LV, 3-Phase Induction Motor
Test Objectives
- To show a case study where the lower sideband is considerably higher than the upper sideband. with a high number of broken bars in a relatively small SCIM 11 kW/14.7 h.p., low inertia rotor.
- To show the effect of load changes with a fixed number of broken bars/prediction of problem severity, etc.
- To make users of MCSA instruments aware of the phenomena demonstrated in this case study.
Figure
b-19
Current Spectrum - 10 broken bars
Nameplate
details
3-Phase, SCIM, 415 V, 20.5 A, 11 kW/14.7 h.p.,
50 Hz, 1430 r.p.m. delta-connected
Additional
information
36 slot stator,
double layer winding, 8/9 coil pitch,
180 turns/phase, 15 turns/coil.
Number of rotor slots = 51 (aluminium die cast).
Direct drive coupling to a d.c. dynamometer load machine
Full-load slip = 0.0467.
MCSA FFT Current Spectrum
Dynamic Range = 80 dB
Frequency Resolution = 15.6 milliHertz/line
(800 line spectrum)
| Test
Results The main 50Hz (actually 49.921 Hz) component = 104.4 dB The twice slip frequency sidebands of ±2sf 1
= ±5.731Hz are evidentThe lower sideband = 82.2 dB (22.2 dB down on f 1)The upper sideband = 76.1 dB (28.3dB down on f 1) |
 Figure b-20 10 broken bars were introduced |
Observations
- When If
l= 20.5A the slip s = 0.0573 which was higher than the rated value from the nameplate speed of 1440r.p.m.:full-load rated slip s = 0.0467
- N = average
dB difference between sideband magnitudes and the main supply component
= (22.2 + 28.3)/2 = 25.25 dB
- At 1470
r.p.m. (s = 0.02) = ± 2sf1 = ± 2 Hz
- The lower
sideband was 30.2 dB down and the upper sideband was 36.7 dB down on
the main supply component of f
1. This is in comparison to 22.2 and 28.2 dB down at the higher load when s = 0.0573
- This shows
that if the motor was monitored at this lighter load then the severity
of the problem looks less than at 1414 r.p.m
- A correction factor needs to be incorporated into the prediction to take account of load effects.
Return to Broken Bars Index |
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