A method of detecting a rotor fault in an electric machine using a data analysis unit, the method comprising: a. measuring magnetic field strength along a first axis and a second axis, wherein the first axis is at a first angle to the axis of the rotor of the electric machine and the second axis is at a second angle to the axis of the rotor of the electric machine; b. determining operating slip frequency of the electric machine using the measured magnetic field strength along the first axis and the second axis, wherein determining operating slip frequency comprises i. generating a first frequency spectrum from the measured magnetic field strength along the first axis and a second frequency spectrum from the magnetic field strength along the second axis; ii. identifying a peak within a predetermined range in at least one of the first frequency spectrum and the second frequency spectrum; and iii. determining a frequency corresponding to the identified peak within the predetermined range, in at least one of the first frequency spectrum and the second frequency spectrum; c. determining an amplitude of the determined operating slip frequency in the first frequency spectrum and an amplitude of operating slip frequency in the second frequency spectrum; and d. detecting the rotor fault by comparing the amplitude of the determined operating slip frequency in the first frequency spectrum and the amplitude of operating slip frequency in the second frequency spectrum against a predetermined threshold. a. measuring magnetic field strength along a first axis and a second axis, wherein the first axis is at a first angle to the axis of the rotor of the electric machine and the second axis is at a second angle to the axis of the rotor of the electric machine; b. determining operating slip frequency of the electric machine using the measured magnetic field strength along the first axis and the second axis, wherein determining operating slip frequency comprises i. generating a first frequency spectrum from the measured magnetic field strength along the first axis and a second frequency spectrum from the magnetic field strength along the second axis; ii. identifying a peak within a predetermined range in at least one of the first frequency spectrum and the second frequency spectrum; and iii. determining a frequency corresponding to the identified peak within the predetermined range, in at least one of the first frequency spectrum and the second frequency spectrum; i. generating a first frequency spectrum from the measured magnetic field strength along the first axis and a second frequency spectrum from the magnetic field strength along the second axis; ii. identifying a peak within a predetermined range in at least one of the first frequency spectrum and the second frequency spectrum; and iii. determining a frequency corresponding to the identified peak within the predetermined range, in at least one of the first frequency spectrum and the second frequency spectrum; c. determining an amplitude of the determined operating slip frequency in the first frequency spectrum and an amplitude of operating slip frequency in the second frequency spectrum; and d. detecting the rotor fault by comparing the amplitude of the determined operating slip frequency in the first frequency spectrum and the amplitude of operating slip frequency in the second frequency spectrum against a predetermined threshold.