**5.1. Rotor failure patterns**

This section shows the failure patterns for rotor problems.

1. Broken Bar: it is the rotor most common problem and the better known pattern. Figure 11 presents this failure pattern, where *f* is the supply fundamental frequency and *s* is the motor slip.

**Figure 11.** Broken bar pattern


Mechanical problems such as rotor misalignment and imbalance can be also inferred in the low spectrum through the analysis of the rotational frequency sidebands. Figure 13 shows this pattern, where fr is rotational frequency.

Predictive Maintenance by Electrical Signature Analysis to Induction Motors 507

**Figure 12.** Static and Dynamic Eccentricities patterns

**Figure 13.** Rotational frequency pattern

### **5.2. Stator failure patterns**

506 Induction Motors – Modelling and Control

before it becomes a failure, even when it is incipient.

This section shows the failure patterns for rotor problems.

A fault in any part of the machine is a decrease in this part performance when compared with the minimum requirements specified. Thus the fault results from natural wear, project errors, incorrect installation, poor use or a combination of all of them. If the fault is not identified in time and increases, failure may ensue (Thorsen & Dalva, 1999). Therefore, failure is the reason why the machine breaks down. This way, one tries to identify the fault

1. Broken Bar: it is the rotor most common problem and the better known pattern. Figure 11 presents this failure pattern, where *f* is the supply fundamental frequency and *s* is the

2. Air gap Eccentricity: it is the condition in which the air gap doesn't present a uniform distance between the rotor and stator, resulting in a region of maximum air gap and another region of minimum air gap. There are two kinds of air gap eccentricity: static and dynamic. Figure 12 shows the patterns for both kinds, where f1 is the supply fundamental frequency, R is number of rotor bars, and CF is the center frequency. a. Static Eccentricity: the minimal radial air gap position is fixed in the space. The stator core is bowed or there is an incorrect positioning between the rotor and the stator generated as a consequence of misalignment. Besides those possibilities, constructive aspects permit an inherent level of eccentricity due to the tolerances of the

b. Dynamic Eccentricity: the minimum air gap turns with the rotor. The main causes are: rotor outer diameter is not concentric, rotor thermal bent, bearing problems, rotor or

Mechanical problems such as rotor misalignment and imbalance can be also inferred in the low spectrum through the analysis of the rotational frequency sidebands. Figure 13 shows

**5. Patterns of failures** 

**5.1. Rotor failure patterns** 

motor slip.

**Figure 11.** Broken bar pattern

manufacturing process.

this pattern, where fr is rotational frequency.

load imbalance.

Most induction motor stator failures are related to the windings. The occurrence of failures in the stator core is less frequent. In spite of being rare, this last problem can cause considerable damages to the machine (Borges da Silva et al., 2009).

**Figure 14.** Stator winding failure modes

The failures related to the stator windings present a diversified set of possible manifestations according to the Figure 14. It is possible to notice their simultaneous occurrence. There are MCSA patterns for the detection of these failures, but EPVA is the most recommended technique to detect electrical imbalance in motors without direct torque control.

Predictive Maintenance by Electrical Signature Analysis to Induction Motors 509

(45)

The MCSA monitors the frequency components related to pulleys (motor pulley and load pulley), belts and gear mesh. It has been observed that load problems can reflect in the transmission system frequency components. This characteristic is one more way of detecting mechanical load failures to be used in addition to the load characteristic frequency components. 1. **Pulleys**: by analyzing the rotational frequency one can detect problems related to the motor pulley. When there is no change in the speed, it is not possible to distinguish the damaged pulley from the healthy one since they have the same rotational frequency. But when a speed transformation is present, one can monitor the load pulley and the attached load through the pattern presented in Figure 16. In this case, *flf* is equal to *fpulley*,

> \_ \_ *motor pulley r*

*D f <sup>f</sup> <sup>D</sup>*

Where *fr* is the rotational frequency, *Dmotor\_pulley* is the diameter of the motor pulley and *Dload\_pulley* is the diameter of the load pulley. The sideband components of the fundamental are

The most common problems are eccentric pulley, pulley with mechanical looseness and unbalanced pulley. Problems related to the load can also reflect in the same frequencies. When this happens, the analyst himself must cross pieces of information from other

2. **Belts**: the first step when monitoring the belt characteristic frequency components is to calculate the belt frequency (*fb*). In this case, *flf* is equal to *fb*, and *fb* is the belt

*motor pulley r* \_

(46)

*belt D f <sup>f</sup> <sup>L</sup>*

*load pulley*

and *fpulley* is the load pulley characteristic frequency given by (45).

*pulley*

*5.4.1. Transmission System Failure* 

at *f*<sup>1</sup> *fp*.

**Figure 16.** Load Pulley Pattern

spectrum regions so as to arrive at a reliable conclusion.

*b*

characteristic frequency given by (46).
