Case Study - Importance of balance speed and plane position in balance operation of flexible rotors.

Several factors are involved in choosing a rotor's speed and plane position balancing operation. I am not going to describe all these factors now, but in this case study, I want to show one of the most important influencing factors.

One of my customers, from an electric motor repair company, called me and raised a problem:

?"We balanced the rotor of an electric motor on the balance machine (low-speed Schenk) according to the routine of other rotors at 600 rpm, but after assembly in the motor body and no-load test at 3000 rpm, the vibration was high! We tried to put a weight on the motor end fan in the field balance method manner to lower the vibration, but the behavior of the rotor unbalance response was not linear at all, and no matter what we tried, we could not reduce the vibration to the appropriate level."

?

They asked me to go there and investigate and solve the problem.

An 1100 kW electric motor with a speed of 3000 rpm with a long rotor, a relatively narrow shaft, and a propeller at the end of the shaft in a position away from the bearing.

At first glance, it seems that this rotor is flexible!

In the first start-up, it was found that the rotor passes two natural frequencies and its working speed is slightly higher than the second natural frequency of the rotor:

This is a flexible rotor, and its working speed is slightly higher than the second natural frequency, and more importantly, the fan is located at a relatively far distance from the bearing, and the smallest imbalance on the fan has a significant effect on both shaft bending modes.

For this reason, correct balancing of this rotor should be done in high-speed balancing machine on its working speed, and if needed must be balanced at each critical speed individually.

However, due to time and cost issues, it was not possible to disassemble the rotor and send it for high-speed balancing, and therefore, an attempt was made to solve the vibration problem of this machine by balancing it on-site at the working speed.

This was done in two stages:

First stage: ?balancing the rotor without fan

The fan was removed from the rotor end and the vibration amplitude was brought below 2 mm/s by performing two-plane balancing on rotor balancing planes. The problem with this work was that the motor doors had to be removed every time the mass was applied, to access the rotor balancing mass installation plane. The number of 10 weights of 70~100 gr was removed from plane 1, which was added in the previous balance! With this balance, the effect of the critical speed in the start-up was also significantly reduced.

Second stage: Fan balancing:

The fan was first balanced on the balance machine at low speed, and then it was installed on the shaft of the motor and balanced in a single-plane manner in such a way that it has the most suitable effect on both bearings simultaneously. Applying the balance weight on the fan strongly affected the vibration of both bearings, and applying the smallest weight change on the fan, changed the vibration of both bearings inversely, It can be said that the effect of the second mode is due to the natural frequency, which is close to the working speed.

Conclusion

In balancing the rotors, it must be determined in the first step whether the behavior of the rotor is rigid or flexible. This information should either be extracted from the machine's documentation, or with a run_up test, the presence of critical frequencies should be determined.

For the balance of flexible rotors, especially in cases where the working speed is close to the critical speed, it is necessary to balance the rotor in its working speed generally is possible in high-speed balancing machine facilities, and low-speed balancing will not be adequate. Although in this case study, I was able to reduce the level of vibration without high-speed balancing machine, but this will not always be possible.