**5.3. The analysis of indicator** *Ispa* **at very low load from no load to 20 % of full load**

The 3rd experiment analyses the changes of *Ispa* in the range of no load to 20 % of full load. Table 5 shows the MCDA broken bars fault indicator *Ispa* decline under 20% of full load. For very low load at *s*=0.44%, *fsp*=0.44 Hz the *Ispa* for 2 broken bars decreases to *Ispa*=31mA, which is approximately the half of its nominal value and it corresponds to *Ispa* for 1 broken bar (Table 2), so great confusion in broken bar diagnostics may come.


**Table 5.** *Ispa* changes from no load to 20% of full load, 2 broken bars, 1.1.kW IM

The decrease of *Ispp* representing PM, and therefore the decrease of MCSA fault indicators *aAPL, aAPH* under 20% are substantially faster than the decrease of MCDA fault indicator *Ispa* (Fig.7–Fig.9, PM in circles).

## **5.4. IM rotor fault diagnostics at time varying load**

538 Induction Motors – Modelling and Control

**Figure 7.** IM spectrum, spectrum of amplitude and phase demodulated current, 25% of full load

**Figure 8.** IM spectrum, spectrum of amplitude and phase demodulated current, 50% of full load

**Figure 9.** IM spectrum, spectrum of amplitude and phase demodulated current, 85% of full load

The aim of 4th experiment was to verify presented theory of the time varying load and its influence on MCDA fault indicator *Ispa*. The time varying load frequency *fload* = 4 Hz was chosen very near to the broken bars fault indicator frequency *fsp* =1.9 Hz.

**Figure 10.** Windows from above: MCS, time course of amplitude demodulated current, and its MCDA spectrum, 25% of full load.

Experimental results are depicted in Fig.10. Additional two new spectral sidebands appear on frequencies *fl ± fload* in MCSA spectrum-upper window and one new spectral peak appears on *fload* in MCDA spectrum –lower window.

Rotor Cage Fault Detection in Induction Motors by Motor Current Demodulation Analysis 541

behind AM -see Fig.11 and validated the

this case the MCS *aAPL aAPH* appeared (the same phenomenon which can appear at great

This phenomenon of PM delay behind AM at low inertia appears when an AC source is not properly dimensioned, its feeding voltage is lower than nominal voltage and therefore it is

It can be concluded that continual increasing of *aAPL* above *aAPH* (out of the range of possible *aAPL* and *aAPH* variations± 2.5dB) generally means, that IM working conditions are out of the

The presented MCDA and following AM and PM synthesis to the full IM current fault indicators *aAPL*, *aAPH* (6) enables the comparison of the demodulation analysis to the direct

The IM stator current demodulation analysis enables to find out the complex changes in the rotor electromagnetic field and MMF. Based on this analysis the new method MCDA was introduced. MCDA comes from the basic fundamental which occurs at dynamic rotor faults – JAPM. MCDA senses the whole stator current, but before the further evaluation as spectral analysis, it extracts time courses of fault currents directly induced by dynamic rotor faults. Anything more accurate than the extraction and direct processing of fault currents cannot

The phase demodulation extracts PM current and can be used for the research of rotor magnetic field changes and oscillation and for the sensorless speed measurement. PM is not very suitable for rotor fault diagnostics, because the fault indicators are dependent on motor

The amplitude demodulation extracts AM fault current. AM is from 20% of full load almost independent of motor load and it is the base for rotor fault diagnostics not dependent on different load and inertia. Broken bar and dynamic eccentricity fault indicators are simple spectral peaks at direct fault frequencies *fsp, fr* (no sidebands). The amplitude demodulation

MCDA is a clear, very simple and reliable method and it is very useful for industrial

Methods of the direct analysis of IM current sense and subsequently process the full IM current. The great disadvantage of the direct IM current analysis is that its fault indicators *aAPL, aAPH* are dependent on IM load and inertia moment. The second disadvantage is that fault frequencies cannot be determined directly, but as a difference from *fl*, which can change. The third disadvantage is the lower resolution both in frequency and amplitude.

inertia). The demodulation detected PM delay *-*

not able to fully excite IM.

full IM current analysis.

normal.

exist.

load.

explanation of *aAPL* > *aAPH* in Fig.4., and also the correctness of (6).

**6. Demodulation analysis versus direct analysis** 

can be easily implemented continually in a real time.

application both for diagnostics and also for IM continual monitoring.

The time course of amplitude demodulated current, so the time course of amplitude modulating (i.e. fault) current is depicted in the middle window. The sum of three harmonic with the fundamental amplitudes *Ispa, Iload* and *Ir* and corresponding frequencies *fsp, fload* and small ripple from *fr* is clearly visible.

The broken bar fault indicator *Ispa* did not change its amplitude (compare with Fig.7). The important conclusion is that the size of MCDA fault indicator *Ispa* is not influenced by the time varying load.
