## 3 Experimental Setup  
All the measurements presented in this paper come from experimental laboratory tests. The experimental setup used is the same as in the previous publications of the authors, such as in [52]. The test bed consisted of an explosion proof squirrel cage induction motor of 1.5 kW, wye-connected, coupled to a magnetic-powder brake through an elastic joint. The induction motor was fed by means of a 2.2 kW open loop Space Vector PWM (Pulse Width Modulation) inverter, model MMX34 AA5D6F0-0 from the brand Moeller. The brake can be easily modulated to obtain variable load torques through a variation of its excitation current. In fact, some tests reported in this paper used an oscillating load torque: a low frequency sinusoidal torque was added to a constant average load torque to obtain the Low Frequency Torque Oscillation (LFTO) condition. This situation can simulate some types of load, e.g., reciprocating compressors, where torque depends on the angular shaft position. The LFTO could also simulate a broken rotor bar defect; in fact, as a consequence of the asymmetry in the air-gap flux distribution due to the broken bar, an oscillating electromagnetic torque is produced. The torque oscillation generates sidebands around the fundamental supply frequency given at the frequency  
\[f_{sb\_osc}=f_{s}\pm k\cdot f_{osc} \tag{5}\]  
While for the case of the broken rotor bars, the sidebands will be at frequency  
\[f_{brb}=f_{s}\pm 2\cdot k\cdot s\ f_{s} \tag{6}\]  
where \(f_{s}\) is the supply frequency, \(f_{osc}\) is the frequency of the torque oscillations, and \(k\) is an integer (usually considered as \(k\) = 1, 2, 3). In addition to the power components, the experimental setup was composed of the following sensors: a current sensor based on a Hall effect, two stray flux sensors (one for the axial flux and one for the radial flux), and two accelerometers. The test bed is shown in Figure 2.  
In this paper, only measurements from the radial stray flux sensor and from the current sensor are analyzed. The stray flux sensor was composed of a C-shaped ferrite nucleus, on which 300 turns of 0.112 mm diameter wire were wound. The sensor was located on the final part of the body of the motor, with its longitudinal axis parallel to the axis of the motor. As stated in a previous work [53], this position has been recognized to be the most effective for the diagnosis of the motor, at least for the detection of stator partial short circuits. The current sensor used was a Hall-effect sensor from the brand Tektronix, model TCP305 (Beaverton, OR, USA). The characteristics of the current sensor are shown in Table 1.  
Figure 2: Laboratory test bed. Red arrows indicate the radial stray flux sensor and part of the current sensor. The data were sampled from the various sensors with a multichannel NI-USB6212 Data Acquisition Board. The chosen sampling frequency was 120 kHz, due to the fact that the switching frequency of the inverter is 6 kHz and, following the procedure already used in [54], to correctly evaluate the harmonic distortion introduced by the inverter, a sampling frequency of 20 times the switching frequency was considered to be more adequate. For antialiasing purposes, a hardware single-order low-pass filter was introduced into the measurement chain, between each sensor and the DAQ board. The bandwidth frequency selected for the filter was approximately 2 kHz. The experimental tests were carried out with two types of bearings and two different faulty conditions of the bearing: generalized roughness (bearing wear) and a single-point defect (specifically a crack) on the outer ring of the bearing. This second condition was evaluated for different angular positions of the crack, similarly to the measurements reported in [20]; in fact, the weight of the rotor affects the intensity of the faulty signals, as reported in [55], so a crack located exactly under the direction of the weight should generate higher vibrations and displacements of the rotor in the air-gap.