Servo Motor Troubleshooting Guide

A servo motor is a linear or rotary actuator that allows for precise control of linear or angular position, acceleration, and velocity. A typical servo motor system consists of a suitable electric motor coupled to a sensor for position feedback. The motors’ control circuit, which includes the position sensor, provides feedback on the real-time position of the motor shaft. With this feedback, the servo motor can rotate with great precision. But for a servo motor system to be complete, it also requires a servo drive that uses the position feedback sensor to precisely turn the servo motor at some specific angles or distances.

Simply put, a servo motor is just a simple electric motor that runs through a servo mechanism. Servo motors are broadly categorized as AC-powered or DC-powered. But there are many other classifications depending on the operating characteristics and gear arrangement of the servo motor in question. The different types of gear arrangements in servo motors allow users to benefit from lightweight, energy-efficient packages with very high torques. On account of these features, servo motors are widely used in a variety of applications including automated manufacturing, robotics, radio-controlled or remote-controlled toy cars, CNC machinery, RC (Radio-Controlled) helicopters, and planes, etc.

Keeping servo motors well maintained and operating in excellent conditions is vital to the success of any industrial or commercial enterprise. As downtime resulting from a faulty servo motor system can be costly, both financially and in terms of manufacturing lead times. However, even with the strictest regimen of servicing and preventative maintenance, a servo motor is bound to experience issues during restart or normal operation. This article provides a list of common servo motor problems and some tips on how to troubleshoot them.

Troubleshooting a Servo Motor 

Troubleshooting a servo motor problem can be a daunting task, especially because it’s part of a closed-loop control system with a servo controller. But most servo motor problems will cause the servo controller to give a fault code, which is communicated on the servo motor’s display or LED indicators. Therefore, the first step in troubleshooting a servo motor is to check for any fault code display or indicator before proceeding with other troubleshooting steps. After which you can refer to the controller’s manual for a description of the shown fault code or indicators and possible solutions.

In addition, you can occasionally diagnose some servo motor problems with a simple visual or physical inspection of the servo motor installation and associated cabling. However, when the issue is not so obvious, isolating the problem between the servo motor and its controller can be more problematic.

Let’s take a look at some of the most common servo motor problems, along with their possible causes and corresponding troubleshooting techniques.

*HIGH VOLTAGE CAUTION* 

High voltage is likely to be present when the servo motor system is powered up; therefore, only experienced engineers or technicians should attempt to troubleshoot such a system with the power on. Otherwise, ensure that the servo system is switched off and the DC bus has no voltage, before troubleshooting or working on its electrical circuits.

A) Servo Motor Overheating

Most servo motors are susceptible to overheating, especially when they’re running for prolonged periods of time. A servo motor can overheat due to a number of reasons including: 

  • A very high supply voltage that exceeds the specified maximum value. 
  • An overloaded motor or the servo motor is started up too frequently. 
  • Extended operating times. 
  • The supply voltage is too low, or the servo rotor is demagnetizing partially leading to excessive motor current. And like any other electric motor, the servo motor is designed to operate at rated load; thus, a large electric current will cause the motor windings to heat up. 
  • Worn bearings or the bearings were replaced with an incorrect type. 
  • Blocked ventilation due to dirt accumulation on the surface of the servo motor. 
  • A high ambient temperature within the motor’s working environment. 
  • Wiring or motor tuning problem. 
  • Lack of phase, where the motor is operating with only two phases. 

Troubleshooting 

  • Reduce the voltage being supplied to the servo motor; for example, by adjusting the tapping point of the servo motors’ supply transformer. 
  • Reduce off-peak or peak loads for load shedding. 
  • If the overload problem is resulting from an undersized motor, be sure to review the servo motor sizing. 
  • Control the number of start-up times and load starts/cycles to be within the recommended number of times.   
  • Increase the supply voltage to the servo motor or adopt a thicker supply line conductor to handle the excessive motor current. 
  • Have the servo motor serviced; repair worn-out bearings, replace the wrong type of bearings and reconfigure the rotor. 
  • Blow out the motor’s internal ventilation or air pathways to clear out any form of blockage. 
  • Clean the cooling system and ensure that the cooling fans are in good working condition. 
  • Reduce the ambient temperature in the motor surroundings by adopting appropriate cooling measures. 
  • For wiring problems, check that the connections/connectors are tight and that there’s no frayed wiring. 
  • Auto-tune the servo motor with the servo controller. 
  • Restore three-phase AC operation, for AC-powered servo motors. 

B) Servo Motor Does Not Turn

Occasionally, your servo motor might fail to rotate or turn even after being powered. But there’s no smoke, peculiar smell, or abnormal sound. Well, this diagnosis may seem like bad news especially because a servo motor has so many components which could make it difficult to pinpoint the exact cause of the problem. But here are some possible reasons why a servo motor may fail to turn: 

  • No supply voltage-at least two-phase supply. 
  • Fuse failure in at least two phases, for a 3-phase supply line. 
  • The servo controller is not receiving command signals i.e. run or speed signals. 
  • Loose servo motor coupling.  
  • Bad connection due to loose or open motor connections, cables, and connectors. 
  • Motor thermal overload has tripped due to motor overheating. 
  • Wiring error of the motor’s control unit. 
  • Unsuccessful/incomplete servo motor debugging.  

Troubleshooting 

  • Check and repair any breakpoint at the rotary switch of the servo motor’s power supply, junction box, and fuse. 
  • Check the type of failed fuse, and the reason for its failure, and replace it with a new one. 
  • Check the input signals to the servo controller. 
  • Check the motor coupling and rectify it if necessary. 
  • Check all motor connections, cables, and connectors for looseness, and tighten them accordingly. 
  • Check all motor thermal relays. Also, check the servo controller for any faults related to motor overheating. 
  • Correct the control unit wiring. 
  • Adjust the settings of the servo motor parameters appropriately or complete any ongoing system debugging. 

C) Desired Servo Motor Acceleration cannot be Obtained

Sometimes, it can be difficult to start up your servo motor (i.e. it doesn’t come up to speed). While in other cases, its speed may remain significantly lower than the rated full speed even when operating at the rated load. This problem is likely to be caused by: 

  • Low supply voltage, or voltage drops within the servo motor system. 
  • The current rating of the servo amplifier is insufficient to supply adequate torque to the servo motor. 
  • Incorrect servo controller settings. For example, the External 1 limit pot may be incorrectly set. 
  • Improper servo motor connections or poor wiring. 
  • Possible tuning problems with the motor’s controller. 
  • The servo motor may be overloaded. 
  • Possible motor overheating issues. 

Troubleshooting 

  • Measure the supply voltage and improve the low voltage condition. 
  • Check the servo amplifier’s current rating. 
  • Ensure that the settings on the servo controller are correct. 
  • Check and correct any misconnections in the servo system, and ensure proper wiring. 
  • Auto-tune the servo motor with the servo controller. 
  • Check and adjust the servo motor’s workload limits accordingly. Or carry out load shedding. 
  • Review the servo motor sizing against the application need. 
  • Perform overheating checks and apply necessary correction measures. 

D) Servo Motor Shuts Off on Reaching Full Speed

In case your servo starts up okay but suddenly shuts off after attaining high or full speed, this could indicate a serious malfunction. A few factors are known to cause this problem, namely: 

  • Blown fuses, or old and dysfunctional fuses. 
  • Rapid motor overheating. 
  • The overload protection system of your servo motor may be defective. 
  • Voltage drops within the servo system due to poor wiring or bad connections. 
  • Defective capacitors. 
  • Malfunctioning RPM meter or inaccurate speed readings. 

Troubleshooting 

  • Replace the fuses. 
  • Check for any causes of motor overheating and eliminate them. 
  • Check for correct overload settings on the servo controller. 
  • Replace the motor overload relays. 
  • Inspect all motor wiring and connections. 
  • Measure the resistance across the servo capacitors, and replace the defective ones. 
  • Test the RPM meter and replace it if necessary. 
  • Auto-tune the servo motor with the servo controller. 

E) Servo Motor Runs Uncontrollably

Causes

  • Possible loss of position and velocity feedback. This could be due to open or incorrect wiring of the velocity command signal or position feedback sensor. 
  • Incorrect setup of the servo controller. 
  • Servo motor or controller wiring issue. 

Troubleshooting 

  • Inspect the feedback sensor and associated connections. 
  • Check servo controller setup or settings. 
  • Check the servo motor and controller wiring. 

F) Abnormal Noise when the Servo Motor is Running 

It’s natural for a servo motor to make a small amount of noise during normal operation-a humming sound. However, if it does make unusual noises or the humming sound is too loud to the point of being obnoxious; then there’s a problem, which is most likely caused by: 

  • Loose servo motor coupling. 
  • Unbalanced supply voltage, which might be too high. 
  • Bearing abrasion or they lack lubricating oil. 
  • Presence of impurities like sand in the lubricating oil. 
  • Presence of noise on the motor feedback signal wires or the controller speed reference signals. 

Troubleshooting 

  • Check and fasten the servo motor coupling. 
  • Check and adjust the supply voltage accordingly, to ensure that the servo motor is receiving appropriate amount of power. 
  • Have the servo motor serviced: (i) Change or clean the bearings. (ii) Change or add the lubricating oil. 
  • Inspect all motor and controller cables, wiring and connections. 
  • Verify that the servo motor is grounded properly. 

G) Excessive Vibration

Sometimes, the vibration of a running servo motor may be quite violent. Some of the possible causes of excessive vibration in a servo motor include: 

  • Uneven or big air gap caused by worn bearings or bearing abrasion. 
  • Misalignment of the servo motor with the load being driven. 
  • The rotating shaft of the servo motor is bending. 
  • Unbalanced rotor. 
  • The coaxial of the servo motor coupling is too low, especially where a belt pulley is used. 
  • Improper servo motor mounting. 

Troubleshooting 

  • Check, repair or change the bearings if necessary. 
  • Check alignment of the motor with driven load, couplings, and for any possible shim movement. 
  • Check and correct both motor and load balance issues. 
  • If the servo motor is belt coupled, remove the belt pulley and see if the coaxiality is still too low or if the motor is still out of balance. 
  • Inspect for worn or loose servo motor mounts. 

H) Servo Motor Produces Smoke or Burning Odor

Whenever a servo motor produces a strong burning odor, it may imply that something is burning. If you observe this or see any sort of smoke, then it indicates that your servo motor is overheating. In that case, inspect its ventilation or cooling system and ensure that it’s not blocked in any way. If it’s blocked by either accumulated dirt or dust particles, be sure to clean it as recommended.

If the step above does not help, check if the bearings are in good working condition. As they could be having several issues such as too much lubricant, overheating or they’re worn. In case you come across any of these issues, ensure that you service the servo motor or have the bearings repaired/changed.

Also, you may encounter an ozone smell that indicates burning servo motor windings or wiring. If that’s the case, ensure that the servo motor is grounded properly. In addition, check all the motor wiring to ensure that the wires are contactless. If there’s a motor winding failure, have the servo motor serviced.

I) Overload Indication when Servo Motor is Running without any Load

If this condition occurs when the servo motor run signal is Access and there’s no Output Pulse: 

  • Check if the power cable of the servo motor is connected properly or if it’s damaged. Replace it if damaged and correct the wiring. 
  • If the servo motor is equipped with a brake system, ensure that it’s turned on. 
  • Check if the setting of the motor speed circuit gain is too high, and adjust it accordingly. 
  • Check if the setting of the integral time constant for the speed circuit is accurate. 

If the condition occurs only during normal operation: 

  • Check if the set gain of the position circuit is too large, and adjust it accordingly.  
  • Check if the setting of the locating completion amplitude is too small, and make the necessary adjustments. 
  • Ensure that there’s no stall on the servo motor shaft, then adjust the servo mechanism again. 
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