## 3 FRA Applications on Induction Motors
### FRA Measurement Results  
FRA measurements have been conducted on the stator winding of the 5.5 HP IM while connected to star and delta configurations. The FRA trace of faulty cases is compared with the measured signature during normal conditions for the two winding configurations as shown in Figures 8 and 9, respectively. For star stator winding, Figure 8a shows a small decrease in the amplitude of the FRA signature due to turn-to-turn fault within the frequency range 200 Hz-100 kHz. The frequency response due to coil-to-coil SC shows more reduction in the amplitude when compared to the turn-to-turn SC fault. In contrast, the phase-to-phase SC fault shows an obvious variation in the magnitude and the resonance point at 50 kHz which is shifted to 100 kHz due to this fault. Figure 8b reveals no change in the FRA signature in the case of neutral-to-ground fault, a slight change in the magnitude due to coil-to-ground fault, and a change in the resonance frequencies due to turn-to-ground fault. As shown in Figure 8c, the OC fault has a significant impact on the FRA signature in the low-frequency range; the magnitude dropped from \(-10\) dB (normal case) to \(-100\) dB (OC case) at 10 Hz while in the high-frequency range over 100 kHz, the two FRA traces align perfectly. It is worth mentioning that these results are obtained based on the measurement  
Figure 7: Schematic connection diagram for IM FRA measurement (**a**) U–N phase-to-neutral for star winding connection (**b**) U–V phase-to-phase for star winding connection (**c**) U–V phase for delta winding connection. connection shown in Figure 7b. The FRA measurements for the connection topology shown in Figure 7a have a similar trend and are given in the Appendix A, Figure A1. For the delta connection, Figure 9a,b show identical FRA patterns of SC faults to the star connection. Only the phase-to-phase SC fault produces a slight variation in the FRA signature as compared to the star connection. This is attributed to the outages of the two SC phases from the circuit in the case of star connection which is not the case for delta connection. It can be also seen that the FRA signature for phase-to-phase SC fault in delta stator winding is similar to that of turn-to-turn SC fault. Turn-to-ground fault FRA signature (Figure 9b) has a similar FRA pattern as the turn-to-ground in star connection. The coil-to-ground fault shows a little reduction in response magnitude within the frequency range of 50 kHz to 100 kHz in the delta connection. The FRA pattern for OC fault (Figure 9c) shows a little variation that is different from the obtained pattern for star connection. The trace is shifted towards the low-frequency range with a slight in magnitude while it coincides with the normal trace at the high-frequency range reduction. This FRA's different pattern is due to the continuity of the supply current through the other two phases in the case of OC fault in one phase of the delta-connected winding. Figure 8: U–V FRA measured responses for the tested IM with stator star connection for (**a**) Normal vs turn–to–turn, coil–to–coil and phase–to–phase SC faults and (**b**) Normal vs turn–to–ground, coil–to–ground, and neutral–to–ground SC faults. (**c**) Normal vs. OC fault. FRA has been successfully used to test transformer mechanical condition. On the other hand, FRA test be extended to be used in rotating machines. Eventually, guidelines on the application of FRA for rotating machines such as IMs can be summarized as follows:  
* End-to-end open circuit method can be adopted. * FRA test should be performed in the manufacturer site and after installation to obtain the machine FRA signature. * In case of no reference signature, users can use phase-to-phase comparison in order to detect any anomalies within the three measured signatures. * Phase-to-phase measurement for delta windings can be conducted like the delta windings in power transformers. * FRA measurement can be performed when there is any abnormal condition or severe fault within the machine.