An electrical machine comprising: a stator; a rotor assembly disposed within the stator and configured to rotate relative to the stator, wherein the rotor assembly comprises: a rotor core comprising a plurality of salient rotor poles that are spaced apart from one another around an inner hub such that an interpolar gap is formed between each adjacent pair of salient rotor poles, with a dovetail-shaped opening being defined by the rotor core in each interpolar gap; and a plurality of inserts positioned in the gaps formed between the plurality of salient rotor poles to fill the entire gap, the plurality of inserts being formed of electrically non-conductive and non-magnetic material; wherein each of the plurality of inserts comprises a mating feature formed an axially inner edge thereof that is configured to mate with a respective dovetail shaped opening being defined by the rotor core; wherein the plurality of inserts comprises an outer shell formed of a first electrically non-conductive, non-magnetic material, the outer shell defining a hollow area there within, wherein: an inner filler is positioned in the hollow area defined by the outer shell, the inner filler being formed of a second non-conductive, non-magnetic material different from that used to form the outer shell and having a lower density than the first electrically non-conductive, non-magnetic material, and the inner filler is a dielectric foam; each of the plurality of rotor teeth includes a pair of axially-extending notches formed in proximity to the radially outermost portion thereof; and the outer shell comprises a shoulder on each end thereof configured to mate with the pair of axially-extending notches formed on the rotor teeth so that the shoulders and the mating feature hold the insert in place against centrifugal force experienced during rotation of the rotor assembly. a stator; a rotor assembly disposed within the stator and configured to rotate relative to the stator, wherein the rotor assembly comprises: a rotor core comprising a plurality of salient rotor poles that are spaced apart from one another around an inner hub such that an interpolar gap is formed between each adjacent pair of salient rotor poles, with a dovetail-shaped opening being defined by the rotor core in each interpolar gap; and a plurality of inserts positioned in the gaps formed between the plurality of salient rotor poles to fill the entire gap, the plurality of inserts being formed of electrically non-conductive and non-magnetic material; wherein each of the plurality of inserts comprises a mating feature formed an axially inner edge thereof that is configured to mate with a respective dovetail shaped opening being defined by the rotor core; a rotor core comprising a plurality of salient rotor poles that are spaced apart from one another around an inner hub such that an interpolar gap is formed between each adjacent pair of salient rotor poles, with a dovetail-shaped opening being defined by the rotor core in each interpolar gap; and a plurality of inserts positioned in the gaps formed between the plurality of salient rotor poles to fill the entire gap, the plurality of inserts being formed of electrically non-conductive and non-magnetic material; wherein each of the plurality of inserts comprises a mating feature formed an axially inner edge thereof that is configured to mate with a respective dovetail shaped opening being defined by the rotor core; wherein the plurality of inserts comprises an outer shell formed of a first electrically non-conductive, non-magnetic material, the outer shell defining a hollow area there within, wherein: an inner filler is positioned in the hollow area defined by the outer shell, the inner filler being formed of a second non-conductive, non-magnetic material different from that used to form the outer shell and having a lower density than the first electrically non-conductive, non-magnetic material, and the inner filler is a dielectric foam; each of the plurality of rotor teeth includes a pair of axially-extending notches formed in proximity to the radially outermost portion thereof; and the outer shell comprises a shoulder on each end thereof configured to mate with the pair of axially-extending notches formed on the rotor teeth so that the shoulders and the mating feature hold the insert in place against centrifugal force experienced during rotation of the rotor assembly. an inner filler is positioned in the hollow area defined by the outer shell, the inner filler being formed of a second non-conductive, non-magnetic material different from that used to form the outer shell and having a lower density than the first electrically non-conductive, non-magnetic material, and the inner filler is a dielectric foam; each of the plurality of rotor teeth includes a pair of axially-extending notches formed in proximity to the radially outermost portion thereof; and the outer shell comprises a shoulder on each end thereof configured to mate with the pair of axially-extending notches formed on the rotor teeth so that the shoulders and the mating feature hold the insert in place against centrifugal force experienced during rotation of the rotor assembly.