That is to allow for thermal expansion.

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Ohm's law
Not just a good idea;
It's the LAW!

I was the customer engineer on a hydro generator rebuild job and the pole jumpers were replaced. The OEM had to consider thermal expansion but more significant was the increase in rim diameter during operating and overspeed events. The hoop stress from the mass of the rim and poles increases the rim diameter and the jumpers must be able to expand without excessive stress which would lead to fatigue failure.

petronila : That particular joint was created for thermal expansion. You can tell because of the orientation of the "flexible" connector. As the cage winding passes current, the two end ring portions expand toward each other, which requires a bit of extra "room" at the joint, if you will. More importantly, the two bars adjacent to the inter-pole gap are going to grow axially at different rates, because the current distribution across the pole head is not uniform. This means one bar will tend to grow slightly longer than the other - with corresponding stress on the end ring joint, if not accounted for by some form of flexible connection.

For rings that require compensation for centrifugal forces, the orientation of the "joint" would be in the other direction, allowing the ring to "flex" radially. More commonly, an extra retaining ring (or rings) of some non-magnetic material are used to keep the shorting ring from excessive radial motion.

Converting energy to motion for more than half a century

Other than thermal expansion, flexible connectors are useful when the damper winding fixing holes are not equally spaced.

Muthu
www.edison.co.in