effect of shaft position on current for sleeve bearing motors 1

[electricpete]

Sleeve bearing motors may have 1/4" to 1/2" endfloat.

In theory, the position of the rotor can affect the current draw. At magnetic center - least current, off-center more exciting current.

Does anyone know how much of a percent change in current might typically be involved in moving from center to the limit of travel?

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[edison123]

pete,

If the coupling allows it, the rotor will position itself axially once a sleeve bearing motor is powered.

I remember in a 50 hp motor with antifriction bearings, the no-load current was very high at around 70% of FLC since the rotor was axially off by about 3/4 inch due to reverse assembly by the OEM. When we reassembled the rotor correctly, the no-load current fell to 30% of FLC.

I do not know if you can quantify the change or increase in no-load current with respect to axial shift.

 

[aolalde]

Electripete:

The increase of magnetizing current will be proportional to the increase of the magnetic circuit reluctance.

Motors with short core length are more sensitive to reluctance change.

In my opinion the mayor concern is the axial thrust imposed to the bearings.
Some bearings can be severely damaged when operated under axial thrust.

 

[mc5w]

edison123,

Thanks for letting me know that installing the stator backwards can affect no load current that much. At one place where I worked we had 3 seemingly indentical punch presses but one had the motor stator installed backwards resulting in backwards lead sequence and nuisance blowing of the fuses. Somebody else had changed the motor bearings within our plant instead of sending it out to a motor shop and reassembled the motor wrong. We even sent it out to a motor shop for evaluation but the motor checked out as having normal no load and full load current.