Analysis of Vibration Causes of Large AC Motor Bearing Housing

【Abstract】During the operation of large current motors, the vibration of bearing housing often occurs, and some vibrations are very serious. This paper mainly analyzes “periodic stability of bearing seat vibration signal”, “analysis of bearing seat vibration causes”, and “simple analysis of large AC motor bearing seat vibration treatment measures”.

1. Cyclic stationarity of vibration signal of bearing seat

The amplitude modulated vibration signal model of the induction motor bearing is:

x(t)=A(1+Bcos(2ft))cos(2ft+),t=0,1,,T-1.bappqL(1)

In the above formula, the carrier amplitude is represented by A; the amplitude modulation factor is represented by B; the carrier frequency is represented by fa; the AM frequency is represented by fb; the initial phase of the carrier is represented by q, and fa≤fb is satisfied. According to the theory of first-order circular statistics.

Obviously, there is a cycle frequency α that is not equal to zero in equation (3), making M¹0lxa. Equation (3) clearly shows that the bearing amplitude modulated vibration signal x(t) is a cyclostationary process.

Simulation verification: for the amplitude modulation signal x(t) described by the model formula (1), the following are the specific values of each parameter: A=1, B=1, f=800Hz, f=150Hz, q=0, ab sampling The length is selected as 4096 points, and 5120Hz is the selected sampling frequency. As shown in Figure 1, it mainly reflects the time domain waveform of the AM signal. From Figure 1, it can be seen that the amplitude modulation of the signal is more obvious. As shown in Fig. 2, it presents two clusters of spectrum peaks with the carrier frequency of 800Hz as the center and the tuning frequency of 150Hz as the interval, and symmetrical on both sides. Therefore, a cyclostationary process can indeed be represented by an amplitude modulated signal, or in other words, a non-stationary signal has cyclostationarity; but from Figure 2, it can be seen that the ability of the modulated signal to be demodulated from the carrier signal is , the circular mean does not have at all.

2. Analysis of the cause of the vibration of the bearing seat

(1) Rotor bending

The axial distance between the front and rear journals of the rotor of the large AC motor is relatively long, and there are factors of the rigidity of the material and the manufacturing quality, and sometimes the rotor is improperly operated during the working process, so that the rotor has a certain static bending, which is unacceptable. Avoided. During the operation of the AC motor, if there is an uneven gap in the air between the rotor and the stator, the rotor is in an uneven temperature field or under the action of periodic electromagnetic force, which can all be used under the action of force (heat). The rotor is bent. In this case, the change of the journal of the rotor is carried out periodically along the axial direction with the rotation speed, and the journal of the rotor is the center point of the oil film bearing in the bearing bush. At the same time, under the action of the bearing center point of the oil film, and due to the elastic characteristics of the support body composed of the bearing seat and the foundation, the bearing seat will deflect axially along the bottom edge, and it is periodic, and this phenomenon is unavoidable. of. This phenomenon is more prominent if the bearing housing connecting bolts do not have enough rigidity.

(2) Asymmetric distribution of bearing seat support stiffness The occurrence of the axial offset phenomenon of the bearing seat may also be caused by the asymmetric bearing stiffness of the bearing, and may cause greater axial vibration. If the contact between the bottom of the bearing seat and the platen does not meet the standard requirements during the installation or maintenance of the motor, the contact area does not reach three quarters, and the contact points cannot be evenly distributed, which will aggravate the axial vibration of the bearing seat. . If the AC motor works in vibration for a long time, the tightening force of each connecting fastener will be loosened, especially the anchor bolts of the base platen, so that the vibration amplitude will increase with the increase of time.

(3) The bearing cap tightening force is too large. When the AC motor is working, shaft bending and shaft vibration will inevitably occur. When the rotor with shaft bending has occurred, it is trying to deflect the bearing bush and bearing seat, but it has not been realized. Deflection, then the axial vibration of the bearing seat will be further strengthened. However, in the actual maintenance, it is often believed that the greater the bearing tightening force, the more the vibration of the bearing can be suppressed. This view is incorrect. In fact, if the bearing cannot follow the deflection movement of the journal, then the shaft vibrates at this time. It will be directly transferred from the bearing to the bearing seat, but the axial vibration amplitude of the bearing seat will increase. Therefore, the tightening force of the bearing cover must be determined according to the working environment and working conditions of each bearing.

(4) Axial resonance of the bearing seat The axial resonance of the bearing seat occurs when the axial natural vibration frequency of the bearing seat is close to the working frequency of the rotor, causing obvious axial vibration.

(5) The influence of the bearing oil blocking edge In order to reduce or even disappear the axial thrust, according to the maintenance regulations, the bearing oil blocking edge should be 0.02~0.03mm lower than the Wujin contact surface. But in practice, this provision has not attracted the attention of the staff.

(6) The ideal oil film in the bearing is not established. If the oil film is not established well enough, coupled with a large friction coefficient, and the rotor does not expand smoothly, axial vibration is likely to occur at this time.

3. Simple analysis of vibration treatment measures for large AC motor bearing seat

The failure of AC motor bearings can be detected by analyzing the cyclic autocorrelation characteristics and their reconciliation. In this paper, the reasons for the vibration of the bearing seat of the large AC motor are analyzed. At the end of this chapter, the author briefly proposes several measures: (1) The platen at the contact point of the bearing seat should be re-grinded to ensure that the bearing seat and the platen are in contact with each other. The contact area meets the requirements; (2) Re-tighten the anchor bolts of the platform and reinforce them with electric welding. For example, the TSI engine not only uses the two fixing bolts on the bearing seat of each cylinder block of the engine to realize the bearing seat of the cylinder block on the cylinder block. Fixing in the vertical direction; on this basis, six transverse bolts are added to increase the transverse fixing of the cylinder block bearing seat on the cylinder block. This structural design can not only reduce the radial movement of the bearing seat of the cylinder block, but also increase the rigidity of the engine, thereby reducing engine vibration and noise; (3) Repair and scrape the contact angle between the bearing bush and the journal to the standard state . And repair and scrape the oil-resistant edge of the bearing bush to make the contact surface with the bearing bush 0.02mm lower; (4) Replace the bearing seat at the load end of the motor, research and match each bearing bush, align each connecting shaft, and dynamically balance the motor rotor. Finally, the vibration of the bearing seat is eliminated, and the motor runs normally.

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