When a low-speed balance isn’t enough
A short, stiff rotor behaves the same at 500 rpm as it does at 3,600 rpm, so balancing it at low speed works. A long, slender rotor doesn’t. As it approaches and passes its critical speeds, it flexes, and unbalance that was invisible at low speed shows up as high vibration. The correction planes you can reach with the rotor installed often sit in the wrong places to fix that bending.
High-speed balancing solves this by running the rotor up through its speed range in a balancing facility and correcting it at the speeds where it actually operates.
What you gain
- Smooth running across the whole speed range, including overspeed, not just at one test speed
- Better correction planes. Weights can go where they do the most good, including locations you can’t get to once the rotor is in the machine.
- A mechanical integrity check. The rotor and everything assembled on it are proven at overspeed before they go back in service.
- Electrical testing of generator rotors at speed, which can uncover faults that only appear when the windings are under centrifugal load
Rotors that should be balanced at speed
The rule of thumb: if something about the rotor’s mass or geometry has changed, it’s a candidate. That’s especially true for long, slender rotors. Typical examples:
- Generator rotors after a full rewind
- Generator rotors after major disassembly, such as pulling retaining rings or coils
- Turbine rotors after re-blading — particularly when the new blades weren’t moment-weighed and sequenced — or after significant blade repair
- Rotors whose journals, couplings or fits have been machined, or whose centers of rotation have changed
- Rotors repaired for a bow from a rub or water induction, lost mass, cracking, or thermal instability
Getting a smooth start-up after the outage
A high-speed balance proves the rotor. It can’t prove the rest of the machine train. Once the rotor is reinstalled, vibration can still come from the way it’s coupled and supported:
- Misalignment at the couplings or bearings
- Runout at the coupling face, rabbet fit or rim
- Coupling bolts that don’t match in size, weight or position
- Field balance weights removed during the shop balance that had been offsetting a neighboring rotor
- Stationary parts that are out of position and rub during start-up or shutdown
Small effects can be cleaned up with trim balancing in the field. Large ones can keep the machine from getting through its critical speeds at all. Before start-up, it pays to confirm:
- Coupling faces are square to each rotor’s running centerline
- Rabbet fits and rims run true to that centerline
- Every rotor in the train is in balance
- Bearing elevations follow the machine’s design catenary
- Couplings are properly aligned
- Stationary components are positioned with correct running clearances
Planning an outage? Call (586) 795-5550 to talk through your rotor and schedule.