This invention relates generally to dynamoelectric machines such as large turbo-generators and more particularly to cooling the end turns of the rotor winding in such machines.
A rotor for a dynamoelectric machine typically comprises a cylindrical forging of magnetic metal having a plurality of axially-extending slots formed therein at circumferentially spaced positions. Conductor bars are disposed in the slots for carrying current. The ends of the conductor bars are suitably connected with conductive end turns to form the required current pattern. Because the conductor bars and end turns give rise to resistive heating, means for cooling the rotor winding are required. Effective cooling of the rotor winding contributes directly to the output capability of a dynamoelectric machine.
Rotor endwindings are difficult regions to cool effectively, as evidenced by the various schemes that have been devised for this purpose. Conventional techniques include providing cooling passages directly in the end turn conductors ("direct cooling") or creating regions of relatively higher and lower pressures to force cooling gas to pass over conductor surfaces ("forced cooling"). These conventional techniques add considerable complexity and cost to rotor construction. For example, directly cooled conductors must be machined or fabricated to form the cooling passages. In addition, an exit manifold must be provided to discharge the gas into the rotor. Forced cooling schemes require the rotor end region to be divided into separate pressure zones, with the addition of numerous baffles, flow channels and pumping elements.
An alternative to these expensive active cooling schemes is passive cooling, sometimes referred to as "free convection cooling." Passive cooling relies on the centrifugal and rotational forces of the rotor to circulate gas in the blind, dead-end cavities formed between the concentric end turns. While passive cooling provides the advantages of minimum complexity and cost, heat removal capability is comparatively weak compared to active cooling because of low circulation velocities inside the cavities. One known approach to increasing gas velocity in the cavities is to machine holes in the retaining ring which encloses the end turns. However, because the retaining ring is under very high stress, openings therein can lead to mechanical failure.
Accordingly, there is a need for a passive cooling system for cooling rotor endwindings with improved heat removal capability without diminishing the structural integrity of the retaining rings.