High-Speed Rotor Balance

Rotors that operate above their first critical speed are often referred to as “flexible” rotors. Generator rotors and some HP-IP rotors are considered flexible rotors (in general the longer and more slender a rotor is the more flexible it becomes). This designation comes from the fact that the rotors are sufficiently flexible to be elastically deformed or distorted by the effect of unbalances, while in operation. Unbalances distributed along the rotor body produce internal moments that elastically bend the rotor; and therefore, requiring multiple balancing planes along the rotor body in order to correct the in balance. On the other hand, rotors that operate below their first critical speed are referred as “rigid” rotors, because rigid rotors do not deform when unbalanced. Corrections can be accomplished using only 2 balancing planes (low speed balancing).

When rotors are spun at low speed both types of rotors behave as rigid rotor. It is only when they are spun at high speed that the centrifugal forces created by unbalances are sufficiently large to bend a flexible rotor changing its behavior from that of low speed; therefore, flexible rotors can only be correctly balanced at high speed. Rigid rotors do not change their behavior regardless of the speed at which they are run; and so, they can be effectively balanced at any speed.

Power plant size generator rotors operate well above their first critical speed and are considered “flexible” rotors. Turbine rotors that operate in close proximity to their first critical speed are "quasi-flexible". Turbine rotors that operate well below their first critical speed are considered “rigid” rotors.

• Guarantees smooth operation of the rotor, through the speed range and
  at rated speed

• The rotor vibration is observed through the entire speed range to overspeed

• All balance weight planes across the rotor body are accessible

• Mechanical integrity is verified with overspeed

• On generators, it allows for electrical testing at operating speed, therefore speed related electrical faults can be detected

Which rotors are candidates for evaluation and high speed balance?

• Rotors that have had their journals, couplings or fits machined.

• Rotors that required changes to their centers of rotation.

• Rotors that have been balanced multiple times over a long operating period or have a history of vibration problems.

• Rotors experiencing changed operating vibration resulting from:

  • Loss of mass due to rubbing or other mechanical means

  • Permanent bowing or deformation due to rubs or water induction

  • Cracking

  • Thermal instability

• Turbine rotors that have been rebladed without blades of like weight positioned 180 degrees opposite one another.

• Turbine rotors that have had blades weld repaired or foxhole rivet repaired – for verification of mechanical integrity.

• Generator rotors that have been completely rewound.

• Generator rotors that have had significant disassembly including removal of retaining rings and at least some of the coils.

Even after a high speed balance, situations arise during installation of a rotor into a shaft train that would create an unbalance.

• Coupling misalignment introduced when installing a balanced rotor will produce an unbalance condition that affects the vibration behavior of the assembled machine.

• Coupling face or locating fit runouts on mating rotor couplings will introduce misalignment when the face and rim are used to align the rotors and the couplings are pulled together.

• Correction of coupling face, rim and fit runout by machining will aid in achieving good alignment conditions, however the coupling bolt hole bolt-circle must also be considered. Correction of a coupling without correcting the coupling bolt hole positions will introduce an unbalance.

• A rotor balanced to correct for a past vibration problem must consider prior balance weight placements in adjacent rotors. Balance weights placed to correct for the prior operating conditions will become an unbalance for the newly corrected and balanced rotor.