High-speed balancing of generator rotors

Fortum Turbine and Generator Services - TGS, runs a workshop designed for full-scope generator maintenance services in Västerås, Sweden. Next to the workshop facilities is the only independent high-speed rotor balancing facility in the Nordic countries, expanding the service capabilities even further. At the balancing facility, we operate two specialised machines that perform high-speed balancing and testing according to the ISO 21940 quality standard. Using the experience gained from working with more than 700 rotors, our engineers perform both hot and cold balancing, as well as electrical tests both at standstill and operational speeds.

Originally commissioned to balance all newly built ASEA and ABB turbogenerator rotors produced in Västerås, the balancing facility now services new and repaired or refurbished rotors of any brand. Over the years, our skilled engineers have successfully balanced everything from the smallest exciter rotor to huge nuclear power turbogenerator rotors.

At the balancing facility, we balance rigid and flexible rotors at low-speed as well as high-speed. We also perform over-speed tests at 120% of nominal speed to confirm structural integrity.

As long as they fulfil the requirements for the balancing units, we handle rotors for motors and generators as well as other applications. Further requirements include bearings, bearing oil seals, and an adapter to the running motor. Our inventory includes stocks of these standard parts. In addition, our experienced engineers are on hand to make any adaptations necessary to fit your rotor; they can design and manufacture whatever parts are required. We can also use operator-supplied bearings, which we fit to our pedestals with adaptation rings.

Rigid vs. flexible rotors

Before a rotor can be balanced, it must first be designated as ‘rigid’ or ‘flexible’. Rigid rotors operate well below their first critical speed, while flexible rotors may have one or even two critical speeds below their operational speed. In this context, ‘critical speed’ is defined as a rotational speed that matches the natural frequency of the rotor. It is essential to designate whether the rotor is rigid or flexible (and for the latter, the number of critical speeds below operational speed) in order to define the balancing procedure and the requirements for declaring the rotor well balanced.

In addition, the rigid vs. flexible designation also affects the regulatory standard applicable to rotor balancing. For both rigid and flexible rotors, the standard ISO 21940 applies today. Oil & Gas industry requirements are usually API 541 or API 546, but there may be others that also apply. These standards also define the balance quality grade to be achieved.

Low-speed balancing can be done (usually at speeds <900 rpm) for rigid rotors that do not require a high balance quality grade. High-speed balancing is needed for all flexible rotors and rigid rotors that require a very fine grade of balance quality. Moreover, due to the very high inertia energy of a rotor turning at high-speed, the balancing machine has to be built into a bunker or a thick concrete housing. Fortum TGS balancing units fulfil this requirement. Below is an example of a balancing test programme for a turbogenerator rotor.

Example of a balancing programme:

• Adjusting bearing pedestals in the balancing room
• Fitting the rotor in the balancing machine
• Run-out check before balancing
• Determination of balancing speeds
• Test run to establish influence coefficient matrix
• Balancing to nominal speed
• Over-speed test to 120% of nominal speed for 2 minutes
• Final trim balancing: G 2.5 quality according to ISO 21940
• Run-out check after balancing
• Securing balance weights
• Returning rotor to the delivery box or cradle

Computer evaluation of unbalance

Our engineering staff helps operators to evaluate the balancing test results and assists with troubleshooting. We have a proven track record in vibration analysis, both at our balancing facility as well as at the operator’s site. We use special computer systems that quickly acquire data and evaluate rotor unbalance. All information gathered is analysed so that balancing is completed in the shortest time possible. If problems like subharmonic excitation or thermal deflection occur, the software quickly analyses the vibration behaviour and generates valuable data for use in decision-making. In addition, data from the rotor’s previous balancing (or from a series of identical rotors) can be used to optimise the process even further.

Electrical testing of turbogenerator rotors

Our balancing facility is also fully equipped to simulate operating conditions and run a wide range of key electrical tests. Below examples of such tests that are performed at standstill and during rotation up to operational speed.

HV test and turn-to-turn insulation check performed with:

• Impedance measurement at standstill and during rotation
• Flux probe measurement during rotation
• RSO measurement at standstill and during rotation
• Insulation resistance at standstill and during rotation
• Resistance measurements at standstill

Balancing documentation

In accordance with the Fortum TGS documentation policy, we fully document the work we carry out. This not only gives operators a written record of the current state of their equipment, it also simplifies future engineering and maintenance work. Examples of balancing documentation are:

• Coast-down curves after balancing
• Over-speed test result with before and after residual imbalance
• Final balancing protocol of attached balancing weights and residual imbalance
• Electrical test protocols
• Run-out measurements before and after balancing

Unit 1 has two bearing pedestals with built-in vibration velocity meters in horizontal, vertical, and axial directions. Hydraulics allow the slide bearings to be varied between soft and stiff (soft pedestals are needed for low-speed balancing, stiff pedestals for high-speed balancing). Note that heat runs with current in the rotor can only be performed at low-speed.

Unit 2 is for rotors above 30,000 kg, which require two or three bearing pedestals. All pedestals have built-in vibration velocity meters in horizontal, vertical, and axial directions. Journal bearings are mounted in stiff pedestals. Shaft movements can be measured.