A variable speed machine assembly comprising: an input shaft; a variable speed magnetically geared generator coupled to said input shaft; an electrical machine coupled to said input shaft; and a power converter coupled to said variable speed magnetically geared generator and said electrical machine, said power converter configured to use electrical power output by said electrical machine to control a frequency of power output by said variable speed magnetically geared generator, wherein said variable speed magnetically geared generator comprises a first stator, a second stator, a rotor having soft magnetic pole pieces located between the first stator and the second stator and coupled to said input shaft, a first set of windings coupled to the first stator and to a grid, and a second set of windings coupled to the second stator and to said power converter and configured to transfer electrical power to said power converter and receive electrical power from said power converter, wherein said power converter is configured to output electrical power to said second set of windings to control and to adjust the frequency of the power output by said first set of windings to a frequency that maintains a grid terminal frequency at a substantially constant value, to receive electrical power from said electrical machine, adjust a frequency of the received electrical power, output the adjusted electrical power to said variable speed magnetically geared generator to control the frequency of the power output by said variable speed magnetically geared generator, and to receive variable-frequency electrical power from said variable speed magnetically geared generator, adjust the variable-frequency electrical power, and output the adjusted electrical power to said electrical machine to control a rotation of said input shaft at said electrical machine to control said variable speed magnetically geared generator. an input shaft; a variable speed magnetically geared generator coupled to said input shaft; an electrical machine coupled to said input shaft; and a power converter coupled to said variable speed magnetically geared generator and said electrical machine, said power converter configured to use electrical power output by said electrical machine to control a frequency of power output by said variable speed magnetically geared generator, wherein said variable speed magnetically geared generator comprises a first stator, a second stator, a rotor having soft magnetic pole pieces located between the first stator and the second stator and coupled to said input shaft, a first set of windings coupled to the first stator and to a grid, and a second set of windings coupled to the second stator and to said power converter and configured to transfer electrical power to said power converter and receive electrical power from said power converter, wherein said power converter is configured to output electrical power to said second set of windings to control and to adjust the frequency of the power output by said first set of windings to a frequency that maintains a grid terminal frequency at a substantially constant value, to receive electrical power from said electrical machine, adjust a frequency of the received electrical power, output the adjusted electrical power to said variable speed magnetically geared generator to control the frequency of the power output by said variable speed magnetically geared generator, and to receive variable-frequency electrical power from said variable speed magnetically geared generator, adjust the variable-frequency electrical power, and output the adjusted electrical power to said electrical machine to control a rotation of said input shaft at said electrical machine to control said variable speed magnetically geared generator. to output electrical power to said second set of windings to control and to adjust the frequency of the power output by said first set of windings to a frequency that maintains a grid terminal frequency at a substantially constant value, to receive electrical power from said electrical machine, adjust a frequency of the received electrical power, output the adjusted electrical power to said variable speed magnetically geared generator to control the frequency of the power output by said variable speed magnetically geared generator, and to receive variable-frequency electrical power from said variable speed magnetically geared generator, adjust the variable-frequency electrical power, and output the adjusted electrical power to said electrical machine to control a rotation of said input shaft at said electrical machine to control said variable speed magnetically geared generator.