3.2 Experimental Setup
3.2.2 Fault Specifications and their Genration Procedure
IMs have been considered in the present study. The procedure of generation of the considered faults in IM is discussed here.
The BRB defects have been manually produced in the workshop by drilling a number of bars in the rotor cage. A solid model of the BRB defect is as shown in Figure 3.7.
BRB
Rotor shaft Cage (bars)
Cage (end ring)
Figure 3.7 A solid model of the broken rotor bar in the IM
The SWF may be due to the turn-to-turn, coil-to-coil, phase-to-phase or phase-to-ground fault as shown in Figure 1.4. Most of the SWF are the consequence of the growth of undetected turn-to- turn faults. Catastrophic failure of IM can be avoided by early diagnosis the turn-to-turn faults within a coil. Therefore, in this study, the turn-to-turn SWF is considered with two severity levels (i.e., SWF1 and SWF2), which was simulated by tapping the stator windings to enable adding an additional load to the winding via an external control box (or rheostats) as shown in Figure 3.8. In this figure, “a”, “b” and “c” denote the three phases of IM, Ias, Ibs, and Ics denote the current in three phases, if1 denotes the current flowing through the short circuit. as1 denotes the normal turns and as2 denotes the shorted turns in phase “a”, and Rf denotes the external resistance.
The variable resistor was used to introduce a varying amount of resistance in the turn-to-turn short between the windings. The control box consists of 0-2 Ω variable resistor. Change of the severity level of the stator inter-turn short fault can be emulated by adjusting the value of this control box, which then be reflected in the variation of the loop current. High resistance simulates a low severe faults and vice versa. The control box also restricts the circulating currents in the shorted portion of the stator winding to a safe level to avoid permanent motor winding damage. However, when the control box is disconnected, the motor becomes a normal. Moreover, other SWF can be simulated using the proposed method. In order to simulate PUF and their severity levels (i.e., PUF1 and PUF2), similar procedure as the generation of SWF was adopted in a separate IM.
c b
a
Fuse Short Circiut
Ias
if1
as1
as2
Rf
Ics Ibs
s
Figure 3.8 Turn fault on a single phase of the IM
The BF may be due to the inner race fault (IRF), outer race fault (ORF), and ball element fault (BEF). In this study, the ORF is considered for the BF, which was artificially developed in the bearing near the shaft end by spalling out or drilling out some material from the outer raceway. A solid model of the ORF in bearing is shown in Figure 3.9.
Outer race
Inner race
Bearing element
Bearing cage or train ORF
Figure 3.9 A solid model of the bearing fault in the IM
The UR was achieved by taking a balanced rotor from the manufacturer and intentionally removing balance weight and/or adding weight. Here, the balanced weight was attached to small aluminum pins protruding from both ends of the rotor. A solid model of the unbalanced rotor of IM is shown in Figure 3.10.
Extra added weight Rotor shaft
Rotor bar
Figure 3.10 A solid model of the unbalanced rotor in the IM
The BR was achieved by carefully bending the rotor at the center, which when produced creates the dynamic air-gap eccentricity. Figure 3.11 shows the centrally bent rotor in the IM.
Stator axis
Centrally bent rotor axis Bearing end
Figure 3.11 The bowed rotor in the IM
The MR can be considered as the parallel misalignment or the angular misalignment as shown in Figure 3.12. The MR creates the static air-gap eccentricity. The parallel misalignment can be achieved by displacing the motor bearing the same amount on each end using four jack bolts attached with the motor. Angular misalignment can be achieved by displacing one end more than the other. In this work, the rotor misalignment is considered as an angular misalignment because the chances of angular misalignment are more than the parallel in IMs.
Stator axis
Rotor axis Bearing end
Parallel misalignment
Angular misalignment
Figure 3.12 The parallel and angular rotor misalignment in the IM
Table 3.2 The discription of motor fault conditions
No. Motor fault conditions Fault description Others
1 No defect condition (ND) Motor with no defect Healthy motor 2 Broken-rotor bar (BRB) 12 broken rotor bar out
of 34
By drilling bar in the rotor cage
3 Phase unbalance level-1 (PUF1)
Less severe fault By adding resistance (max)
4 Phase unbalance level-2 (PUF2)
High severe fault By adding resistance (min)
5 Stator winding fault level-1 (SWF1)
Less severe fault By adding resistance (max)
6 Stator winding fault level-2 (SWF2)
High severe fault By adding resistance (min)
7 Bearing fault (BF) Outer race fault A spalling on the outer race way
8 Unbalanced rotor (UR) Motor with unbalanced rotor
By attaching balanced weight
9 Bowed rotor (BR) Or dynamic air gap eccentricity
Motor with centrally bent rotor
Bent the rotor in the center
10 Misaligned Rotor (MR) Or static air gap eccentricity
Angular misalignment By displacing the motor bearing at one end more than the other.
In total, ten different faulty conditions were artificially created in eight different IMs. It is noted that, in actual case, levels of these faults may be different, but basic features of such faults would remain the same in dynamics and electrodynamics. All the considered faults in this study are common faults of IM failures that occur in the industry. They cover more than 90% of all fault modes occurs in the IM.
3.2.3 Measurement Sensors