The Magnetic Center in Electric Machines.

In Electric Machines such as Motors and Generators, the shaft tends to maintain a distinct axial position when running. This position may be different from the position of the shaft at rest and, in the case of motors, when the machine is running with a coupled load.

The Magnetic center is caused due to the magnetic forces between the rotor and stator attract each other. These magnetic forces tend to ensure that the gap between the stator and the rotor is as small as possible. Hence, if the axial position of the machine at rest(mechanical center) is different from the magnetic center, the rotor of the machine may slightly move axially to the magnetic center when running without load.

Causes of shift of the magnetic center from the mechanical center.

There are many causes for the shift of the rotor axially when running. Some of them are

Effect of the cooling fan when running (air flow)
Different in the core stack length of the motor causes magnetic forces in the drive-end and the non-drive end to be unequal. These forces tend to balance each other by shifting the rotor axially.

In most machines, the magnetic center is indicated by an external indicator which is fitted on the stator and which points to a groove on the rotor. Correct positioning of this indicator ensures proper magnetic centering

Consequences of wrong Magnetic centering.

If the magnetic center is not set properly when the machine is reassembled after any maintenance work, the rotor may tend to shift beyond the axial limits permitted by the bearings. This is particularly true for sleeve bearings. This may cause the rotor to rub against the thrust collars of the bearing.

Adjusting the Magnetic Center

The Magnetic Center can be brought to the indicated position when the machine is at rest by either moving the bearings of by moving the stator depending on the provision of the manufacturer.

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