Servo Motor Services

Minimizing Noise on Your Servo System

Your servo suffers from intermittent failure, feedback loss, runs choppy, or trips out on some sort of encoder error. Could electrical noise be the culprit?

 

Electrical noise is present in every electrical system. It can disrupt the feedback signals (especially with resolvers) on the motor and cause the drive to misread information and not perform as instructed. Noise is a little thought, high impact problem that is relatively easy to minimize.

 

Cable selection is crucial.  Try to buy cable with the correct number of wires.  Use twisted pairs for the send and return of each circuit if possible.  Keep your cables as short as possible, and do not just coil up the extra cable and leave it in a cabinet.

 

Use shielded cabling always.  Just having a shield inside the cable is not enough; it needs to be properly grounded at either the motor or the drive but not both.  Shielded cabling can actually make your problem worse if you do not have a solid ground.

 

Keep feedback cables from looping around or running closely parallel to high voltage cables.  Maximizing distance between communication and high voltage cabling will minimize the impact of noise on your system.

 

Motors/Drives/Encoders are often misdiagnosed as bad when they are really just being affected by noise.  Minimizing noise will save you downtime and unnecessary repair costs.

Oil and Coolant, A Killer of Servomotors

Your machine is down and you pull out a faulty servomotor and send it out for repair.  The diagnosis is quick, there is coolant or oil in your motor and it needs to be rebuilt.  Your motor had a seal on the shaft, sealed plugs, and gaskets between all of the exposed parts.  How did your servo motor fill with oil?

There are a number of common routes liquids can take to get into your motor.  You need to know these routes in order to prevent liquid intrusion from happening again.

• Replace the shaft seal every time your motor is worked on such as during a bearing change.  It is common for maintenance personal to reuse old seals versus replacing with new ones due to time constraints, budget, or laziness.

• Even environmentally sealed plugs will leak over time.  Replace worn plugs especially if there are loose pins that liquid could get around. 
  

• Even if your servo motor connectors are sealed, they need to be mated with matching sealed cable connectors for proper protection. Unsealed cable connectors will allow contaminants into gap between motor and cable connectors.  As no seal is perfect, this unnecessary exposure to the plug surface will lessen the time it takes for contaminants to infiltrate the connector.
  

• AC servo motors have a stator that is made up of stacked laminations which will allow liquid to slowly leak through them.  Some brands, such as Yaskawa, put a thin metal case around the stator to prevent liquid intrusion.  Many other manufacturers such as Fanuc do not and just paint over the laminations.  We have found that sealing the laminations with a two part epoxy paint significantly reduces the chance of contamination through the laminations.

• Make sure that your servo is not being directly sprayed by oil or coolant.  If it is not possible, there are many custom solutions out there to modify the motor or cabling to limit exposure.

These are some general tips on preventing liquid intrusion into your servo motor.  If your servo motor is flooded and you need help send it in to us and we will repair your servo and work with you to find a way to prevent flooding in the future.

DC Servo Motor Brush Wear

DC servo motors very often come in for repair because their brushes have worn out.  This seems like a straight foward problem with an easy solution; install new brushes!  And while this could be true, premature brush wear can be a sign of larger problems with your servo motor.  There are a few points you can check to narrow down the cause of your premature brush wear.

• Is the head of the brush broken or chipped?  If it is then your could have a high bar or piece of mica on your commutator which means you will need to clean up your comm or possibly rewind your armature.

• Are there grooves in the brushes?  If there are thin grooves in the brush then there are probably matching thin grooves in the commutator. This problem is called threading because the grooves resemble the threads of a bolt. This could be an indicator that you have the wrong type of brushes installed or are running the motor with too light a load.  This is a common problem with long life brushes.

• Does the brush show signs of burning or arcing?  If the servo motor's electrical neutral is not properly set then there will be voltage potential between the brush and the commutator bar before the brush is contacting that bar, which will cause arcing when they get close to each other.

Servotech uses Helwig brushes for almost all of our servo motor repairs.  Check out their site for more detailed information on brush wear.  www.helwigcarbon.com

 

 

 

Knowing why your brushes are wearing will help you determine if your DC servo motor needs repair or if you can modify the brushes you use and save money and downtime.  Contact us if you need any assistance or services done to your DC servo.