Tony Young explains the key points to consider when deciding whether to repair or replace a servo motor
The servo motor has an air of mystique around it, even after such a long service life in industry. As a result, there are plenty of maintenance engineers who choose not to repair such equipment themselves. There are others who do try to fix their own servo motors, often with disappointing results. The third group simply replaces faulty equipment – which can lead to repeated problems or unnecessary downtime if a repair is more appropriate.
When making a decision about a faulty servo motor there are three points to bear in mind. These are the cost of the repair versus the cost of the replacement, the speed with which the overall system needs to be up and running again and the availability of spares from the servo manufacturer or its agent.
The first of these points is a simple decision based on economics. A good service partner will always provide two quotes – one for repair and one for replacement so you can compare easily. You should never be driven down one route or the other by the repair partner. The second point is normally a question of downtime. If you have a paper mill incurring thousands of pounds of downtime costs an hour, the decision to simply take the quickest route is a no brainer. However, the question of the availability of spares is slightly more complex. If a manufacturer requests that you deal with its agent to replace an encoder or resolver for instance there can be a delay due to stock levels (or conversely, some agents might speed the process up). The crux of the issue is that while you may have the skills to repair a servo, if you don’t have the contacts you might still incur unnecessary costs.
When making these decisions, the principles remain the same throughout industry; the size and rating of the servo doesn’t make much difference, nor does the application in which it is used (except in so far as these factors will influence the three points I have already outlined).
In addition to these basic choices, there can be benefits to a repair that aren’t associated with a replacement, and vice versa. If you choose to send a servo to be repaired, your maintenance partner might discover that the route cause of the problem is elsewhere and that the malfunctioning servo is only a symptom of a greater ailment. Because the motor is the portion that revolves, when it isn’t working one assumes that the problem lies with it. However, the motor is often fine when it is tested; this tells us that the problem is elsewhere. Normally, such further repairs are confined to the servo drive or controller but it isn’t unknown for this analysis to diagnose a bearing misalignment, for example.
Of course, if a problem with another piece of equipment is spotted, there is a cost associated with replacing or repairing it. However, there is also a saving attached to not having to replace the servo again a few months down the line. In any case, if a servo is replaced by one from another manufacturer then the associated drive and control electronics will often have to be replaced at the same time.
As a director of a company that performs these repairs, I’m very conscious that I’m protecting the good name of the OEM when I make any recommendation. If a customer elects to replace a motor and then discovers further problems like those I’ve already outlined, he may decide that the fault lies with the goods and not their use. This could unfairly bias him against a particular manufacturer.
If the decision to repair a servo motor is made, it’s essential that it is sent to the repair partner ‘as failed’. It can then be set it up on a test rig to get an accurate diagnostic before it’s dismantled. This also provides a footprint of the positioning of the electronic commutation device in relation to the motor windings. As a result, it can be precisely realigned after the repair - exactly as it was when it left the manufacturer. This isn’t to say that if a motor has had some work done on it in-situ it can’t be resurrected, simply that the process will be more difficult, longer and more expensive in terms of both charges levied and downtime incurred. It’s a bit like calling in a PC Doctor to your home – if a virus had created problems in the settings on your computer it would be better to leave the virus where it was so that the IT engineer can get a good idea of what caused the problem and thus of all the associated side effects. The principle applies across all areas of repair really.
To conclude, the utopian solution when a servo motor problem is identified is to both repair and replace. This is especially true if you have a number of similar systems using identical servos. It provides the quickest solution and insurance against any future problems – because you have an identical spare on site that can be slotted into place.
Tony Young is a service, maitenance and replacement specialist with CP Automation.