Settings issue

Published:  11 September, 2007

The answer to this month’s trouble shooting teaser is supplied by Gerry Burn, a director of service, maintenance and replacement specialist CP Automation.

 

Q:

As a plant manager, it makes sense for me to be able to swap equipment around across lines in the event of a breakdown or maintenance issue. However, I often find that the DC drives attached to permanent magnet motors on different production lines have different settings – making it hard to interchange things easily. Any idea why this might be?

 

A:

I think you will find that the settings are only different on DC drives attached to motors that have been serviced or rewound at some point during their lifetime. This is because there is something unique to be borne in mind when repairing or servicing a DC permanent magnet motor. The standard philosophy within the rewind industry is to remove the armature and replace it with a ‘keeper’; to stop the levels of the magnets degrading. However, older magnets, and the material used to manufacture some newer magnets, will degrade while the armature is being removed and the keeper is being put in its place. The same applies when the armature is put back in after re-winding or service. The result is increased RPM and lost torque, which may mean the motor draws more current, costing money and creating a greater environmental impact.

As a result, someone in-house will have to re-set the DC drive to cope with the changes to the motor. Thus, all the systems in the plant will eventually become unique and cease to be interchangeable. The only solution is to de-magnetise the motor before repairing it and then re-magnetise to the correct level afterwards. The calculation for this can be derived from the speed and EMF (Electro Motive Force) settings on the name plate of the motor. Using these figures, the drive settings can be set to the correct parameters. Naturally, I would always recommend that you get a specialist with the appropriate skills and equipment to handle the process for you.