How can we prevent broken magnets in a servomotor?

In this article we are going to talk about a failure, which does not happen very often, but is really important to prevent.

Servomotors can suffer serious damage because of magnet detachment or breakage.

We assume that if a servomotor rotates properly and there is no error message in the driver, it is working correctly. However, occasionally servomotors do not perform as well as we think  and some failures begin to appear without warning until it is too late to do anything about them.

This type of failure tends to need a lengthy repair, and if a spare servomotor is not available we can experience long and costly downtimes and stoppages.

Picture 1 – A clear example of a servomotor’s broken magnet.

Picture 2 – The broken magnet in picture 1 has caused wear in the stator

The three most likely causes of magnet breakage.

1.- High temperatures. If a servomotor has been working for long periods at an excessively high temperature, the magnets lose their properties and can detach themselves from the rotor.

Sometimes the servomotor blocks without causing any damage, but  on occasion the magnet or magnets break and lead to more serious failures, such as damage in the rewinding coil ( picture 2).

Above all, this can cause the copper wire to break, which means the servomotor has to be rewound.

2.- The electrical current flow through the servomotor.  This can cause the magnets to detach themselves and seriously damage the bearings. To check this electrical flow an SKF TKED1 can be used, which allows us to follow up the extent of the electrical flow in our machines.

If the magnets break and we do not have the spare parts, the servomotor cannot be repaired.

Picture 3 – Magnet detached from the rotor.

3 – Bearing breakage.  This can also be a cause of magnet detachment and breakage, because when the bearing breaks it provokes friction between the magnets and the stator.

Picture 4 – Broken bearing.

The factors that can cause one of these three failures are very different, which means that it is crucial to carry out a proper check of our machines both mechanically and electrically.

If we take into account that all the parts are connected, it is essential to perform a regular review of them.

Good wiring facilitates proper servomotor functioning and communication with the driver, which is the other vital part that we must keep in a place free of dust, without damp and with  good ventilation.

The mechanical parts need to be greased and serviced frequently, so that they help the servomotor not to overwork and always keep it at an optimum working temperature.

For the electrical flow the machines need to be electrically insultation using an appropriate earth cable, their corresponding filters and monitoring the static electricity.

To keep the bearings in good condition, apart from controlling the points mentioned in this article, we should carry out a good preventive maintenance and replace them from time to time.

Small temperature increases might be telling us that some part(s) of our machines is not working properly. Regular checks with an SKF TKTI 21 thermal camera can help us to anticipate and prevent unexpected production downtime and stoppages.


In the video below there is real case of a Bosch Rexroth  MKD041B-144-KG0-KN servomotor with detached magnets.