At Motion Tronic, we repair and offer a 12 month warranty on all major AC and DC Servo motors and Controllers. For those of you looking to repair the motor yourselves, here’s our advice and a step-by-step guide.
Before you start to repair a servo motor, please note:
- It is important to exercise the utmost care when conducting any disassembly of a servo motor or servo drive unit.
- You should always use specialised equipment to carry out such a repair.
- You need to be able to read the electrical angle of the feedback device and simulate running conditions of the motor. This is an area the Motion Tronic team has perfected!
Know how your servo motor works
All servo motor and servo drives are mechanically calibrated between multiple facets of the unit to ensure the proper positioning of the device when it is commissioned. Failure to properly dis-assemble a servo motor or drive correctly can cause the unit to become unusable in the future.
The inner workings of servo motors and servo drives are separate and, therefore, they should be treated separately. Here, we will focus on troubleshooting the servo motor that is connected to the servo drive.
Servo motors are specifically designed with what is known as ‘feedback’ capabilities so that the servo drive can maintain positioning of the motor within fractions of a degree and, many times, even more accurately. Servo motors are specifically designed to withstand high amounts of holding current as well as extremely fast-start stop procedures so that accuracy can be maintained at all times.
This feedback often comes in the form of an encoder or resolver, depending on the type of encoder or resolver you might have. Either way, there are many more aspects to a servo motor that can go wrong, so it is best to start at those places.
The Motion Tronic repair process
Following, our professional approach to repairing servo motors:
Step 1: Take Notes
Many people pass over this step when attempting to repair industrial equipment but it is important for future reference. When a unit first hits our bench, we make note of various aspects of the equipment, including but not limited to:
– Serial Number
– Reason for Service
– Urgency (Rush Overtime or Standard)
– Visual Inspection of External Device
Step 2: Check The Shaft
It is important to establish whether or not the servo motor shaft has been bent, damaged, or broken. If this is the case, a new shaft may need to be ordered or machined in order to recommission the unit. In some cases, a bent shaft cannot be replaced and this then becomes a quote for replacement.
A bent shaft on a servo motor can cause extreme wear to the device over time due to vibration and heat. A bent shaft can also cause slip on a gear box and loss in positioning, resulting in unexpected faults from the servo drive.
You may need to use a calliper to establish the proper positioning of the shaft within the closest accuracy possible.
Step 3: Check the encoder and motor cables
You should have the communications cable with the unit so you can test the pin-out of the cable to ensure all of the wiring has a strong signal. In instances such as these, you may need to pull up pin-out information from the servo motor manufacturing website to ensure you are testing the appropriate cables. In many cases, an encoder cable will have a pin for each channel of A, A not, B, B not, Positive Voltage, and Common. In some cases, it will also have a Z and Z not pulse as well as a shield.
If you do not have the cable (for example, if you are working with a spare unit), then you really have no choice but to move on to the next step. Finding a bad cable early on is a great way to solve servo motor feedback issues without disassembling the servo motor.
For the motor cable, you are looking at 3 separate phases, 2 armature wirings, and sometimes commutation wiring. Using your meter, test the end of each cable to ensure there are no shorts between any of those connections. You also need to read the electrical angle of the feedback device as this is critical for reassembly and for the motor to work correctly. Failure to do so will make the motor non-repairable. Motion Tronic has invested heavily in specialised equipment to test all encoder types including ENDAT and to read the alignment information correctly.
Step 4: Check the Bearings
The shaft coming out of the servo motor should rotate freely with little resistance. This is assuming you have actually disconnected the cables from the original servo drive. The drive often causes resistance when rotating the shaft if it is connected to the servo drive through the motor cables.
If there is ‘bouncing’ when attempting to rotate the shaft then it is likely due to a short within the motor or a bearing beginning to fail. In either case, the servo motor will need to be disassembled for further diagnosis.
Step 5: Test the motor for shorts
This step is essential to further diagnostics. At this point, you need to test the motor to ensure there are no shorts within it. Using your meter, you will want to test from phase to phase to ensure the connections are open. Do the same with the armature connections. There should be no shorts between them. If you find a short at this point, it is likely the cause of the failure.
There are a number of reasons for servo motor shorts, including contamination, overheating, and wear and tear – the only real way to find the source of the short is to disassemble the unit and establish the cause of failure.
Step 6: Rotate the Motor
Assuming the prior tests have all been passed, apply a small voltage starting from 0 volts to the armature winding of the servo motor. Slowly increase this voltage using your variable power supply until the shaft begins to turn. At this point, you can further evaluate whether there is unnecessary vibration of the servo motor during rotation or whether the shaft appears to wobble or ‘bounce’ due to a bent shaft or bad bearing.
Rotating the motor prior to disassembling is a good idea if possible – this will let you know that the servo motor at least functions on some level prior to establishing the original cause of the failure.
Step 7: Contact the Customer
If everything has tested ‘healthy’ by this step, we contact the customer and get more details as to the original cause of failure. It is quite common with servo motor and servo drive applications that the cause of the failure was external to the motor itself.
Most people do not realise that an outside application change or environmental change can cause fluctuations in the configuration of the servo motor that can cause faults. It is also important to establish whether or not the servo motor was properly tuned for the application to begin with. Sending a field service tech and getting the servo motor properly tuned may cause the issue to go away. Need help? Contact us on 083 6411 487.
Step 8: Carefully disassemble the servo motor
When we say carefully, we mean carefully! You should mark the location of every detached piece with a punch or file to ensure you reassemble the unit exactly as it was assembled. Failure to do this will often cause the servo motor to produce an excessive level of vibration which can cause further failures or dysfunction. It is also sometimes wise to take photographs as you are disassembling the unit so you have a log of how the servo motor looked prior to each phase.
This cannot be emphasised enough – as you remove more and more pieces of the servo motor, take a closer visual inspection of each element within the motor including the commutator, armature, shaft, and brushes. Try to locate any possible cause of failure before continuing to disassemble the drive. The sooner you find the cause of failure, the better, as it means less reassembly is required.
Once you have established the cause of failure, your response will depend on both the manufacturer of the servo motor and the original cause. In many causes, certain brands will not allow you to purchase parts for their servo motors and a replacement may be required. There are certain aspects of a servo motor that can be replaced or repaired by third parties such as the armature winding or brushes.
Step 9: Send a technician
If our customers cannot establish the failure on any other aspect of the machine and the servo motor appears to be in good shape after disassembly, then it may be necessary to get a field service technician on-site to establish the cause of failure.
Our field service techs are trained to troubleshoot any issue ranging from standard AC Motor Speed Controllers to advanced robotics and PLCs. They are trained to establish the cause of failure as quickly as possible.
Let us handle your servo motor repairs
For the past 13 years, Motion Tronic has used specialised techniques to establish one of the best reputations for servicing the industrial industry. Our methods for testing servo motors are well-established and have resulted in the repair or replacement of hundreds of industrial servo drive motors.
Want an expert technician to have a look at your servo motor? Call us on 083 641 1487 or 031 701 1620 for a quote.