Improvements relating to cooling of electric machines

An electric machine comprising:

• a stator including stator windings, and a rotor;
• wherein the rotor includes a rotor core mounted on a shaft, and a plurality of first hollow conductor bars carried by the rotor core, wherein each of the first hollow conductor bars has a first bar end and a second bar end and a fluid conduit extending between the first and second bar ends;
• wherein each first hollow conductor bar is configured such that the respective fluid conduit has an average flow area (B′) along its length,
• the electrical machine further comprising:
• a first short-circuit ring proximate the first bar ends of the plurality of first hollow conductor bars, and a second short-circuit ring proximate the second bar ends of the plurality of first hollow conductor bars,
• wherein the first short-circuit ring is configured to define an annular sump volume, and wherein first short-circuit ring is configured such that the annular sump volume is in fluid communication with an inlet end of each respective fluid conduit of the plurality of first hollow conductor bars;
• wherein an outlet end of each of the plurality of first hollow conductor bars, feeds a respective first restricted orifice,
• wherein the respective first restricted orifice is configured to have a flow area (A′) that is less than the average flow area (B′) of the respective fluid conduit of the respective first hollow conductor bar, such that, in use, the flow of cooling fluid through the outlet end of the plurality of first hollow conductor bars is balanced with cooling fluid delivered to the annular sump volume to maintain a flow of fluid through the respective fluid conduits.