Excessive vibration caused by frequency inverters can cause motor and structural failures, reducing their service life. In one of its success stories, Kot Engenharia and solved abnormal vibrations on an industrial platform through field measurements and simulations using the Finite Element Method (FEM). Keep reading to understand how the integration of experimental and computational analysis can ensure the performance and reliability of your equipment.

 

 

1. Effects of frequency inverters on the vibration of motors and structures

Frequency inverters are devices used in drive systems to control the torque and speed of alternating current motors by controlling the magnitude and frequency of the supply voltage. Therefore, to obtain an alternating voltage with the desired characteristics, inverters act by converting the mains voltage into direct voltage, and then converting it back into alternating voltage, controlling the parameters of this second conversion, as illustrated in Figure 1.

Figure 1: Voltage conversion in an inverter (Source: adapted from Citisystems)

The output alternating voltage is generated from direct voltage pulses, and by varying the frequency, width and signal of these pulses, the amplitude and frequency of the output alternating voltage also change, as can be seen in Figure 2. This technique is called pulse width modulation, and is better known by its acronym PWM.

Figure 2: PWM modulation of a frequency inverter (Source: adapted from Citisystems)

One problem that can be caused by this feature of inverter operation is the excitation of torsional vibration modes of the motor shaft and/or the structure in which it is located. This is because these pulsations in the motor drive voltage can cause pulsations in the motor torque, which in turn can resonate with the structure.

Therefore, this article addresses one of Kot's success stories, in which an investigative analysis of the vibration of a shoe feeder system platform was performed. The platform in question supports two feeders, see Figure 3, whose drive motors are controlled by frequency inverters, and presented vibration of the floor, the base of the motors, and frequent failure of the elastic elements of the couplings.

Figure 3: Feeder platform. (Source: Kot Engenharia collection)

In order to determine the cause of the vibration, Kot carried out visual inspections, measurements and computer analysis using the Finite Element Method (FEM).

2. Vibration analysis with visual inspection, accelerometry, and structural simulation

First, the activities began with a visit to the equipment, during which a visual inspection was carried out, measurements were taken with a motion amplification camera, and accelerometry was performed at points of interest on the structure. The main objective of this stage is to collect data for the formulation of hypotheses.

Once hypotheses about the cause of the vibration had been established, computer simulations of the structure were performed to ascertain whether the proposed cause would be capable of producing dynamic behavior similar to that observed. For this purpose, two distinct models were developed: a local model of the drive base and a global model of the platform, see Figure 4. The local model was used to perform a more detailed modal and transient dynamic analysis of the drive base, aiming to quantify the magnitude of the excitation associated with the measured vibrations. The global model, due to the simplifications inherent to the beam elements and the stiffness of their connections, was used to understand and evaluate the overall behavior of the structure when subjected to loads calibrated by the detailed model.

Figure 4: Models. (Source: Kot Engenharia collection)

Finally, once there was consistency between the hypothesis raised and the results of the simulations, an intervention was carried out on the equipment, followed by new measurements, to assess the effectiveness of the proposed modifications.

3. Identification of torsional vibration and validation with modal analysis

During the inspection, it was observed that the platform only showed noticeable vibration when the level of material in the system was above a certain threshold, and when the motors were operating at a specific rotational frequency. In addition, it was noted that the failure aspect of the motor couplings was characteristic of torsional vibration.

Thus, through measurements, it was observed that the vertical response at the base of the motor has a similar magnitude to the horizontal response, and that both are more pronounced than the longitudinal response, as shown in graph 1. This behavior may be caused by radial or torsional vibration of the motor; however, given the failure aspect of the couplings, the second option was credited.

Graph 1: Vibration measured at the base of the motor. (Source: Kot Engenharia collection)

Furthermore, it was noted that both the platform and the motor base vibrated at the same frequency, as shown in graph 2, which was much higher than the motor rotation frequency. Thus, this pronounced response is not associated with any typical behavior of mechanical problems in the equipment in question, given its construction characteristics. Video 1 shows footage of the platform vibrating, captured using a motion-amplification camera.

Graph 2: Comparison between measurements on the platform and at the base of the engine – Vertical speed. (Source: Kot Engenharia collection),

Video 1: Footage of the vibrating platform – Camera with motion amplification. (Source: Kot Engenharia collection)

Based on the characteristics highlighted, it was hypothesized that the cause of the vibration would be a pulsation in the torque of the motors, caused by the speed modulation by the frequency inverter.

The modal analysis of the base of the drives showed that the torsional natural frequency of this structure coincides with the measured vibration frequency. The associated vibration mode is shown in video 2. Furthermore, in the transient dynamic analysis, it was shown that a pulsation of the drive torque, oscillating at this frequency and with a magnitude lower than the motor's nominal torque, would be capable of causing a vibration in the structure with intensity and characteristics similar to those observed in the measurements. Graph 3 shows a comparison between the vibration measurements on the motor base and the results obtained by the model.

Video 2: Torsional vibration mode of the motor base. (Source: Kot Engenharia collection)

Graph 3: Comparison between measurement and model – Speeds at the motor base. (Source: Kot Engenharia collection)

Thus, the results of the frequency response analysis showed that, when excited by a pulse in the drive torque with a frequency of approximately 4 times the motor rotation, the dynamic response of the platform has characteristics similar to those observed in the measurements, as shown in graph 4.

Graph 4: Comparison between measurement and model – Speeds on the platform. (Source: Kot Engenharia collection)

Given that the results of the computer analysis supported the hypothesis raised by Kot, an intervention was carried out on the equipment, in which the frequency inverter's operating parameters were adjusted by the manufacturer. After the intervention, the vibration intensities measured on the platform and at the bases of the drives, which had previously been higher than the normative admissible limits, were significantly reduced and became lower than the normative limits. In addition, the responses began to occur at the motor's rotation frequency and at its second harmonic, which is an expected behavior for the equipment. Graph 5 shows a comparison of the horizontal vibration measured at the base of the motor before and after the intervention, showing the change in response frequencies and the reduction in magnitude, which in this case was 92%.

Graph 5: Comparison between before and after the intervention – Horizontal velocity at the base of the motor. (Source: Kot Engenharia collection)

4. Diagnosis of vibration caused by frequency inverters: measurement and simulation

Frequency inverters are a point of attention in vibration problems, and it is important to discern when the characteristics of the vibration point to mechanical or electrical causes.

Through vibration measurements, as well as visual inspection, it was possible to adequately characterize the vibration of the structure, making it possible to identify the inverter as the main cause of the problem, which highlights the importance of correct instrumentation of structures and a well-executed data survey.

Finally, computer analysis proved to be an important tool in validating the hypothesis raised, ensuring greater certainty as to the effectiveness of the proposed intervention, thus helping to minimize losses and rework in the field.

Vibration analysis is with Kot Engenharia

If you, like our more than 150 clients, are looking for specialized solutions in structural analysis or failure prevention such as deformation, vibration, and corrosion, consult our team and count on Kot Engenharia.

Since 1993, we have been offering engineering consultancy services through technical studies using non-destructive testing, field instrumentation and computer simulations (FEM, DEM and CFD) for highly complex diagnoses of concrete and metal structures and industrial equipment.

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