**2.1. Classification of the maintenance activities**

"Maintenance" can be understood as the action to repair or to execute services in equipment and systems. It can have its activities classified in four main groups:

a. Corrective maintenance: this is the most primary form of maintenance. It occurs after a failure carried out. Usually, it becomes the unavailable equipment for use. Many disadvantages of this type of maintenance are clear. As examples, the systematic occurrence of not-programmed stops, lesser time of useful life for the machine, bigger consumption of energy (since with the presence of the failure the motor needs more current keeping the constant torque) can be cited.

Predictive Maintenance by Electrical Signature Analysis to Induction Motors 491

interesting and little explored field surfaces, which is the introduction of predictive

Consider the following brief description of the most common failures that can be avoided

a. Bearings Faults: can be caused by incorrect lubrication, mechanical stresses, incorrect assembling, misalignment, etc. They can affect all the bearing parts such as inner and

b. Stator Winding Faults: normally a consequence of overheating, contamination, project errors, etc., possibly causing shorted turns, shorted coils (same phase), phase to phase, phase or coil to ground and single phasing. Such failures cause stator electrical imbalance as well as variations in the current harmonic content. Mechanical problems can also occur in the stator such as loosen edges, but this is statistically less frequent. c. Rotor Faults: usually caused by broken bars or broke end rings, rotor misalignment and

Faults in the coupling (pulley, belt and gear mesh) and in the attached load also can be diagnosed. The failures are also related to the petrochemical process different characteristics. For example, at off-shore plants, the motors start directly from the mains. This demands high start currents and causes pulsating torques which contributes to the origin of rotor and stator faults. Furthermore, outdoors motors present more incidence of failure than indoor motors. The same statistic holds for high voltage motors and high speed

The main focus of problems in three-phase induction motors are in the stator and the supports. The main causes of failures are: superheating, imperfections in the isolation, mechanical bearings and electric failures. Figure 2a presents a division of the failures in three-phase induction motors with squirrel steamer and power of 100 HP or higher (Bonnett

**Figure 2.** Problems in: (a) Three-Phase Induction Motors, (b) Motor Stator, and (c) Motor Rotor.

(a) (b) (c)

motors when compared with low voltage motors and low speed motors.

**3.1. Abnormalities in three-phase induction motors** 

maintenance techniques based on electrical signature analysis.

through the adoption of condition monitoring methods:

outer races, cage and balls or rolls.

imbalance.

& Soukup, 1992).

**3. Common failures in three-phase induction motors** 

