SKF AWAIV-12, User Manual, Page: 155 / 250

We can understand rotor loading when we consider the rotor as a secondary of a
transformer. When one phase being surged has a different number of rotor bars under its
stator windings than the other phase being surged and compared, there is a different
transformer action existing for each phase. When we compare them, the wave patterns on
the display indicate this difference by displaying separated wave patterns.
Not all motors exhibit this characteristic. It is most prevalent in smaller, high efficiency motors
with small tolerance air gaps. Separation of wave patterns that are due to rotor coupling can
be determined when the wave patterns separate from the first positive peak downward, cross
one another at the bottom (first most negative point), and separate again as they go upward
(positive).
Figure 127. Wave pattern comparison for motor with rotor in place.
We recommend the following procedure for testing assembled motors where rotor coupling
may occur.
1) Surge test phase No. 1 of the motor.
2) Surge test phase No. 2 of the motor. During the test, carefully turn the rotor until
the wave pattern superimposes that of phase No. 1 on the display.
3) Repeat the steps for phase No. 3.
If you cannot turn the rotor, carefully observe the wave pattern as the test voltage slowly
rises. Watch for a sudden shift to the left, instability, or flickering, which can indicate an arcing
winding fault. Many winding insulation failures will not be visible at low voltages, but become
apparent at a higher voltage.
The rotor coupling does not impede the surge impulse from stressing the turn-to-turn or
phase-to-phase insulation. It only causes the rapid damping of the wave pattern. This rapid
damping decreases sensitivity in interpretation of solid faults. Unstable, flickering wave
patterns clearly indicate a fault in assembled motors whether rotor coupling is present or not.
Figure 128. Motor with rotor in place; fault windings showing one trace shift significantly to the left.
SKF Static Motor Analyzer—Baker AWA-IV User Manual 141
Motor testing theory and reference