Risk of opening a stepper motor?

[McFLUFFELS]

Good afternoon,
Got a pretty straight forward question. I have heard opening a stepper motor like a Nema 17 will result in a power loss for the motor.
Does this happen when the rotor is removed or the cap at the back because my repairs will not need the rotor to be removed.

[Towger]

It is when the actual motor is opened up.
 In saying that, I have opened NEMA17 motors, replaced bearings, removed (and resealed) iron filings etc and they are still working fine years later.

[Armadillo]

Nah!.... not that I heard off.
Just make sure you don't bend the shaft, lost the spring washer etc...
Do the marking, mark the position of the body, screw, bearings etc so that you will be able to put it back to same location and manner.
Otherwise how to change bearings if cannot be opened up.
Use proper tools to remove bearings off-course, yeah!. Don't use hammer to bang away.

[SeanB]

Generally no, so long as you remember the orientation of parts ( mark body and cas as described above) , put any spacers and washers back in the same position and same orientation ( some use a wave washer and others use Belleville springs, where direction does matter) and tighten the screws up in an even way, no one at a time till tight, each one a turn or two at a time going round till all are tight.

Generally the hardest part is getting the locked screws out in the first place without breaking them or destroying the head.

[Jeroen3]

Since they are often permanent magnet rotors the moment you pull the rotor from the bearing it will get pulled to the housing.
You'll need some way to maintain the airgap and slide it out.

[McFLUFFELS]

So if i keep the rotor in the housing all should be fine. I just want to open the back cover and replace a damaged wire.

[Ian.M]

It depends on the magnet material's coercivity and the field strength.  If the rotor is removed, the magnetisation of the poles may shift from normal to the pole surface to transverse, towards the adjacent pole, following the lines of flux, significantly reducing the effective field strength when it is reassembled.  Whether or not this will happen depends on how the rotor was magnetised and assembled.  Higher power, lower mass motors are more likely to be affected.   The issue can be avoided by sliding the rotor into a closely fitting tubular magnetically soft shunt, thick enough to carry all the flux as it is removed.   For PM motors with magnetised stators the shunt is slid in in place of the rotor.

As long as you can leave the rotor in situ and don't disturb the magnetic circuit, increasing the rotor gap, you wont have any problems.  For high performance motors, you may need to rigidly clamp the shaft and body so that the rotor doesn't go off-center  increasing the gap at one or ,more poles when you remove an end plate supporting a bearing.   Once you have access, you can shim the rotor with not-magnetic material so you can un-clamp the motor to work on it then reverse the process to reassemble.

[Armadillo]

It takes alot of energy to magnetise a magnet not to mention rare earth magnet unlike very old ferric and Alnico types.
If I follow correctly, then the counter reactive magnetic force due to back emf when driving the motor can also kill the magnet, if I follow per se.

Whenever therefore, you ask for paper, it is unlikely we going to see one.
So I would say foremost, its kind of a ghost story spread by words of mouth around.

Look, I might be wrong until I see research printed paper.   
Thank you with appreciation.