Servo duty cycle and overdriving

[theycallmevirgo]

TL;DR

How can I evaluate duty cycle of small servomotors?

What kind of service life can I expect from small hobby servomotors? Can I exceed current rating for increased power? How will that affect duty cycle?

Thanks so much in advance

Joe

 

[jrmichler]

My experience is with industrial permanent magnet servomotors, not hobby motors, so take this accordingly. Industrial servomotors have two torque limits - RMS (average) torque and peak torque. The RMS torque is the allowable torque for continuous operation. The peak torque is the maximum allowable torque for a short time.

Operating at torques greater than RMS torque requires calculating the motor temperature rise over a worst case operation cycle. Shaft load calculations are also required.

Operating at greater than peak torque will instantly demagnetize the permanent magnets. Then the motor is junk, and cannot be repaired.

 

[theycallmevirgo]

My experience is with industrial permanent magnet servomotors, not hobby motors, so take this accordingly. Industrial servomotors have two torque limits - RMS (average) torque and peak torque. The RMS torque is the allowable torque for continuous operation. The peak torque is the maximum allowable torque for a short time.

Operating at torques greater than RMS torque requires calculating the motor temperature rise over a worst case operation cycle. Shaft load calculations are also required.

Operating at greater than peak torque will instantly demagnetize the permanent magnets. Then the motor is junk, and cannot be repaired.

Thanks so much, very helpful

 

[hutchphd]

Operating at greater than peak torque will instantly demagnetize the permanent magnets.

Is this because of exceeding the Curie temperature?

 

[jrmichler]

No. Permanent magnets are manufactured as a non-magnetized block. That block is placed in an electromagnet, which is then energized. The magnetization process is instantaneous, plus the time to build and collapse the magnetic field. One microsecond is more than enough time to magnetize or demagnetize a permanent magnet, so temperature is not involved.

Temperature can also demagnetize a permanent magnet. This is a good discussion of temperature effects on neodymium magnets: https://www.kjmagnetics.com/blog.asp?p=temperature-and-neodymium-magnets.

As a child, I learned how to fix a compass that pointed south. My father wrapped some wire around the compass, and flashed that wire across the car battery. After that, it pointed north.

 

[hutchphd]

As a child, I learned how to fix a compass that pointed south. My father wrapped some wire around the compass, and flashed that wire across the car battery. After that, it pointed north.

But demagnetizing (degaussing) an object with remanence (hysteresis) is not so easy. Hence the typical AC degaussing field tapering to shrink the hysteresis loop to zero. So I am confused by how this works. Are you not demagnitizing but just "flipping" the field?
And some of the very newest rare earth megnets have very low Curie Temps....I was shocked.

 

[dlgoff]

(degaussing)

Brings back memories of degaussing TV picture tubes and computer monitors.
From: https://en.wikipedia.org/wiki/Degaussing

 

[TonyStewart]

I use Arrhenius Law to inter/extrapolate for a 40'C riseto a 80'C rise. This is how all e-caps are rated for accelerated failure rates. I would avoid the limits of RMS power because there is no cooling mechanism for these products and give margin to the max RMS power being applied. Typically we know life span reduces 50% for every 10'C rise above ambient until some catastrophic melting point or oxidation occurs that causes contamination or whatever.

 

[DaveE]

Are you not demagnitizing but just "flipping" the field?

Yes, I think this is correct. But you are flipping the field in the wrong pattern for good motor operation, particularly if the rotor is moving. Taken as a whole, the magnets may be effectively de-magnetized.