# How do large turbines deal with asymmetry and beat arising from those?

A large turbine used in an electric plant can easily have 10+ meters in diameter and rotate at 70+ RPM. Should any asymmetry occur the rotating turbine will produce beat which will sooner or later destroy the plant. Crafting a turbine that large and perfectly symmetrical looks like a tough challenge.

How is this problem solved? Why doesn't the beat from turbine asymmetry cause mechanical damage?

• NB that 70+ is a bit vague - some run in the thousands of rpm (e.g. MS9000 gas turbine - 3000 rpm) Commented Feb 24, 2015 at 14:20
• The large power grid generators don't run at just 1.2 Hz. That is way too slow. They generally run at the line frequency, which is 50 or 60 Hz depending what part of the world they are in. This is the slowest they can run since one revolution is then one complete output cycle. Commented Feb 24, 2015 at 16:36
• @OlinLathrop unless there are multiple magnetic inversions per rotation (unlikely though) Commented Feb 24, 2015 at 17:40
• @ratchet: Multi-pole generators do exist, but the large utility-scale generators are single pole as far as I know. Commented Feb 24, 2015 at 18:14

It is done carefully. Lots of other rotating machinery has the same problem, and whole systems exist just to deal with it.

For example, jet engines are usually smaller, but also usually spin much faster. Balancing a jet engine is something that gets lots of attention at manufacturing, and again any time when it is put back together after having been sufficiently disassembled. I have worked on metrology systems designed to measure a few µm non-roundness of circular parts a meter or more in diameter. Large maintenance facilities have such systems, in addition to the manufacturers.

One method of balancing I saw used a stackup of multiple "plates". These plates were ring-like structures that would be part of the spinning system. No plate could be manufactured exactly right. The metrology system would accurately measure the asymmetries of each plate, then calculate what orientation the multiple plates have to be mounted together to make one larger object that was balanced to high precision.

There are also other techniques, like build it best you can, then balance afterwards. Actually in practise multiple techniques are employed, with final assembly balancing always one of them. This works a lot like dynamic balancing of tires. The turbine is spun slowly, and the vibrations measured synchronous to the rotations. From that a computer system calculates how much weight to add or remove where.

The most common scheme I've seen is drilling holes or indentations in a certain area of metal left available for that purpose. The computer will tell you where to drill and what diameter based on the analysis of the vibrations. In another case we has a electronic product that spun as part of its normal operation. In this case we left a few thru-hole solder pads at different angles from the shaft. The vibration analyzer then suggested which pads to fill in with solder, which was done manually by a technician in this case.

This overall process of measure and adjust is repeated until the vibrations are below some specified level. Despite the best measurements and algorithms, any one adjustment doesn't ever seem to null out the vibrations quite as much as it should in theory.