Servo Motor Repairs

Servo Motor Repairs; why electrical, electronic and mechanical skills are needed

Servo Motor Repairs

Servo motors are very specialist and high precision motors. They contain both precessions mechanical and miniature electronic components. Repairs to them require a specialist set of skills in electronic, electrical and mechanical repair along with specialised test facilities.Fletcher Moorland is unique in that we are a fully integrated electromechanical and electronic repair centre with a dedicated servo repair department. We started our servo motor repair department 25 years ago. We are continually improving what we do through investment in training and equipment.Doing repairs to these with different repair teams for each discipline or in different department or work sites is a recipe for disaster, faults will be missed and repair times will take much longer. These motors operate as a complete closed loop system, and you can only do a successful, reliable and cost effective repair by repairing and testing it as such with a dedicated engineering team who fully understand them.To ensure a successful servo motor repair, you need to be proficient in all areas of electro-mechanical repair and electronic repair. Consider what can go wrong with a servo motor, here are some common problems we see:

Mechanical faults

  • Bearings and bearing housings.
  • Grease, oil or cutting fluid contamination inside the motor.
  • Grease, oil or cutting fluid contamination inside the brake.
  • Shaft keyway or coupling failure.
  • Rotor magnet failures.
  • Out of balance rotors.
  • Motor brake wear.
  • Worn brushes or commutators (DC types).
  • Carbon builds up (DC types).

Electrical faults

  • Motor winding failures.
  • Brake coil failures.
  • Shorted wiring connections.
  • Broken wiring connections or faulty connectors.
  • Electromechanical tacho faults.

Electronic faults

  • Encoder output or position count faults.
  • Encoder alignment.
  • Encoder Memory faults or corruption.
  • Electronic Tacho faults.
  • Memory battery failure.
  • Resolver failure.
  • Faulty components on PCB’s.
  • Op-amp failures.
  • Driver IC failures.
Some faults that occur are due to problems with the integration of these various disciplines. For example, a servo motor that is noisy in operation, is it one or a combination of the following or a bad bearing?

Possible reasons for faults

  • A noisy tacho?
  • A worn or loose encoder, resolver or tacho mechanical coupling?
  • Misalignment of the encoder, resolver or tacho?
  • Broken screen on the encoder or resolver cable?
  • The low output from the resolver?
  • Loose brake disc?
  • Incorrect brake air gap clearance?
  • Bad earthing connection in the connector?
  • Contamination in the air gap between rotor and stator?

All of these we have found can cause unusual noises inside a servo motor. Only by having a dedicated team that fully understands servo motors as a complete closed loop system can you quickly isolate these unique faults and repair them successfully.Illustration

We don’t use separate engineers for mechanical and electronic work. We have a specialist servo engineering team trained in both disciplines with all the electronic and mechanical tools required to perform reliable and cost effective repairs to your motors. These servo engineers have constant access to our companies dedicated mechanical and electronics experts too where required for advice as well as to the OEM’s, internationally recognised bodies and international experts. This level of integration within a single building at Fletcher Moorland ensures that your motor is repaired reliably and quicker, reducing your costs.

An in-depth understanding of how servo systems and feedback devices work is essential. Servo motors come in a variety of forms, from AC & DC, but more importantly what type of feedback system is incorporated in each type? These can be one or a combination of the following;

  • Incremental Encoders.
  • Slotted optical or hall effect commutation sensors.
  • Absolute Encoders
  • Serial Data Encoders (En-Dat, SSI, OEM Specific codes).
  • Serial Data Encoders with the model, data, parameter and position memories.
  • Multi-Turn encoders.
  • Resolvers.
  • Tacho generators, electronic and electromechanical.

Each type requires different alignment and test equipment specific for each manufacturer. The data output from encoders can be in a variety of forms, from simple “A&B” quadrature outputs or parallel outputs to bi-directional serial communications in binary, hexadecimal, grey or an OEM specific code. This data needs to be checked and verified to ensure the feedback device is working properly and is correctly aligned relative to the rotors magnetic field. Where the feedback data is in a specialised communication form specific to the OEM we have built specialist test equipment to read the data and run the motor at it working speed. This is even before a repair is started!

Even the mechanical repair needs to be carried out with specialist equipment. Rotors should never be removed from a servo motor by hand. Dragging the rotor magnets over the stator will cause damage to them as they are often very brittle.

Correct alignment of feedback devices is crucial, we have built our own equipment, some of which was built in conjunction with OEM’s to align, resolvers, encoders, commutation sensors etc to the manufacturer’s specifications

The most important aspect of a servo motor repair is the ability to fully test and verify the operation. Being able to run the motor is the absolute minimum for testing. Servo motors will not run on a standard 3 phase supply, you can destroy the magnets that way. One method of testing is to have the associated PSU and servo drive coupled to the motor. This is then linked to a specially built motion controller to communicate to the drive and then run the motor to simulate the conditions that would be seen on a CNC or robot. We use this method of testing for Siemens, Bosch, NUM, Indramat, Control Techniques, Vickers, Reliance, Rexroth, Allen Bradley, Baldor, Lenze, ABB and various other manufacturers’ motors.

Certain servo motors require more complex methods of testing, i.e. Fanuc, Kawasaki, Yaskawa, Sanyo Denki and Nachi as these motors communicate directly with the machine controller and not the drive. We have developed and engineered our own equipment various facilities to meet the demanding test requirements for these motors and run them at rated speed and load.

Finally, to test and verify a servo motor it needs to be run against a dynamic load. We use our own designed and built dynamic load rig. We have the capability to run a servo motor through a motion controller, against a changing load similar to the conditions that would be encountered on site. This test is critical as the motor reaches its operating temperature where it’s operating performance at its rated load and power output can be monitored. The servo engineer can select various tests, speeds and loads using an LCD touch screen which also shows them all the key performance parameters of the motor including the motors torque constant Kt and back EMF constant Ke.

Here’s an example of the test rigs we build to ensure that we can fully test servo motors (this one is for the entire Siemens servo motor range). Read about how we tested the rigs here.

Set up

We also carry out a full test of any brake fitted to the motor using the same load test facility. We can apply a load to the brake and measure the point at which slipping occurs, displaying it on the touch screen. The engineer can also select an automated brake bedding in the program to bed in any new brake discs or components that were fitted. Can other repairers do that? We don’t think they can.

Do we know it all? No, we don’t. New and more complex servo motors are coming to the market every year. Our R&D engineers are looking to the future. No servo motor repair company can afford to stand still and not to be continually developing their repair capabilities. We are always improving our already vast testing facility by designing and developing new improvements to our in-house designed test rig with data acquisition system and touch screen interface controller which automates our testing facility. We have also bought the ‘Mitchell’ TI3000 and TI5000EX servo motor repair and test equipment and integrated these into our test benches. These off the shelf items are good in verifying the repair of a servo motor however our R&D team have redesigned them to give current monitoring of each phase and have more than doubled the load range of the existing test.

Servo motors are complicated, and we are servo motor repair specialists. Major OEM’s put their confidence in us to repair and service their servo motors. Some on a recognised basis and some on a confidential basis.

We are now repairing thousands of servo motors. Whoever you choose to use for your servo motor repairs, demand the results of the final run and load test so you are sure it will work.

If you want any further information on servo motor repair or you wish to see our facility in action, just call us and arrange a visit. 01782 411021 or visit