technical Data: Critical Test Points When Buying Used Servo Motors

Field Considerations: Critical Test Points When Buying Used Servo Motors

• Visual Inspection and Physical Condition Check:

  Examine the motor’s general physical condition. Check for dents, scratches, cracks, or corrosion on the housing. Damage, especially on mounting flanges and shaft connection points, can compromise the motor’s mechanical integrity. Inspect the integrity of cable glands and check for crushing, cuts, or wear on the cables. Verify that the label information is legible and original. If it is a fan-cooled model, check the integrity of the fan covers and blades. Look for any deformation or signs of wear on the visible parts of the brake mechanism.

• Mechanical Tests:
  • Shaft Rotation and Bearing Condition: Rotate the shaft by hand to test the smoothness of its rotation. Any sticking, friction noise, excessive play (radial or axial), or grinding can indicate bearing failure. The shaft should rotate freely and silently.
  • Shaft Runout: Use a dial indicator to measure the runout at the end of the shaft. Runout exceeding permissible tolerance values (typically between 0.02-0.05 mm) indicates a bent shaft or a serious problem with the bearings.
  • Brake Test: Check that the brake releases when brake supply voltage is applied (the shaft should rotate) and that the brake engages, locking the shaft when power is cut. The brake is expected to make a clear “click” sound when engaging and disengaging. You can test if the brake fully locks the shaft by applying torque by hand.
• Electrical Tests (with Power Off):
  • Winding Resistance Measurement: Using a multimeter or a low-resistance meter (milliohm meter), measure the resistances between the motor’s phase windings (U-V, V-W, W-U). All phase resistances should be very close to each other (within tolerance). A significant difference between values can indicate winding damage.
  • Insulation Resistance (Megger Test): Using an insulation test device (megger), measure the insulation resistance between each phase winding and the motor housing (ground). A test voltage of typically 500VDC or 1000VDC is applied. The value should be at least several Megaohms (MΩ) or higher. Low insulation resistance indicates that the windings have been affected by moisture, contamination, or that the insulation has deteriorated. This carries the risk of the motor short-circuiting.
  • Inductance Test: Measure the inductance values between phases with an LCR meter. Like winding resistance, inductance values are expected to be balanced between phases. Unbalanced inductances can indicate uneven damage to the windings.
  • Feedback Cable Check: Check that the pins in the feedback cable connector are not oxidized and are intact, and that the outer sheath of the cable is not damaged. A cable continuity test can be performed.
• Feedback Unit (Encoder/Resolver) Tests:
  • Signal Check (with Oscilloscope): After connecting to the servo drive, slowly rotate the motor shaft and check the A, B, and Z (index) signals from the encoder with an oscilloscope. The signals should be square wave, clean, noise-free, and have the correct phase angle (90 degrees between A and B). For absolute encoders, serial communication (SSI, EnDat, Hiperface, etc.) or analog sine/cosine signals should be checked. For resolvers, the amplitude and phase balance of the sine and cosine outputs are important.
  • Resolution and Accuracy Check: Connect the motor to a drive and make it travel a certain distance to test if the feedback unit provides accurate position information. Observe positioning accuracy in repetitive movements.
• Dynamic Tests (with Servo Drive Connection):

  These tests are critically important for understanding how the motor performs under actual operating conditions and require a servo drive.

  • No-Load Operation Test: Connect the motor to the drive and run it at different speeds without a load. The rotation should be smooth, silent, and vibration-free. Excessive noise, abnormal vibration, or high no-load current draw can indicate problems.
  • Torque Production and Acceleration/Deceleration: If possible, run the motor under a load close to its nominal torque (e.g., a dynamometer or suitable mechanical load). Test acceleration and deceleration ramps to observe the motor’s dynamic response. Verify that the motor can produce its nominal torque and responds appropriately to the parameters set by the drive.
  • Temperature Control: After running the motor under nominal load for some time, measure its surface temperature with an IR thermometer. Excessive overheating (above label values or abnormal hot spots) can indicate winding damage, bearing friction, or insufficient cooling.
  • Current Draw Check: Monitor the current drawn by the motor under different load and speed conditions via the drive or with a clamp meter. Abnormally high current draw can indicate mechanical friction, winding damage, or feedback unit problems.
  • Positioning Accuracy and Repeatability: If testing in an application environment, test the motor’s ability to move to a specific position and repeatedly achieve that position accurately. Jitter or positioning error indicates a potential problem with the feedback unit or motor mechanics.

Common Problems and Solutions: Critical Test Points When Buying Used Servo Motors

Common problems encountered in used servo motors are typically mechanical, electrical, or electronic in origin. Detecting these issues beforehand saves costs and increases operational reliability.

1. Bearing Failures: One of the most common problems. Friction, play, grinding, or abnormal noises (humming, hissing) felt when rotating the motor shaft by hand, or heard during operation, indicate that the bearings have reached the end of their life. Excessive vibration and localized heating on the motor housing are also symptoms. The solution is replacement with appropriate type and quality bearings. Bearing replacement is a delicate procedure requiring special tools and precision, and should be performed by experts.

2. Winding Damage (Short Circuit, Open Circuit, Ground Fault): Imbalances or low values in electrical tests (resistance, insulation resistance) indicate such problems. In a short circuit, the motor draws excessive current, heats up, and experiences power loss. In an open circuit, the motor does not run at all or runs erratically. A ground fault carries the risk of electric shock and can cause the drive to malfunction. The solution is rewinding the motor’s coils or completely replacing the motor. Winding is also a specialized process.

3. Feedback Unit (Encoder/Resolver) Failures: Manifests as lack of position information, incorrect position information, signal noise, jitter, or inability to control the motor. Signal distortions are observed in oscilloscope tests, or feedback error messages are received from the drive. The solution is to repair or replace the feedback unit. Especially since encoders are sensitive devices, selecting the correct type and resolution during replacement and proper alignment with the shaft are critically important.

4. Brake Failures: Situations such as the motor shaft not releasing (brake remains continuously engaged) or not locking the shaft when power is cut (brake does not hold) can occur. Excessive heating of the brake or abnormal noises are also signs of failure. The solution is to repair/replace the brake coil or mechanical parts. Proper brake operation is essential for both safety and position holding accuracy.

5. Shaft Damage (Bending, Breaking): High values in the shaft runout test or excessive vibration during motor operation indicate that the shaft may be bent or damaged. This can put excessive load on the bearings, leading to more serious failures. The solution is to replace the shaft, which is often a complex process requiring complete disassembly and reassembly of the motor. In some cases, shaft damage may have ended the motor’s economic life.

6. Connector and Cabling Issues: Loose connections, oxidized pins, or damaged cables can cause the motor to not operate, operate intermittently, or transmit faulty signals. These can be detected by visual inspection and continuity tests. The solution is to clean the connectors, repair or replace the cables. Feedback cables, in particular, are very important for signal integrity.

Conclusion and Expert Advice: Critical Test Points When Buying Used Servo Motors

Purchasing a used servo motor offers an attractive alternative for industrial automation budgets, but rigorous pre-inspection and comprehensive testing processes are essential to truly leverage these advantages. Unseen defects or overlooked small details can transform short-term cost savings into much higher long-term repair, maintenance, and production loss costs. Therefore, before purchasing a used servo motor, it is critically important to fully implement the visual, mechanical, electrical, and dynamic tests outlined in this guide to minimize potential risks and ensure the motor will deliver expected performance. Tests on the motor’s “sensitive points,” such as the feedback unit and winding insulation, are decisive for the motor’s lifespan and reliability. It should be remembered that reliability, efficiency, and precision are fundamental expectations in automation systems, and the integrity of servo motors directly impacts the fulfillment of these expectations. If you lack the in-house equipment and expertise to perform these tests, the most appropriate approach is to source used motors from trusted suppliers with references who can provide test reports, or to have them tested by an independent expert before purchase. Always remember: “Trust nothing you haven’t measured.” A detailed testing process is not just a cost, but a smart investment against potential future operational disruptions.