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AC servo motors, unlike induction motors, have magnets on their rotors. Permanent magnets are what give AC servo motors such high torque specs. Magnets are made out of rare earth materials like Alnico or Neodemium which cannot be fastened to metal rotors by welding or soldering.

Re-Epoxying Magnets onto an AC Servo Motor Rotor

The traditional method most manufacturers use to attach magnets to rotors is epoxy (a kind of super high strength glue). However, epoxy isn’t enough. Over time as the rotor turns the centrifugal force of rotational motion can dislodge the magnets and cause them to detach.

To counteract the force of inertia a kind of special tape, known as banding material, is wrapped around the magnets to hold them on. Unfortunately even with good epoxy and banding material, both considerations can break down over time.


Servo Motor Magnet Repairs From Servotech

These photos depict best what we do to remedy a rotor that’s magnets have come undone. We peel the old banding material off to get down to the raw magnet and remove the magnets that are loose (the magnets don’t actually fall off because they are magnetic and stick to the rotor).

Using a precision device, known as a gauss meter, we find the magnet polarity and strength, marking pole on the magnet itself and strength readings in a separate table of values. Once we have the magnet data for the whole rotor, we mix up our industrial strength epoxy, and clamp the magnets back onto the rotor. We then let the epoxy rest until it is completely cured. Once cured we undergo the banding process, which uses banding material to make sure the repaired magnets have been properly secured.

The rotor is now ready to be magnetized. Conditions that cause servo motor magnets to become dislodged are typically the same conditions that cause a loss in magnetism: namely heat and shock or impact. We re-magnetize the newly banded rotor in our electron discharge magnetizer, and recheck the values of each magnet in the gauss meter.

AC Servo Motor Repair Testing

The final test of the magnet strength cannot be conducted until the servo motor has been reassembled and a proper BEMF reading can be taken.

Servotech tests all repaired motors for quality and performance before they leave the shop, ensuring your motor is returned in “like new” condition.