What you should know about a motor's magnetic center

Electrical Apparatus, Jan 1998 by Nailen, Richard L

Answers to questions you may not have thought to ask

Q: What is a motor's "magnetic center"?

A: Sometimes called "electrical center," it's the axial position that the shaft/rotor assembly will attempt to maintain during steady-state running at rated voltage and frequency, without the influence of any coupled load.

In the usual ball- or roller-bearing machine, the shaft position is fixed by the bearing mounting. But in sleeve bearing motors, the bearing construction inherently permits mechanical movement or "float" (typically 1A" or 2", depending upon motor size). Low sliding friction allows the shaft to drift readily back and forth to seek whatever position is dictated by the axial forces acting on the rotor.

Q: What creates those forces?

A: Like an elastic band, the magnetic field linking rotor and stator across the air gap will be stretched if the rotor is pulled axially out of alignment with the stator. A restoring force will tend to pull it back. Movement in the opposite direction will be similarly resisted. That location for which such forces are balanced is called "magnetic" center because those forces are magnetic in nature (although, as we will see, other forces may be present as well).

Q: Why isn't magnetic center necessarily the same as mechanical center?

A: In a perfect machine, the rotor/shaft would find its rest position exactly midway between the limits of mechanical movement allowed by the bearing and journal relationship. Here are the main reasons why that may not happen:

Differences between the magnetic structures of stator and rotor-in overall length, in flatness and squareness of the core ends, and in the relative positions of radial air vents in the two structures. These result from necessary tolerances in the manufacturing process. For example, core stack length in a large machine is allowed to vary as much as a quarter inch. The result is axial variation in the magnetic field "stretch," causing the rotor to move until the forces in one direction are balanced by those in the other.

Non-electromagnetic forces that are not balanced endfor-end, because of differences in rotor fan action. Winding connections typically cause air flow paths-and therefore fan pressure/flow characteristics-to differ between the two ends of the machine. This effect is most often observed in 3600 rpm motors.

Q: What difference does it make?

A: As long as the rotor does not seek a magnetic center that forces a shaft journal into continuous running contact with a bearing thrust face, no harm will result from a difference between magnetic and mechanical centers. Running uncoupled in a properly built motor, the rotor will never move far enough to "close up" all the mechanical end play provided.

At the factory, if the magnetic center lies outside (or even dangerously close to) the boundaries set by the mechanical end play, three corrective measures may be employed depending upon how the machine is constructed. The most common is to shift the bearings axially in their housingsmoving the mechanical center closer to the magnetic center. Some sleeve bearings are fitted with adjustment screws for that purpose.

A second fix is to move the stator in its housing. Some designs permit that; most do not. Similarly, the rotor core might be moved on the shaft-but large, high-speed machines seldom permit that. Reworking or replacing the shaft itself, to change the journal positions, is likely to be required.

Q: How much endwise force is involved when a motor is restrained against finding its magnetic center?

A: The value is typically quite small (see Figure 1). Measurement is diffcult because holding a spring scale against the end of the rotating shaft introduces its own axial force. But motor bearings aren't designed to withstand even these low values. Bearings with thrust capability could be provided-at a price.

Q: Is thrust the only issue?

A: No. Figure 2 shows another effect, largely resulting from the greatly increased magnetizing current associated with the distorted magnetic field. But it is of little importance, because no properly constructed sleeve bearing motor could allow such extreme displacements; they result only from gross manufacturing errors.

Q: What about shaft movement during starting?

A: The rotor of an accelerating motor will usually "hunt" or "bump" back and forth between its mechanical limits. That does no harm because it doesn't last long enough. At full speed, most rotors will settle into one safe position. Others, particularly some 3600 rpm designs, may continue to drift slowly back and forth, which some users find unsettling. However, proper use of a limited-end float coupling (in accordance with NEMA standards) will constrain the shaft to run in a safe position while the load is being driven.

Q: Doesn't that constraint involve some endwise thrust against a bearing in the driven machine?

A: Yes. But the magnitude is too low to constitute any threat to bearings normally provided in such machinery.

In conclusion: We emphasize again that motor magnetic center often is not, and need not be, coincident with mechanical center. The only requirement for safe operation is that the magnetic center fall somewhere within the limits of the mechanical end play. To show that, users sometimes request a "magnetic center indicator" as a motor accessory. This metal pointer, bolted to the bearing chamber, shows at a glance whether or not the rotating shaft is "floating" within the end play limits (shown by scribed lines on the shaft; the motor manufacturer may also provide a third scribed line indicating the magnetic center as well, either in place of or in addition to the pointer).