How to scrap your steppers/servos quickly!

[ESjaavik]

Today I have seen 3 different postings indicating that there are a few important things people should consider regarding stepper motors and BLDC servo motors.

DONT! Disassemble the motor! This will demagnetize it resulting in severe loss of torque.

DONT! Run too much current through the windings! Long term overcurrent will heat the motor, but that's not such a big problem because when experimenting you will feel the heat long before the windings are damaged. If below 100 centigrade most motors will be just fine. Check by spitting on it. What is worse is a current many times the labeled current as this will demagnetize the motor. And this happens in microseconds. It can happen if you connect the motor directly to a battery or a power supply with smoothing capacitors. If you do this, it is safe only as long as the voltage is lower than the labeled voltage. Because then the internal resistance of the windings will limit the current. "Connecting to" also means if it's done through a FET or bipolar transistor as seen in some bad designs on Internet that have no current limiting resistor or current sensing chopper.

This posting does not go in-depth on the subject. I feel that should be done in a tecnical bulletin on this board as it seems there are many visiting it that are not aware of these possibilities of destroying the motor without knowing it. And also because a lot of bad designs float around on the net. Some even SELL this crap for money!!!

The BLDC servos are not nearly as much in danger as they require quite a bit more knowhow to even get them moving.

[Konstantin]

This info is quite interesting. I have one question though:
How disassembling will demagnetize a permanent magnet rotor?

[Konstantin]

Why do I always have to learn by the hard way, lol.

This snip is from Stepping motho physics by Douglas Jones
Interesting info for those whos motors stall.

It is also worth noting that the best way to demagnetize something is to expose it to a high frequency-high amplitude magnetic field. Running the control system to spin the rotor at high speed when the rotor is actually stalled, or spinning the rotor at high speed against a control system trying to hold the rotor in a fixed position will both expose the rotor to a high amplitude high-frequency field. If such operating conditions are common, particularly if the motor is run near the curie temperature of the permanent magnets, demagnetization is a serious risk and the field strengths (and expected torques) should be reduced accordingly!

[Ferenczyg]

I do not know for true the main reason of the steppers demagnetization, but I know they are magnetized once assembled and disassemble seems modify the magnetic domains of the rotor-stator assembly. When reassembled, they work, but with less strength and that implies less torque for the stepper.

[ynneb]

Well now I am worried, I took off the end-cover so that I could tap it and avoid getting filings into the servo. when I took off the end-cap I noticed the rotor rested on one of the magnets. I have reassembled the servo and it seems OK.

I know with magnets you are meant to have keepers on them. Maybe the redirecting of the magnetic flux has something to do with damaging them. Maybe it is something that happens over a period of time and not for a momentary dismantling.

In my instance I would imaging filings been far worse than a small loss in magnetization.

When you think about it a servo was dismantled b4 it was made, but it still works when assembled.

I can only think that such a warning is referring to having them disassembled over a period of time.

[ESjaavik]

If you just took off the end cover but left the rotor in the motor, there should be no damage done as the flux path through the stator was not interrupted. BTW: The magnet is in the rotor, not in the cage as it is in a brushed DC motor.

I don't know the procedure, but I think they are assembled first, then magnetized. See also Ferenczyg's post.

[HuFlungDung]

I know that if you take apart a simple magnetic dial indicator base, that the magnetic strength is instantly reduced. It takes a bit of force to actually push the magnet endwise out of the "stator". But it wrecks it

[Stevie]

took mine apart a few day ago because chips got in there and i could not turn in anymore
Got it back together and it still works

[Zagroseckt]

Ahh somthing i can help with

Guys when you pull a stepper apart you do muck up the internal magnetisem of the drive.
This results in unfathful steps loss of torq and incresed power consumption.

Solution.
Remagnatise (or realine the flux in the device)

You can do this (odly enouhg) with a degousing wire. (yep same thing youd use for a CRT)

And you sweep it the same way.
Posisiton degousing wrap nere the back activate and sweep it fowerd and away from the device.
if problems presist reverce the direction.

I lerned this fixing old high speed printers.

Umm as a note (REMOVE THE STEPPER FROM THE ASIMBLY) unless you want your gantry atracting things

[Konstantin]

Zagroseckt great info!!!
What was the diameter of the degaussing wire you used and stepper size?

Can you please elaborate more on this, maybe on a new thread.
Please please please.

One thing though, a degaussing wire can restore multipole alignment?


On another thought will it be possible to use the flyback high voltage to "flash" the stepper with a pulse?

Konstantin.

[Zagroseckt]

EESH umm
..................
I'll see if i can find the work log i made on that printer for you but it was 3 yars ago.
And it was a Hevy stepper.
the modle# will be in the worklog if i can find it.

Degousing wire can eleveat stray magnetic fields by scrambling them.
More often that not a steprs reliability is do to Magnatasaton of the serounding serfices.
As long as you dont over do the degousing you wont damige the internal unipoles or the perminent magnets inside the steper.

Non permenint magnet stepers wont be effected by dissesimbling unles thers iron or nikle in them and degousing will repair that as well.

i do know wich degousing wire i used i still have it

32"long 20wind per inch 5.5amp at 115V/AC 3second spike or 10 second at half power 2.2amps
It's design was for 12-15" CRT's case wrap (you wrap the whole case) ive degoused 32" tv's with it tho using it as a loop degouser (make a loop and pull it away from the crt)


Sorry bout the bad spelling I'm on my laptop and dont have a wordprosesser instaled on it

As a note i've also made and used bilinear gousing line.
This will inflict a hevy magnetic field on an object.
but there hard to make and they burn up alot unles you make a pulse system.
This kind of line is used when dealing with "simy" permint magnetic moters. these moters loos power over time or if there scrambled /(droped)\ you first degouse compleatly then gouse in the apropet pole directions.
ok thats all i can think of for now i'll try to find that worklog in the morning

[fahque99]

I know this is a old topic, but has anyone else successfully done the degaussing?