1.4 Condition Monitoring and Fault Diagnosis of Induction Motors
1.4.1 Various Types of Faults in Induction Motor
1.4.1.1 Mechanical Faults in Induction Motors
About 45-55% of IM failures occur due to mechanical faults. Now, various mechanical faults are described below:
A. Bearing fault: The most IM use either the ball or roller bearing, which consists of the inner race, outer race, rolling element and train (or cage) as shown in Figure 1.5. The bearing fault (BF) occurs due to damage in the outer race, inner race, rolling element and train. The main causes of the bearing failure are: high vibration of the rotor due to large output load torque that ultimately leads to high fatigue stress, improper installation of bearing, deterioration of lubrication due to shaft voltage directing to high current in bearing, due to heat conducted through the shaft, and ultimately friction and contamination. The consequence of bearing fault is the motor rotor becomes eccentric in the stator bore, causing unbalanced magnetic pull (UMP), and placing more load on bearings. The bearing fault is one of the reasons of excessive vibration in the motor as the shaft dynamics are affected by the altered air-gap between the rotor and stator, and the bearing stiffness.
The ultimate effects of bearing faults are rotor bar failures, which lead to a premature failure of IMs.
Outer race Inner race
Rolling element Bearing train
Figure 1.5 A typical ball bearing of an IM
B. Rotor related fault: In a healthy motor, the rotor is centrally aligned with the stator and the axis of rotation of the rotor, and is the same as the geometrical axis of the stator. This results in identical air gap between the outer surface of the rotor and the inner surface of the stator. However, if the rotor is not centrally aligned or its axis of rotation is not the same as the geometrical axis of the stator, then the air-gap will not be identical and the situation is referred as the air-gap eccentricity.
In fact, the air-gap eccentricity is a common rotor fault in an IM. Eccentricities can be divided into two types, i.e. the static and the dynamic, as shown in Figure 1.6. In the static eccentricity the position of the minimal radial air-gap length is fixed in space, while in the dynamic eccentricity the position of minimal air-gap rotates with the rotor. When eccentricity becomes large, resulting unbalanced radial forces can cause the stator to rotor rub, and this can result in damage of the stator and the rotor. The eccentricity may be caused by a relative misalignment of rotor and stator in the commissioning stage, wrong placement of bearing, misalignment of load axis and rotor shaft, unbalanced load, bearing wear, and mechanical resonance at the critical speed.
Stator Rotor geometric center Rotor
Stator geometric center Air gap
(a) (b) (c)
Figure 1.6 The air-gap eccentricity in the IM (a) normal motor (b) motor with static eccentricity (c) motor with dynamic eccentricity
(i) Rotor Misalignment: The rotor misalignment or misaligned rotor (MR) is of two types, i.e. the parallel and angular misalignments. In the parallel misalignment, the minimal air-gap between the rotor and the stator is fixed, and it is due to incorrect positioning of the rotor and the stator core at the commissioning stage. In the angular misalignment, the center of rotor does not coincide with the center of rotation of the rotor, and the minimal air-gap rotates with the rotor; and it is due to several reasons like the bent rotor shaft, unbalance, and bearing wear. Maximum 10 % air-gap eccentricity is permitted in IMs. The MR creates the static air-gap eccentricity. When it crosses a permissible limit, the resulting unbalance force can cause rotor to the stator rubbing, core damage and ultimately the destruction of the IM.
(ii) Bowed rotor: The bowed rotor (BR) and bent rotor are actually same phenomenon, but only difference is that the bowed rotor is measurable inside the machine housing while the bent rotor can be observed outside the machine also. It creates the dynamic air-gap eccentricity. The BR is
caused by local rubbing (resulting in permanent metallurgical changes), local expansion and yielding (resulting in permanent bowing and cracking), weight of the rotor by sitting stationary for a long time, and residual stresses. The ultimate effect of BR is the rotor misalignment, rotor-to- stator rubbing, and ultimately damage of the motors.
(iii) Unbalanced rotor: The unbalanced rotor (UR) can be defined as an uneven distribution of mass on the rotor. This is also a primary cause of vibration in IMs. The UR consists of the static and dynamic unbalances. The static unbalance is due to only unbalanced forces, while the dynamic unbalance is due to the unbalance force as well as the unbalance couple. The UR produces excessive centrifugal forces, vibrations that reduce the life of rotor, bearings, coupling, seals, and gears. The small amount of UR may cause severe problems in high speed induction motors. In actual practice, a rotor can never be perfectly balanced because of the manufacturing defects, non- uniform density of material, and gain or loss of material during operation. The unbalance rotor is caused by mainly broken the rotor bar, resulting total damage of the motor.