Rotors and stator rewinds


Rewinding a rotor is a typical retrofit procedure performed as part of a lifetime extension program.

One good example is rewinding a rotor belonging to the ASEA/ABB GTP generator series. The design of this type of generator – parallel slotted and with shaft ends bolted to the rotor body – is unique.


Turbo-set with two generators connected to a radial-flow steam turbine.


CAD model of a parallel-slotted rotor, e.g. ASEA/ABB GTP-type. 

Our rewinding expertise with this type of rotor has led to many improvements in the rewinding process, as well as the development of specialized tools needed to perform it. The overall result of this vast practical know-how can be found in a long list of rewind reference cases. Please contact us for a complete reference list.

A further example of rotor rewinds we perform is that based on the more modern type with radial rotor slots. Thanks to close co-operation between our engineering department and our workshop personnel, we have improved not only the process but also the tools needed for rewinds with so-called spiral coils (see below). In a recent rewind project, we also incorporated improved rotor connection details for ASEA/ABB-type GTA 975CV rotors.


For stator rewinds, we are familiar with stator bars featuring both resin-rich and VPI insulation systems. We use either stator bars with strand transposition in the bar itself (so-called roebel bars) or stator bars where transposition is made when the half-bars are joined together (see below). The technology applied depends on what system was used originally, what is technically feasible based on user requirements after the rewind, and which of the two alternatives offers the most time and cost effective solution.


Assembly of stator slot wedges in order to fixate the winding in the stator slot.

One key issue when performing a stator rewind is ensuring a good electrical as well as mechanical fit of the new winding in the stator slots. Lack of good mechanical contact means a high risk of vibration and partial discharge during operation. This can lead to breakdown of the insulation system and, in a worst case, long-lasting forced outage.

Our ripple-spring wedging system ensures a long-lasting fixation force and good electrical contact between stator core and stator bar.


Section of at stator slot showing the stator bar fixation system.

We also use a system (round-packing) that we wrap around the stator bar before inserting it into the slot (see below). This ensures that the sideways fit of the stator bar in the slot is optimal, and that the long-term electrical contact is good.


Wrapping round-packing on a stator bar.

Ripple-spring wedging and round-packing have been used by our engineers for more than 30 years. Their track record, for both turbogenerators and hydro-generators designed and built in Västerås, Sweden, is very good.

New Generator (rotor/stator/exciter)

We supply new components, such as rotors, stators and exciters. In many cases, especially when downtime is costly, fitting a new component can be more cost effective than a rewind. Examples of a new stator and rotor are shown below.


New component stator for a turbo-set containing one radial flow steam turbine and two generators.


New component turbo-rotor with brushless exciter.

When designing new components such as a rotor or stator, we always consider the materials to use and whether or not the old component has any generic design weaknesses. Both are important aspects to address, even if the new component is to be ‘the same’ as the old one. Certain materials, for instance, become obsolete or may no longer be available.

Excitation equipment upgrades

We enjoy close cooperation with Voith Hydro AB for upgrading excitation equipment. Voith Hydro has in-house capabilities for the design, installation and commissioning of new excitation systems, as well as for diagnostics of existing systems. Our electrical testing and diagnostics engineers also test excitation systems as part of their on-site programs. We serve both static and brushless excitation systems.

Power upgrades

Upgrading turbines to deliver higher active power also requires a generator upgrade in order to get the power out onto the electrical grid.

The first step involves analyzing the existing generator from electro-magnetic and temperature points of view, and assessing the power upgrade possibilities. VG Power Turbo uses in-house expert software for electro-magnetic and ventilation calculations, backed up by specialists with many years’ experience in designing new generators as well as upgrading existing ones.

The second step analyzes the components or parts of components that may limit the possibility of a power upgrade, e.g. rotor winding temperature or stator core end temperature. These components are then re-designed, if necessary.

The third step is to recalculate the electro-magnetic and ventilation data to see that temperatures are within allowed limits. The figure below shows generator power capability curves for three situations:

  • Black – original generator
  • Red – generator with improved rotor cooling
  • Blue – generator with new rotor and refurbished stator

Generator capability curve (active vs. reactive power) for a typical power upgrade.

New generators

VG Power Turbo can deliver new replacement generators. Shaft height, generator footprint, layout of terminals, etc., are adapted to existing foundations and turbine layout. Generator performance and capabilities are designed by experts with years of experience in designing new generators and upgrading existing ones. Our in-house developed software for electro-magnetic and ventilation design is at their disposal. When designing a new replacement generator, we also analyze the old one and incorporate upgrades to overcome known generic weaknesses or problems associated with old materials becoming obsolete.

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A new air-cooled turbogenerator for an existing foundation.