Figure 1 - Spectrum Showing Pattern
Of Peaks Separated By 2xLine Frequency (Sidebands) In High Frequency Range
bars- Extremely unusual and never found in
cast rotors. As a loose rotor bar passes a winding slot, the magnetic force
causes it to momentarily lift and then drop. The frequency, then, is the
number of windings slots x RPM (Winding Slot
Pass Frequency or WSPF). The number of winding
slots will be between about 25 and 100. The bad news is that you will not
know the number of winding slots and it is very difficult to find out.
The good news is that the vibration is accompanied by a precise sideband
- 2 x FLine.
at a very high frequency (WSPF, but we don't know what it is) accompanied
by sidebands at 2 x FLine. This symptom
is not unusual and at low amplitudes - below 0.1 ips or 2.5 mm/sec - often
means little more than an imperfection in the machine. In
fact, it usually has more to do with a potential winding problem than rotor
bar looseness (see the next page).
Vibration at 2
x WSPF and even 3 x WSPF w/ sidebands at 2 x FLine. These are much more
unusual and indicate a much more potentially severe problem.
to the next problem - looseness in the windings (make sure both are understood
before recommending any action for either problem).
analysis to determine severity. If a healthy rotor is found, it is more
likely a potential winding problem (next page).
in addition to rotor testing will provide for a comprehensive eletrcial