An electric machine comprising: a primary mechanical output or input; a plurality of electric sub-machines (5002, 5004, 5006) configured to selectively drive, or be driven by, the primary mechanical output or input respectively; a controller configured to: monitor a parameter of the electric machine indicative of the power of the electric machine; selectively engage or disengage one or more of the plurality of electric sub-machines (5002, 5004, 5006) dependent upon the monitored parameter, so as to increase the number of electric sub-machines (5002, 5004, 5006) simultaneously driving, or being driven by, the primary mechanical output or input respectively at higher electric machine powers; characterised in that the controller is configured to: engage a first electric sub-machine (5002, 5004, 5006) of the plurality of electric sub-machines (5002, 5004, 5006) at a first electric machine power range from zero to Preq/N where Preq is an established power requirement of the electric machine and N is the number of the plurality of electric sub-machines (5002, 5004, 5006); engage a further electric sub-machine (5002, 5004, 5006) based on a power interval equal to Preq/N, in which each electric sub-machine (5002, 5004, 5006) has a maximum rated power of at least twice Preq/N such that each electric sub-machine (5002, 5004, 5006) has an operating power range spanning its maximum efficiency power point; and disengage one of the engaged electric sub-machines (5002, 5004, 5006) based on said power interval Preq/N. a primary mechanical output or input; a plurality of electric sub-machines (5002, 5004, 5006) configured to selectively drive, or be driven by, the primary mechanical output or input respectively; a controller configured to: monitor a parameter of the electric machine indicative of the power of the electric machine; selectively engage or disengage one or more of the plurality of electric sub-machines (5002, 5004, 5006) dependent upon the monitored parameter, so as to increase the number of electric sub-machines (5002, 5004, 5006) simultaneously driving, or being driven by, the primary mechanical output or input respectively at higher electric machine powers; characterised in that the controller is configured to: engage a first electric sub-machine (5002, 5004, 5006) of the plurality of electric sub-machines (5002, 5004, 5006) at a first electric machine power range from zero to Preq/N where Preq is an established power requirement of the electric machine and N is the number of the plurality of electric sub-machines (5002, 5004, 5006); engage a further electric sub-machine (5002, 5004, 5006) based on a power interval equal to Preq/N, in which each electric sub-machine (5002, 5004, 5006) has a maximum rated power of at least twice Preq/N such that each electric sub-machine (5002, 5004, 5006) has an operating power range spanning its maximum efficiency power point; and disengage one of the engaged electric sub-machines (5002, 5004, 5006) based on said power interval Preq/N. monitor a parameter of the electric machine indicative of the power of the electric machine; selectively engage or disengage one or more of the plurality of electric sub-machines (5002, 5004, 5006) dependent upon the monitored parameter, so as to increase the number of electric sub-machines (5002, 5004, 5006) simultaneously driving, or being driven by, the primary mechanical output or input respectively at higher electric machine powers; characterised in that the controller is configured to: engage a first electric sub-machine (5002, 5004, 5006) of the plurality of electric sub-machines (5002, 5004, 5006) at a first electric machine power range from zero to Preq/N where Preq is an established power requirement of the electric machine and N is the number of the plurality of electric sub-machines (5002, 5004, 5006); engage a further electric sub-machine (5002, 5004, 5006) based on a power interval equal to Preq/N, in which each electric sub-machine (5002, 5004, 5006) has a maximum rated power of at least twice Preq/N such that each electric sub-machine (5002, 5004, 5006) has an operating power range spanning its maximum efficiency power point; and disengage one of the engaged electric sub-machines (5002, 5004, 5006) based on said power interval Preq/N.