what is magnetic center as related to motor alignment? I posted this question once before however that was right about the time the site moved. Thanks

Very simplified, a motor's rotor and stator work somewhat like magnets, opposing each other. As the rotor rotates, it tends to find the magnetic center, or the electrical "middle", both radially, and axially.

Especially on sleeve bearing motors, this can alow the motor to either move radially a little, which will change the alignment, or axially, which will "push or pull" on the coupling.

The term magnetic centering force is usually applied to axial movement. The magnetic force generally pulls the rotor toward center of the stator axially. This may be modified slightly by rotor and stator air ducts which also want to align with each other.

You can determine mag center of sleeve bearing motor by running motor uncoupled and scribing the position. Then when coupling it is desirable to couple on magnetic center. Also important is that the coupled position is not too close to the limits of travel since the motor bearing thrust shoulders are not designed to carry continuous thrust. Some people just couple up on mechanical center for this reason. I think that usually works as well but apparently there are some cases when motor will hunt if coupled off magnetic center and when axially flexible coupling is present.

The radial component of magnetic force does not have a centering effect. If rotor goes slightly off-center, the magnetic force tends to pull it further off-center.

Pete said it a lot better than I did! We usually blue the shaft, and scribe it while it's running, to find magnetic center.

Bike Freak,

As said below, blue the shaft first. On a sleeve bearing motor, push the shaft all the way to the outboard (opp. drive) end. Scribe a line around the shaft by holding a scribe up against the inboard bearing housing and then rotate the shaft.
Then pull the rotor all the way out as far as it will go toward the drive side. Scribe another line.
Start the motor unloaded and uncoupled. Scribe a line after the shaft quicks "jacking" in and out. This line is your magnetic center.
It should be approximately halfway between the previous two lines you scribed.
We usually scribe all three lines while the motor is running. You can place a hammer handle in the middle of the shaft from the front and back and move the shaft much easier while it is running than stopped.
If you have had a shaft replaced, it is good practice to check the magnetic center. I have seen shafts not put back in the right distance.
1/16" will make a difference. As Pete says, those shoulders of the motor bearing are not meant to take a continous thrust of a motor with rotor off mag center and thrusting up against the bearing shoulder trying to get to where it can't

Dave

Good comments.

I also forgot to say that if the machine has a coupling like gear coupling allowing some limited end-float.then you have to couple it far enough from the limits of shaft travel (close enough to the mechanical center) to avoid contacting the shoulder when coupling moves to its limits.

Also, it's a good idea to check that the rotor is "level" before starting this procedure. A machinists' level works well, but the new digital levels also work fine. If a sleeve bearing motor is too far out of level, gravity can make it run downhill, pulling it off magnetic center.

I saw this once on a large motor that had an intermittent "bump" in the coupling. They had me check the alignment which was fine, but when I checked level, it was significantly low at the front end. We leveled it and then realigned it (shimmed fan bearings), and the problem was solved.

Also, as has been stated here before, the magnetic centering "force" is usually overestimated. Problems are often attributed to "running off magnetic center" which are really due to something else.

Good point. I would recommend to use a level with a round channel on the bottom to straddle the round contour of the shaft for correct positioning of the level perfectly axial. Radius of curvature of the level channel ideally slightly larger than shaft radius.

Once I tried to use a flat level and I got bad readings indicating non-level. Our machinist came and used this level and found it perfeclty level. When I played with the flat level on a round shaft, I found it is very easy to misposition it so it looks right but reads wrong.

The machinist called it a "spirit level" but I'm not sure if that is the right name.

This message has been edited. Last edited by: electricpete,