Page 118 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

NOTE ON ELECTRIC MOTORS

In general, the electric motor is considered as the movable
machine when performing alignment of equipment trains. Often
this is due to constraints caused by piping or other connections to
the driven equipment. Usually this creates little or no problem,
since the electrical connections to the motor are most often termi-
nated with a short piece of flex conduit.
However, some potential problems can occur when aligning
the motor to a stationary piece of equipment. In some cases, the
motor may be aligned in such a manner that its magnetic center
and its gravitational center do not coincide.
This internal misalignment of centers can cause the motor’s
rotor to pulse as it operates. This is due to its hunting for the
magnetic center while being pulled to its normal center of gravity.

Motor Rotor Operating
In Its Magnetic Center

Figure 7-12. Rotor Operating in its Magnetic Center

The motor rotor can be pulled from its magnetic center by the
equipment it is coupled to. That is, the driven equipment may
produce a force across the coupling that attempts to either push or
pull the motor rotor from its magnetic center. In either case, the
magnetic field will try to force the rotor back to its magnetic cen-
ter, while the other force tries to force it away from the magnetic
center. This force imbalance often results in the motor rotor surg-
ing back and forth, pounding the thrust bearing of the driven
equipment.
This problem is easily identified, and easily cured. The motor
should be stopped and disconnected from the coupling of the
driven equipment. The motor is started and allowed to reach full