Dynamic balance analysis of rotors for small and medium vertical hydro-generators

I. Vibration of the hydro-generator unit and the vibration of the dynamic unbalanced vertical hydro-generator unit. Although there are three reasons for hydraulic, electrical and mechanical, most of them are caused by the imbalance of the rotor quality of the generator. Because the hydro-generator rotor is large in volume and heavy in weight, it is composed of multiple parts. It is difficult to ensure balance in the processing and assembly process, and it is difficult to perform static balance test. Even if the static balance test is performed, it is difficult to ensure dynamic balance. The rotor does not produce an unbalanced moment when it is at rest. When the rotor rotates at a certain angular velocity, an unbalanced couple is generated. This kind of imbalance, the static balance test can not be found, it only appears when the rotor makes a rotary motion, so it is called dynamic imbalance.
Second, the dynamic balance test conditions Rotor dynamic balance test must first have vibration test conditions, vibration test methods generally have three kinds: the dial gauge vibration test; strain beam vibration test method and vibration pickup vibration test method. In the middle of the first method, each power station has the condition that the stationary weight is suspended by the bridge machine, and the weight is sucked up by a dial gauge with a magnetic watch base, and the measuring rod of the watch is vertically placed on the object to be tested. To select the appropriate weight of the test weight, when selecting the weight of the test weight, the vibration of the unit should be significantly different than the original, but in order to avoid severe vibration, it should not be added too heavy, generally according to the rated speed. The allowable value of the centrifugal force generated by the test weight may also be determined according to the magnitude of the vibration value of the unit.
1. Determining the weight of the test weighting block The weight of the test weighting block is determined by the centrifugal force generated by the test weighting block, which is about 0.5% to 2.5% of the weight of the generator rotor.

In the formula, P tries to weight the weight, N
G—generator rotor weight, N
G-gravity acceleration, cm/s2
R—test weighting block fixed radius, cm
N—rated speed, r/min
For low-speed units, 0.5%~2.5% of the formula takes a small value, and for high-speed units, it takes a large value.
2. The centrifugal force generated by the test weighting block is about half of the actual maximum unbalanced force, and the maximum unbalanced force is difficult to determine before the test, and the vibration can be increased by 0.01 mm for each centrifugal force of 1% of the weight of the added rotor. To determine the size of the test weighting block, namely:

In the formula, P tries to weight the weight, N
Μ—the maximum amplitude of the unit when no test weight is applied, mm
G—rotor weight, kN
G-gravity acceleration. Cm/s2
R—test weight block fixed radius, 10-2mm
N—rated speed, r/min
This formula is more suitable for high speed units.
Third, the dynamic balance test method The three test weighting method is a common method for the hydropower station to dynamically balance the generator rotor. When the unit is not oscillating and idling, the vibration of the unit is measured. If the unit vibration is too large and exceeds the specified allowable value, the dynamic balance test should be carried out.
The three-time test emphasis method is to sequentially add the test weights at three points of the same radius of the generator rotor at 120°, respectively, to start the unit to the rated speed, and measure the amplitude values ​​of the racks in the bearing together with the unweighted The measured amplitude values ​​have a total of four values, and the unbalanced forces and azimuths of the rotor are obtained from the four amplitude values.
However, in actual work, it is often difficult to accurately separate the three equal parts of 120° in the same plane and the same fixed radius due to the limitations of the rotor itself. The measured vibration value must have errors, sometimes resulting in failure to press. The formula calculates the weight of the weight to be added. For example, the data in Table 1 is the dynamic balance test data of September 3, 2006 in Tianshi Pillow Hydropower Company, Enshi Prefecture, Hubei Province.

Calculate the vibration value μP generated by the test weight according to the formula:

In the formula, μP—the vibration value μ1 generated by the test weight plus—the vibration value μ2 generated by the first point test plus the weight—the vibration value μ3 generated by the second point test plus the weight—the vibration value μ0 of the third point test plus the weight The vibration value is not meaningful after substituting the measured vibration value into the above formula, and the reason is that one of the selected test weights is small, and the second is the selected test weight fixed point due to the condition. Restrictions, neither in the same plane nor in the same radius, the number of test rotor poles is 10, there is no way to divide the equal weight into three equal parts to fix the weights, according to the field conditions, the fan holes that are not in the same plane and the same radius are also As a fixed point. In view of this, the weight of the weight is fixed according to the formula (μ1 is the maximum vibration value):

The obtained fixed angle α does not necessarily have a fixed condition at the site. According to the above test data, a suitable test block fixing point is found within a range in which the vibration minimum value is shifted to the intermediate value by 60°, and the measured vibration value is 0.028 mm. According to the rated speed, the different unbalanced centrifugal force on the rotor can be reflected on the load-bearing frame to measure the corresponding radial vibration value. For the same unit, the vibration value is proportional to the magnitude of the unbalanced force, ie μ0: μ1: μ2: μ3 = P0: P1: P2: P3, according to this relationship: 5.2: P0 = 2.8: (P0-2.5) According to this, the test weighting block P0=5.42 kg was obtained.
After the 2.5kg test weight is replaced by 5.42kg permanent weight, the final measured vibration value is 0.007mm, which is in line with the current requirements of GB/T 8564-2003, which is quite ideal.
IV. Conclusion In the actual situation, the vibration of the unit cannot be caused by a single dynamic imbalance, and the electromagnetic and hydraulic effects also play more or less, which is a comprehensive factor. No matter which method is adopted, the calculation error will be generated, and its value has a great relationship with the magnitude of the interference of other aspects and the accuracy of the measured value, and it is impossible to completely eliminate the vibration phenomenon. Although this kind of dynamic balance test method deviates from the theoretical calculation, the basis is sufficient. It is a method worthy of reference for many small and medium-sized hydropower stations. Compared with large and medium-sized power stations, the technical strength of small and medium-sized power stations is weak, and the field test conditions are relatively poor, although the test The test may be a little more frequent, but it can still effectively solve the problem of generator rotor imbalance in operation.

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