several case studies here are presented in Vibration Analysis in electric motors.
The three-phase induction electric motor is the most common drive and an unexpected malfunction in such machines can be very onerous economic implications. This means that today is often the use of preventive maintenance based on time approach as the maintenance of these machines. As the MTBF of this type of machine eight years in the petrochemical industry ( for a working 8760 hours / per year) it is common to use very-intervals shorter. But also are already common facilities where preventive maintenance is performed only based on knowledge of the machine's working condition. This latter approach, due to economic and inserted also needs in modern maintenance philosophies, also results from the fact that maintenance managers of facilities where this approach is practiced, feel comfortable with the most common condition monitoring techniques.
Case 1 – Paw Assay for thigh-identification conditions as a cause of eccentricity situations in Static Induction Motors AC
this essay, performed with the engine running under normal operating conditions, It aims to identify the existence of leg-thigh conditions that often are the source of the static eccentricity between iron AC induction motors. That way, intended to eliminate excess vibration that this problem may generate, increasing life two components, preventing unscheduled stops due to premature deterioration or even breakage of some of the engine components or coupling union.
Case 1 Example test performed on an engine
An engine running at 2964 presented rpm vibrations above the established limits for this type of equipment. The results of vibration analysis in electric motors observed that predominated in the spectrum the second harmonic frequency of the electrical network (100 Hz). The filtered amplitude had a value 6,7 mm/s RMS. this feature, illustrated in Figure 1, It is caused by an eccentricity static condition in the air gap.
Figure 1 - frequency spectrum recorded during routine measurement
Case 1 – measures after loosening legs
After loosening the first motor paw, We observed that there were no major changes in the spectral characteristics, namely, the amplitude of frequency 100 Hz (Figure 2).
Figure 2 - Frequency Spectrum recorded after unscrewing the front paw on the left
The most significant changes in the amplitude of the second harmonic of the frequency of the power grid occurred when unscrewing the back of the left leg and when unscrewing the front leg on the right side. In the latter case, the amplitude of the frequency dropped to 0,2 mm/s RMS. In the figure 3, registered frequency spectra are shown after loosening each of the four paws engine where one can observe significant changes in the amplitude of the frequency 100 Hz.
Figure 3 - spectra recorded frequency after loosening each of the four legs
With the test done, It was possible to identify objectively, vibrométricos that elevated levels in the presence engine were associated with leg-thigh condition that originated static eccentricity between the motor iron. The correction / deletion paw-thigh will greatly reduce vibrométricos levels at which the engine is submitted, reflecting on increased component life.
case study 2 –vibration analysis in electric motors – Analysis resonance vibrations in the drive of a pulper
A vertical engine that drives a pulper via belts, such as shown in Figure 4, with 600 kW of power and 1000 rpm, after the assembly and general improvement, It showed high levels of vibration voted, As shown in Table I below presents.
Figure 4 – Scheme equipment with the location of the measuring points
Case 2 - first measurements
The vibration analysis results in electric motors, shown in Table I, It reveals that the recorded levels are vibrométricos, mostly, considered "Severe" according to the evaluation criteria adopted. Only the engine support, side pulley, according to the measurement and directions Vertical Axial, vibrométricos the levels are considered good / acceptable. data collected and presented in Table I., It reveals that the recorded levels are vibrométricos, mostly, considered "Severe" according to the evaluation criteria adopted.
Global Vibration levels (mm.s-1 RMS)
In support of the engine, side pulley, The data presented above show a significant difference between the amplitudes vibrométricas recorded in two radial measuring directions (Vertical and horizontal). The spectral analysis performed showed that vibration levels are present in the engine, about everything, influenced by the operating frequency range (1xRPMmotor=16,65 Hz). This condition is illustrated in the frequency spectrum shown in Figure 5.
Figure 5 – Analysis resonance vibrations – frequency spectrum collected on the motor support opposite the drive, horizontal direction
Given the asymmetry revealed by vibrométricos levels observed between the two radial measuring directions (Vertical and horizontal), the data presented so far were inconclusive as to the cause for the dynamic behavior registered.
Case 2 - Measurements in transitory regime
In order to confirm the existence of conditions of resonance, Assays were performed in transitional regime, to determine the natural frequencies of the system.
The tests performed revealed in the horizontal direction at a natural frequency 919 CPM, due to the variation of the phase angle between about 90 919 CPM and 999 CPM. this feature, illustrated in Figure 6, shows a resonance condition, due to the fact that the natural frequency is close to the engine operating frequency (1000 CPM).
Figure 6 – Analysis resonance vibrations – Bode diagram taken in the horizontal direction
Case 2 - Recommendations made
Depending on the data collected, It was recommended rectification motor support base and the slide rails of the belts.
Case 2- Measurements after repair
After rectification of the bases and the reconstruction of massive, was a new measurement performed, whose values are shown in Table II.
Global Vibration levels (mm/s RMS)
Comparing the values shown in Tables I and II, There has been a significant improvement in vibration levels. The spectral analysis performed showed that the improvement is, about everything, due to the reduced range of motor operating frequency (16,59 Hz), As can be seen in the spectra shown in Figure 7.
Figure 7 – Analysis resonance vibrations – Spectra of frequencies collected at the point 1 Vertical, before and after the intervention on the whole support base
In order to justify the improvement, Two tests were performed in transitional regime (in the vertical direction and the horizontal direction according to), to determine the natural frequencies of the system (Figure 8 e 9).
Figure 8 – Analysis resonance vibrations – Bode diagram taken in the vertical direction
Figure 9 – Analysis resonance vibrations – Bode diagram taken in the horizontal direction
case study 2 – -Conclusão resonance vibrations Analysis
The inspection carried out concluded that the cause of the high levels in the presence vibrométricos the machine was associated with a resonance condition, caused by the proximity of the natural frequency of the system and the motor operating frequency (1xRPM).
As found in this case, MRI is one of the causes that may lead to high amplitudes 1xRPM. But not only: other abnormalities / damage not only the imbalance, can give increased amplitude 1xRPM, as surely we will be able to address in future issues of this Newsletter.
case study 3 – Analysis resonance vibrations in an electric motor
We are quite often we are asked about the causes that are behind some atypical behavior of rotating machines. Today, with the "pressure" to reduce energy costs, is quite often we find driven motors with frequency inverters. This trivialization can, in some conditions, cause certain problems in the equipment. One of the most common problems, from other, is that these machines that operated with low vibration values (without variator), go to vibrate more at some speeds (with speed variation). The data that follow are presented below, They were collected in a pump identical to that shown in Figure 10.
Figure equipment 10-photography with the identification of measurement points
case study 3 - Initial Measurements
Analyzing the values presented in Table III, We found that the vibration values registered in the machine to speed 1121 rpm, They are clearly superior to the values collected when the engine was at maximum speed (1479 rpm).
Like this, easily conclude that the slower speed (1121 rpm) The levels are higher vibrométricos, compared to values recorded at higher speeds (1479 rpm). This behavior may be considered atypical because the machines tend to vibrate more, as the rotation speed is higher.
The spectral analysis performed to the collected elements, revealed that the majority of the measuring points, the severity recorded at a speed of vibrométrica 1121 rpm was, about everything, due to the operating frequency range of the group (18,68 Hz). this feature, illustrated in Figure 11, imbalance arises normally associated with.
Figure 11 – Analysis resonance vibrations – Frequency spectrum collected at point 1 Vertical
Yet, analyzing the figures in detail we can see a significant difference in values, between the two measuring directions (V- Vertical e H- Horizontal).
this difference, associated with the fact that, with increasing speed vibration values decrease (the opposite situation to an imbalance condition), It led us to suspect that the equipment might be a resonance condition.
Case 3 – further testing in the vertical direction
Additional tests carried out, namely the realization of impact instrumented test hammer, They revealed the presence of a natural frequency 18,8 Hz (1128 cpm) according to the direction Vertical (Fig. 12).
Figure 12 – vibration analysis in electric motors – Analysis resonance vibrations – Figure transfer and phase angle function recorded during impact with the instrumented test hammer (Vertical direction)
Caso 3 – further testing in the horizontal direction
Toward Horizontal, The same tests also revealed a natural frequency 20,5 Hz (1230 cpm), As can be seen in Figure 13.
Figure 13- vibration analysis in electric motors – Analysis resonance vibrations – Figure transfer and phase angle function recorded during impact with the instrumented test hammer (Horizontal direction)
Alongside these frequencies, also confirms a phase angle range of about 90 (characteristic resonance condition).
Caso 3 – Considerations on the outcome of the tests
With the completion of the impact testing, It was possible to determine the natural frequencies of the system, and conclude that high levels vibrométricos that the equipment was submitted, when he was running the 1112 rpm, were generated by a resonance condition according to the horizontal direction.
Resonance occurs when one of the vibrational frequencies in the presence on the machine (often forced) It is close to the natural frequency of the system.
In other words, we could say that a resonance condition is not more than the coincidence of the encounter between two frequencies (the natural frequency of the system and one of the vibrational frequencies in the presence on the machine). These frequencies are always present in machines and only become a problem when they are close to each other.
Case 3 - Notes on resonance
It should also be noted that, often, prior to characterize these resonance phenomena, most responsible for equipment try to minimize vibration levels, making successive equilibragens the rotary joint, and / or alignment of the various checks, where applicable.
These attempts usually have no effect, since the equilibrium condition or alignment are not the root of the problem. Depending on features such as the amplification factor and the damping system, the actions mentioned above can reduce the levels in the presence vibrométricos, but not enough that the machine can operate for long periods, without their reliability is affected. De facto, these actions are just trying to change the magnitude of the exciting force, not eliminating the resonance condition.
case study 3 – Conclusion resonance vibrations Analysis
For most builders / installers of equipment, the problems associated with resonance conditions are difficult to solve, due to the fact, sometimes, are not aware of the actual reason for the high levels vibrométricos presence in machine. Characterized this condition, sometimes, the resolution of the problem is relatively simple.