Baker D30R User Manual - page 60
Surge Test Applications
12/3/2009 | 71-022 V6
D30R
60
degree of fault sensitivity.
Rotor loading (coupling) when testing assembled motors
When testing assembled motors, the rotor can influence the shape of the surge wave pattern.
These influences are as follows.
1)
Loss of wave pattern amplitude: The inductive loading of the rotor causes rapid
dampening (little to no cycles of the ringing pattern) of the wave pattern.
2)
Separated wave pattern comparisons for good windings: Imbalance in the inductive
coupling between the rotor and stator winding causes the wave patterns of two good
phases to appear separated when they are compared. By turning the rotor, this coupling
effect can be balanced out so the wave patterns superimpose.
Rotor loading can be understood when the rotor is considered 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. The wave patterns on the display indicate this
difference by displaying separated wave patterns when they are compared.
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).
If the rotor cannot be turned, carefully observe the wave pattern as the test voltage is slowly
raised. Watch for a sudden shift to the left, instability, or flickering which could indicate a
winding fault. Many winding insulation failures will not be visible at low voltages but become
apparent at a higher voltage.
Note
: 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.
Testing assembled motors from the switchgear
The surge and HiPot tests are valid tests when testing from the switchgear at the motor
control center. Not only are the windings of the motor tested, but the insulation on the
connections and feeder cables phase-to-phase and phase-to-ground are tested.
Fig 8-7: Motor with rotor in place & with faulty (shift to left) winding