temperature effects on brushless AC servomotors

[cokeguy]

Does temperature degrade the performance of brushless servomotors? I was told by someone at a motor repair shop that the permanent magnet of a servomotor can lose its strength if operated continously at high temps.

I didn´t pay much attention to those remarks, but some of our motors are always running at around 100 degC, and it is practically impossible to add external cooling to them. However, one of the motors failed just a few days ago, we had to change it after about only a year of operation, since at full load/speed it no longer had enough torque and causes tracking errors.

I don´t know if this was an isolated incident or if we can expect more of these premature failures. Manufacturer indicates that these motors are designed for high temperatures, but also that the cooler they run the longer they´ll last, and doesn´t give us much more quantitative information with regards to high temp effects.

Any comments?

 

[sreid]

It mostly depends on the motor manufacturer. If lower grades of Neo magnets are used and thinner magnets are used, the magnets can be demagnetized at high temperatures and high stator currents.

 

[7anoter4]

Hi cokeguy
There are a lot of types of permanent magnets .The usual material for brushless motor is
HARD FERRITE MAGNETS
From Arnold Magnetic Technology [HARD FERRITE MAGNETS manufacturer]:
TEMPERATURE EFFECTS
Temperature variation can result in both reversible and irreversible changes in magnetization. A reversible change occurs at the rate of approximately -0.2% per degree centigrade. That is, as temperature rises above ambient, induction (Br) will decrease. Coercivity, a measure of resistance to demagnetization, changes at a rate of about 0.27% per degree centigrade. As temperature rises, a ferrite magnet will increase in coercivity!
Irreversible changes can result from exposure to very low temperatures, and the magnetic quality is restored only by remagnetization. Irreversible changes can be avoided by providing an adequate permeance coefficient.
See:

http://www.arnoldmagnetics.com/products/ferrite/index.htm

Regards

 

[Clyde38]

You say your motors "are always running at around 100 degC". How are you determining this. If you are measuring the case temperature, then it is quite possible that the PM rotor is at a higher temperature. High temperature combined with high currents can cause irreversable demagnetization.

 

[sreid]

The usual magnet material for brushless motors is Rare Earth Neodymium Boron Iron.

 

[cokeguy]

Thank you all for your comments. Clyde, the 100 degC temperature I mention is the one indicated by the motor internal sensor (in the winding, I suppose), and displayed by the drive along with drive temperatures and i2t values.

I haven´t measured motor casing values.

I have seen other servomotor specs stating upper temperature limits of 130 or even 150 degC, but for how long, they don´t say.

Anybody has experience with servos running above 100 degC continuosly for several years?