AC Induction Motor Problems:
Loose Rotor Bars

Figure 1 - Spectrum Showing Pattern Of Peaks Separated By 2xLine Frequency (Sidebands) In High Frequency Range (30-90xRPM)
Loose rotor 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.
Loose Rotor Bar Symptoms:
  • High amplitude 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.
  • Symptoms identical to the next problem - looseness in the windings (make sure both are understood before recommending any action for either problem).
Recommended Actions:
  • On-line current analysis to determine severity. If a healthy rotor is found, it is more likely a potential winding problem (next page).
  • Winding testing in addition to rotor testing will provide for a comprehensive eletrcial PdM program.

Figure 2 - Motor Construction

Figure 3 - Winding Construction