Analysis for Motor B
From the spectrum : 2sf1 = 0.719Hz

Operational slip, s = 0.719/(2(50.156) = 0.00717

Actual Rotor Speed at the reduced load,
Nr = (f1 x 60/p)(1-s)

    = (50.156 x 60/2) x (1-0.00717)

    = 1494 r.p.m.
       (compared to the full-load nameplate
        speed = 1485 r.p.m.)

This gives further confirmation of the MCSA diagnosis and shows the influence of the actual supply frequency and the operating slip.

Test Results for Motor B
Figure b-4 - Spectrum from unhealthy rotor cage,
Motor B

This current spectrum shows there are ±2sf1 sidebands = ±0.719Hz.

The +2sf1 sideband is 32.5dB down on f1
The –2sf1 sideband is 36.5dB down on f1

EXPERT COMMENT

• Estimation of problem severity and reasons for them being of a different magnitude?
  
  
• Due to the broken rotor bars this causes speed and torque oscillations at twice slip frequency (2sf1).
  
  
• The magnitude of this speed oscillation is a function of the drive system inertia.
  
  
• This oscillation can cause a reduction in the lower (-2sf1) current sideband and it also causes an upper current sideband (+2sf1) to be induced.
  
  
• This upper sideband can be enhanced in magnitude by the third time harmonic flux.
  
  
• It has been shown via industrial case histories that an average of the dB difference (Nav) between the two sidebands and the magnitude of f1 should be taken as a measure of the problem severity.

  In this case history:
  Nav = average dB difference = (32.5+36.2)/2 = 34.35dB
  

This corresponds to an unhealthy rotor winding