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Did I damage these servo motor magnets by removing the rotor from the stator?
3

Did I damage these servo motor magnets by removing the rotor from the stator?

Did I damage these servo motor magnets by removing the rotor from the stator?

(OP)
The three images below show the rotor of a Fanuc Servo Motor Model O, Type A06B-0513-B002, originally built in 1987. I do not know what material the permanent magnets are made of, but I don't think the rare earth magnets were in wide commercial use yet in 1987. As you'd expect, the raised steel pole pieces alternate north and south around the circumference of the rotor, for a total of 4 pole pairs.

The servo motor had taken pretty severe mechanical damage, and I opened it up to repair it (cleaning, seals, bearings, and encoder). Shortly after pulling the rotor out from the stator, I remembered that some (not all) pre-rare-earth rotors would demagnetize when they were removed from their stators without a keeper installed. I'm hoping that the end plates shown bolted through the pole pieces suffice for a keeper, but I don't know for sure.

Can someone tell from the pictures below if I can continue on with the rebuild and installation, or have I demagnetized the rotor and need a new servo motor ($$$)? I don't have the information or equipment to remagnetize the rotor; if I've damaged it I'm up the proverbial creek.

Thank you very much for looking at my problem!
Dave



RE: Did I damage these servo motor magnets by removing the rotor from the stator?

I can't answer your magnatizing question but let me ask this:

You say you took the encoder off? Was it a glass encoder? If so you're likely snookered already. Those encoders are used by the drive unit to commutate the stator windings. It has to be precisely located. You usually need an oscilloscope and 'the magic recipe' to re-align it. I have never been successful at it. I've tried several times.

Victims often pull the encoder to replace the bearings. Screwed!

Keith Cress
kcress - http://www.flaminsystems.com

RE: Did I damage these servo motor magnets by removing the rotor from the stator?

They look to be ferrite so you are likely OK in that department.
But Keith hit the real issue, alignment.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Did I damage these servo motor magnets by removing the rotor from the stator?

Demagnetization was an issue with tach-generators 50 years ago.
They were factory calibrated and removing the armature from the permanent magnet field would de-magnetize the field enough to destroy the calibration.
If you are unable to re-align the encoder you may be able to use the machine for an alternator.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Did I damage these servo motor magnets by removing the rotor from the stator?

(OP)
Well, this is fine kettle of fish, Ollie!

First off, thank you EdStainless; I'm very glad to know that the magnets are probably ferrite and therefore OK. The prospect of remagnetization would have unquestionably led to the motor's status being reassigned to "boat anchor".

As to the alignment of the encoder, though, I plead guilty to charges of capriciously removing the encoder module from the back of the motor. I think we've got all the instruments that might be needed, but right now I need to get the secret recipe from Fanuc.

Repurposing as an alternator instead of a boat anchor is clearly a step in the right direction.

Thank you all very much! I will post again with full details when the servomotor is working. (Such confidence!)

Dave

RE: Did I damage these servo motor magnets by removing the rotor from the stator?

If you get the recipe out of them I'll be amazed! It would be very welcome.

I get these motors about once a year from people, "It was working when I replaced the bearings, now it turns about 1 revolution and throws a fault."

I found A recipe on the web after extensive searching but it didn't work. What I really need is a correctly setup unit so I can see what 'I should be seeing' so I know 'what to see'.

Three things I believe are needed:
1) Which winding sinewave to use
2) How the sinewave should relate to the keyway
3) How the encoder HOME mark should relate to the keyway

Keith Cress
kcress - http://www.flaminsystems.com

RE: Did I damage these servo motor magnets by removing the rotor from the stator?

There's some pretty smart electrical people that hang out here:

https://www.practicalmachinist.com/

I'd post the encoder question in the "General" forum.

RE: Did I damage these servo motor magnets by removing the rotor from the stator?

I should have added a bit more discussion. What you see as poles are pole pieces made of steel. The magnets are the rectangular pieces recessed between them. This offers a longer magnetic path (putting them higher on the demag curve) and physical protection.
Regardless of material you should be fine as long as you have handled them carefully. The real test will be when you put it back together and see if you have a symmetrical waveform.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Did I damage these servo motor magnets by removing the rotor from the stator?

I suspect that those end plates are non-magnetic.
Possibly stainless steel.
Otherwise they would shunt a lot of the flux away from the pole faces.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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