Figure b-14
Time domain current signal, base frequency = 50Hz

Nameplate details
3-Phase, SCIM, 415 V, 20.5 A, 11 kW/14.7 h.p., 50 Hz, 1470 r.p.m. delta-connected.

Additional information
36 slot stator,
"Double layer lap wound, 8/9 coil pitch"
180 turns/phase, 15 turns/coil
Number of rotor slots = 28
(aluminium die cast - deep bar design)

For a slip, s = 0.01

No twice slip (±2sf1) frequency sidebands present.

For a slip, s = 0.01 (i.e. 1485 r.p.m.)

Twice slip frequency sidebands are obvious at :
±2sf1 = ±1 Hz

For a slip, s = 0.02 (i.e. 1470 r.p.m.)

Twice slip frequency sidebands are obvious at :
±2sf1 = ±2 Hz

Note
The sideband magnitudes in dB are higher (N, the dB difference, is therefore lower) due to the increase in load. This demonstrates the effect of load changes with a fixed number of broken rotor bars - compare Figures b-16 and b-17

Figure b-18
Current spectrum - three broken bars
(Nr = 1685 r.p.m.)

The sidebands (±2sf1) are the same frequency as was the case with Figure b-16 at 50 Hz

Speed of rotating magnetic field from the stator winding
Ns= (40x60)/2 = 1200 r.p.m. compared to 1500 r.p.m. for the 50 Hz case and 1800r.p.m. for a 60 Hz case.

Slip at 1185 r.p.m.
s = 0.0125 or 1.25%

Twice slip frequency sidebands are obvious at :
±2sf1 = ±2 x 0.0125 x 40 = ±1 Hz

Test Conditions (for the spectra above)
MCSA FFT Current Spectrum
Dynamic Range = 80 dB
Frequency Resolution = 12.5 milliHertz/line

EM Diagnostics Ltd,
20 Greystone Road,
Alford, Aberdeenshire, AB33 8TY
Scotland, UK